Unleashing the real power of Core Data with the elegance and safety of Swift



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• Swift 5.9: iOS 13+ / macOS 10.15+ / watchOS 7.4+ / tvOS 13.0+
• Previously supported Swift versions: Swift 5.4, Swift 5.3, Swift 5.7

Upgrading from previous CoreStore versions? Check out the 🆕 features and make sure to read the Change logs.

CoreStore is part of the Swift Source Compatibility projects.

Contents

• TL;DR (a.k.a. sample codes)
• Why use CoreStore?
• Architecture
• CoreStore Tutorials (All of these have demos in the Demo app project!)
  • Setting up
    • In-memory store
    • Local store
  • Migrations
    • Declaring model versions
    • Starting migrations
    • Progressive migrations
    • Forecasting migrations
    • Custom migrations
  • Saving and processing transactions
    • Transaction types
      • Asynchronous transactions
      • Synchronous transactions
      • Unsafe transactions
    • Creating objects
    • Updating objects
    • Deleting objects
    • Passing objects safely
  • Importing data
  • Fetching and querying
    • From clause
    • Fetching
      • Where clause
      • OrderBy clause
      • Tweak clause
    • Querying
      • Select<T> clause
      • GroupBy clause
  • Logging and error reporting
  • Observing changes and notifications
    • Observe a single property
    • Observe a single object's updates
    • Observe a single object's per-property updates
    • Observe a diffable list
    • Observe detailed list changes
  • Type-safe CoreStoreObjects
    • New @Field Property Wrapper syntax
      • @Field.Stored
      • @Field.Virtual
      • @Field.Coded
      • @Field.Relationship
      • @Field usage notes
    • VersionLocks
  • Reactive Programming
    • RxSwift
    • 🆕Combine
      • 🆕DataStack.reactive
      • 🆕ListPublisher.reactive
      • 🆕ObjectPublisher.reactive
  • 🆕SwiftUI Utilities
    • 🆕SwiftUI Views`
      • 🆕ListReader
      • 🆕ObjectReader
    • 🆕SwiftUI Property Wrappers
      • 🆕ListState
      • 🆕ObjectState
    • 🆕SwiftUI Extensions
      • 🆕ForEach
• Roadmap
• Installation
• Changesets
• Contact
• Who uses CoreStore?
• License

TL;DR (a.k.a. sample codes)

Pure-Swift models:

class Person: CoreStoreObject {
    @Field.Stored("name")
    var name: String = ""
    
    @Field.Relationship("pets", inverse: \Dog.$master)
    var pets: Set<Dog>
}

(Classic NSManagedObjects also supported)

Setting-up with progressive migration support:

dataStack = DataStack(
    xcodeModelName: "MyStore",
    migrationChain: ["MyStore", "MyStoreV2", "MyStoreV3"]
)

Adding a store:

dataStack.addStorage(
    SQLiteStore(fileName: "MyStore.sqlite"),
    completion: { (result) -> Void in
        // ...
    }
)

Starting transactions:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let person = transaction.create(Into<Person>())
        person.name = "John Smith"
        person.age = 42
    },
    completion: { (result) -> Void in
        switch result {
        case .success: print("success!")
        case .failure(let error): print(error)
        }
    }
)

Fetching objects (simple):

let people = try dataStack.fetchAll(From<Person>())

Fetching objects (complex):

let people = try dataStack.fetchAll(
    From<Person>()
        .where(\.age > 30),
        .orderBy(.ascending(\.name), .descending(.\age)),
        .tweak({ $0.includesPendingChanges = false })
)

Querying values:

let maxAge = try dataStack.queryValue(
    From<Person>()
        .select(Int.self, .maximum(\.age))
)

But really, there's a reason I wrote this huge README. Read up on the details!

Check out the Demo app project for sample codes as well!

Why use CoreStore?

CoreStore was (and is) heavily shaped by real-world needs of developing data-dependent apps. It enforces safe and convenient Core Data usage while letting you take advantage of the industry's encouraged best practices.

Features

• 🆕SwiftUI and Combine API utilities. ListPublishers and ObjectPublishers now have their @ListState and @ObjectState SwiftUI property wrappers. Combine Publisher s are also available through the ListPublisher.reactive, ObjectPublisher.reactive, and DataStack.reactive namespaces.
• Backwards-portable DiffableDataSources implementation! UITableViews and UICollectionViews now have a new ally: ListPublishers provide diffable snapshots that make reloading animations very easy and very safe. Say goodbye to UITableViews and UICollectionViews reload errors!
• 💎Tight design around Swift’s code elegance and type safety. CoreStore fully utilizes Swift's community-driven language features.
• 🚦Safer concurrency architecture. CoreStore makes it hard to fall into common concurrency mistakes. The main NSManagedObjectContext is strictly read-only, while all updates are done through serial transactions. (See Saving and processing transactions)
• 🔍Clean fetching and querying API. Fetching objects is easy, but querying for raw aggregates (min, max, etc.) and raw property values is now just as convenient. (See Fetching and querying)
• 🔭Type-safe, easy to configure observers. You don't have to deal with the burden of setting up NSFetchedResultsControllers and KVO. As an added bonus, list and object observable types all support multiple observers. This means you can have multiple view controllers efficiently share a single resource! (See Observing changes and notifications)
• 📥Efficient importing utilities. Map your entities once with their corresponding import source (JSON for example), and importing from transactions becomes elegant. Uniquing is also done with an efficient find-and-replace algorithm. (See Importing data)
• 🗑Say goodbye to .xcdatamodeld files! While CoreStore supports NSManagedObjects, it offers CoreStoreObject whose subclasses can declare type-safe properties all in Swift code without the need to maintain separate resource files for the models. As bonus, these special properties support custom types, and can be used to create type-safe keypaths and queries. (See Type-safe CoreStoreObjects)
• 🔗Progressive migrations. No need to think how to migrate from all previous model versions to your latest model. Just tell the DataStack the sequence of version strings (MigrationChains) and CoreStore will automatically use progressive migrations when needed. (See Migrations)
• Easier custom migrations. Say goodbye to .xcmappingmodel files; CoreStore can now infer entity mappings when possible, while still allowing an easy way to write custom mappings. (See Migrations)
• 📝Plug-in your own logging framework. Although a default logger is built-in, all logging, asserting, and error reporting can be funneled to CoreStoreLogger protocol implementations. (See Logging and error reporting)
• ⛓Heavy support for multiple persistent stores per data stack. CoreStore lets you manage separate stores in a single DataStack, just the way .xcdatamodeld configurations are designed to. CoreStore will also manage one stack by default, but you can create and manage as many as you need. (See Setting up)
• 🎯Free to name entities and their class names independently. CoreStore gets around a restriction with other Core Data wrappers where the entity name should be the same as the NSManagedObject subclass name. CoreStore loads entity-to-class mappings from the managed object model file, so you can assign independent names for the entities and their class names.
• 📙Full Documentation. No magic here; all public classes, functions, properties, etc. have detailed Apple Docs. This README also introduces a lot of concepts and explains a lot of CoreStore's behavior.
• ℹ️Informative (and pretty) logs. All CoreStore and Core Data-related types now have very informative and pretty print outputs! (See Logging and error reporting)
• 🛡More extensive Unit Tests. Extending CoreStore is safe without having to worry about breaking old behavior.

Have ideas that may benefit other Core Data users? Feature Requests are welcome!

Architecture

For maximum safety and performance, CoreStore will enforce coding patterns and practices it was designed for. (Don't worry, it's not as scary as it sounds.) But it is advisable to understand the "magic" of CoreStore before you use it in your apps.

If you are already familiar with the inner workings of CoreData, here is a mapping of CoreStore abstractions:

A lot of Core Data wrapper libraries set up their NSManagedObjectContexts this way:

Nesting saves from child context to the root context ensures maximum data integrity between contexts without blocking the main queue. But in reality, merging contexts is still by far faster than saving contexts. CoreStore's DataStack takes the best of both worlds by treating the main NSManagedObjectContext as a read-only context (or "viewContext"), and only allows changes to be made within transactions on the child context:

This allows for a butter-smooth main thread, while still taking advantage of safe nested contexts.

Setting up

The simplest way to initialize CoreStore is to add a default store to the default stack:

try CoreStoreDefaults.dataStack.addStorageAndWait()

This one-liner does the following:

• Triggers the lazy-initialization of CoreStoreDefaults.dataStack with a default DataStack
• Sets up the stack's NSPersistentStoreCoordinator, the root saving NSManagedObjectContext, and the read-only main NSManagedObjectContext
• Adds an SQLiteStore in the "Application Support/" directory (or the "Caches/" directory on tvOS) with the file name "[App bundle name].sqlite"
• Creates and returns the NSPersistentStore instance on success, or an NSError on failure

For most cases, this configuration is enough as it is. But for more hardcore settings, refer to this extensive example:

let dataStack = DataStack(
    xcodeModelName: "MyModel", // loads from the "MyModel.xcdatamodeld" file
    migrationChain: ["MyStore", "MyStoreV2", "MyStoreV3"] // model versions for progressive migrations
)
let migrationProgress = dataStack.addStorage(
    SQLiteStore(
        fileURL: sqliteFileURL, // set the target file URL for the sqlite file
        configuration: "Config2", // use entities from the "Config2" configuration in the .xcdatamodeld file
        localStorageOptions: .recreateStoreOnModelMismatch // if migration paths cannot be resolved, recreate the sqlite file
    ),
    completion: { (result) -> Void in
        switch result {
        case .success(let storage):
            print("Successfully added sqlite store: \(storage)")
        case .failure(let error):
            print("Failed adding sqlite store with error: \(error)")
        }
    }
)

CoreStoreDefaults.dataStack = dataStack // pass the dataStack to CoreStore for easier access later on

💡If you have never heard of "Configurations", you'll find them in your .xcdatamodeld file

In our sample code above, note that you don't need to do the CoreStoreDefaults.dataStack = dataStack line. You can just as well hold a reference to the DataStack like below and call all its instance methods directly:

class MyViewController: UIViewController {
    let dataStack = DataStack(xcodeModelName: "MyModel") // keep reference to the stack
    override func viewDidLoad() {
        super.viewDidLoad()
        do {
            try self.dataStack.addStorageAndWait(SQLiteStore.self)
        }
        catch { // ...
        }
    }
    func methodToBeCalledLaterOn() {
        let objects = self.dataStack.fetchAll(From<MyEntity>())
        print(objects)
    }
}

💡By default, CoreStore will initialize NSManagedObjects from .xcdatamodeld files, but you can create models completely from source code using CoreStoreObjects and CoreStoreSchema. To use this feature, refer to Type-safe CoreStoreObjects.

Notice that in our previous examples, addStorageAndWait(_:) and addStorage(_:completion:) both accept either InMemoryStore, or SQLiteStore. These implement the StorageInterface protocol.

In-memory store

The most basic StorageInterface concrete type is the InMemoryStore, which just stores objects in memory. Since InMemoryStores always start with a fresh empty data, they do not need any migration information.

try dataStack.addStorageAndWait(
    InMemoryStore(
        configuration: "Config2" // optional. Use entities from the "Config2" configuration in the .xcdatamodeld file
    )
)

Asynchronous variant:

try dataStack.addStorage(
    InMemoryStore(
        configuration: "Config2
    ),
    completion: { storage in
        // ...
    }
)

(A reactive-programming variant of this method is explained in detail in the section on DataStack Combine publishers)

Local Store

The most common StorageInterface you will probably use is the SQLiteStore, which saves data in a local SQLite file.

let migrationProgress = dataStack.addStorage(
    SQLiteStore(
        fileName: "MyStore.sqlite",
        configuration: "Config2", // optional. Use entities from the "Config2" configuration in the .xcdatamodeld file
        migrationMappingProviders: [Bundle.main], // optional. The bundles that contain required .xcmappingmodel files
        localStorageOptions: .recreateStoreOnModelMismatch // optional. Provides settings that tells the DataStack how to setup the persistent store
    ),
    completion: { /* ... */ }
)

Refer to the SQLiteStore.swift source documentation for detailed explanations for each of the default values.

CoreStore can decide the default values for these properties, so SQLiteStores can be initialized with no arguments:

try dataStack.addStorageAndWait(SQLiteStore())

(The asynchronous variant of this method is explained further in the next section on Migrations, and a reactive-programming variant in the section on DataStack Combine publishers)

The file-related properties of SQLiteStore are actually requirements of another protocol that it implements, the LocalStorage protocol:

public protocol LocalStorage: StorageInterface {
    var fileURL: NSURL { get }
    var migrationMappingProviders: [SchemaMappingProvider] { get }
    var localStorageOptions: LocalStorageOptions { get }
    func dictionary(forOptions: LocalStorageOptions) -> [String: AnyObject]?
    func cs_eraseStorageAndWait(metadata: [String: Any], soureModelHint: NSManagedObjectModel?) throws
}

If you have custom NSIncrementalStore or NSAtomicStore subclasses, you can implement this protocol and use it similarly to SQLiteStore.

Migrations

Declaring model versions

Model versions are now expressed as a first-class protocol, DynamicSchema. CoreStore currently supports the following schema classes:

• XcodeDataModelSchema: a model version with entities loaded from a .xcdatamodeld file.
• CoreStoreSchema: a model version created with CoreStoreObject entities. (See Type-safe CoreStoreObjects)
• UnsafeDataModelSchema: a model version created with an existing NSManagedObjectModel instance.

All the DynamicSchema for all model versions are then collected within a single SchemaHistory instance, which is then handed to the DataStack. Here are some common use cases:

Multiple model versions grouped in a .xcdatamodeld file (Core Data standard method)

CoreStoreDefaults.dataStack = DataStack(
    xcodeModelName: "MyModel",
    bundle: Bundle.main,
    migrationChain: ["MyAppModel", "MyAppModelV2", "MyAppModelV3", "MyAppModelV4"]
)

CoreStoreSchema-based model version (No .xcdatamodeld file needed) (For more details, see also Type-safe CoreStoreObjects)

class Animal: CoreStoreObject {
    // ...
}
class Dog: Animal {
    // ...
}
class Person: CoreStoreObject {
    // ...
}

CoreStoreDefaults.dataStack = DataStack(
    CoreStoreSchema(
        modelVersion: "V1",
        entities: [
            Entity<Animal>("Animal", isAbstract: true),
            Entity<Dog>("Dog"),
            Entity<Person>("Person")
        ]
    )
)

Models in a .xcdatamodeld file during past app versions, but migrated to the new CoreStoreSchema method

class Animal: CoreStoreObject {
    // ...
}
class Dog: Animal {
    // ...
}
class Person: CoreStoreObject {
    // ...
}

let legacySchema = XcodeDataModelSchema.from(
    modelName: "MyModel", // .xcdatamodeld name
    bundle: bundle,
    migrationChain: ["MyAppModel", "MyAppModelV2", "MyAppModelV3", "MyAppModelV4"]
)
let newSchema = CoreStoreSchema(
    modelVersion: "V1",
    entities: [
        Entity<Animal>("Animal", isAbstract: true),
        Entity<Dog>("Dog"),
        Entity<Person>("Person")
    ]
)
CoreStoreDefaults.dataStack = DataStack(
    schemaHistory: SchemaHistory(
        legacySchema + [newSchema],
        migrationChain: ["MyAppModel", "MyAppModelV2", "MyAppModelV3", "MyAppModelV4", "V1"] 
    )
)   

CoreStoreSchema-based model versions with progressive migration

typealias Animal = V2.Animal
typealias Dog = V2.Dog
typealias Person = V2.Person
enum V2 {
    class Animal: CoreStoreObject {
        // ...
    }
    class Dog: Animal {
        // ...
    }
    class Person: CoreStoreObject {
        // ...
    }
}
enum V1 {
    class Animal: CoreStoreObject {
        // ...
    }
    class Dog: Animal {
        // ...
    }
    class Person: CoreStoreObject {
        // ...
    }
}

CoreStoreDefaults.dataStack = DataStack(
    CoreStoreSchema(
        modelVersion: "V1",
        entities: [
            Entity<V1.Animal>("Animal", isAbstract: true),
            Entity<V1.Dog>("Dog"),
            Entity<V1.Person>("Person")
        ]
    ),
    CoreStoreSchema(
        modelVersion: "V2",
        entities: [
            Entity<V2.Animal>("Animal", isAbstract: true),
            Entity<V2.Dog>("Dog"),
            Entity<V2.Person>("Person")
        ]
    ),
    migrationChain: ["V1", "V2"]
)

Starting migrations

We have seen addStorageAndWait(...) used to initialize our persistent store. As the method name's ~AndWait suffix suggests though, this method blocks so it should not do long tasks such as data migrations. In fact CoreStore will only attempt a synchronous lightweight migration if you explicitly provide the .allowSynchronousLightweightMigration option:

try dataStack.addStorageAndWait(
    SQLiteStore(
        fileURL: sqliteFileURL,
        localStorageOptions: .allowSynchronousLightweightMigration
    )
}

if you do so, any model mismatch will be thrown as an error.

In general though, if migrations are expected the asynchronous variant addStorage(_:completion:) method is recommended instead:

let migrationProgress: Progress? = try dataStack.addStorage(
    SQLiteStore(
        fileName: "MyStore.sqlite",
        configuration: "Config2"
    ),
    completion: { (result) -> Void in
        switch result {
        case .success(let storage):
            print("Successfully added sqlite store: \(storage)")
        case .failure(let error):
            print("Failed adding sqlite store with error: \(error)")
        }
    }
)

The completion block reports a SetupResult that indicates success or failure.

(A reactive-programming variant of this method is explained further in the section on DataStack Combine publishers)

Notice that this method also returns an optional Progress. If nil, no migrations are needed, thus progress reporting is unnecessary as well. If not nil, you can use this to track migration progress by using standard KVO on the "fractionCompleted" key, or by using a closure-based utility exposed in Progress+Convenience.swift:

migrationProgress?.setProgressHandler { [weak self] (progress) -> Void in
    self?.progressView?.setProgress(Float(progress.fractionCompleted), animated: true)
    self?.percentLabel?.text = progress.localizedDescription // "50% completed"
    self?.stepLabel?.text = progress.localizedAdditionalDescription // "0 of 2"
}

This closure is executed on the main thread so UIKit and AppKit calls can be done safely.

Progressive migrations

By default, CoreStore uses Core Data's default automatic migration mechanism. In other words, CoreStore will try to migrate the existing persistent store until it matches the SchemaHistory's currentModelVersion. If no mapping model path is found from the store's version to the data model's version, CoreStore gives up and reports an error.

The DataStack lets you specify hints on how to break a migration into several sub-migrations using a MigrationChain. This is typically passed to the DataStack initializer and will be applied to all stores added to the DataStack with addSQLiteStore(...) and its variants:

let dataStack = DataStack(migrationChain: 
    ["MyAppModel", "MyAppModelV2", "MyAppModelV3", "MyAppModelV4"])

The most common usage is to pass in the model version (.xcdatamodeld version names for NSManagedObjects, or the modelName for CoreStoreSchemas) in increasing order as above.

For more complex, non-linear migration paths, you can also pass in a version tree that maps the key-values to the source-destination versions:

let dataStack = DataStack(migrationChain: [
    "MyAppModel": "MyAppModelV3",
    "MyAppModelV2": "MyAppModelV4",
    "MyAppModelV3": "MyAppModelV4"
])

This allows for different migration paths depending on the starting version. The example above resolves to the following paths:

• MyAppModel-MyAppModelV3-MyAppModelV4
• MyAppModelV2-MyAppModelV4
• MyAppModelV3-MyAppModelV4

Initializing with empty values (either nil, [], or [:]) instructs the DataStack to disable progressive migrations and revert to the default migration behavior (i.e. use the .xcdatamodeld's current version as the final version):

let dataStack = DataStack(migrationChain: nil)

The MigrationChain is validated when passed to the DataStack and unless it is empty, will raise an assertion if any of the following conditions are met:

• a version appears twice in an array
• a version appears twice as a key in a dictionary literal
• a loop is found in any of the paths

⚠️Important: If a MigrationChain is specified, the .xcdatamodeld's "Current Version" will be bypassed and the MigrationChain's leafmost version will be the DataStack's base model version.

Forecasting migrations

Sometimes migrations are huge and you may want prior information so your app could display a loading screen, or to display a confirmation dialog to the user. For this, CoreStore provides a requiredMigrationsForStorage(_:) method you can use to inspect a persistent store before you actually call addStorageAndWait(_:) or addStorage(_:completion:):

do {
    let storage = SQLiteStorage(fileName: "MyStore.sqlite")
    let migrationTypes: [MigrationType] = try dataStack.requiredMigrationsForStorage(storage)
    if migrationTypes.count > 1
        || (migrationTypes.filter { $0.isHeavyweightMigration }.count) > 0 {
        // ... will migrate more than once. Show special waiting screen
    }
    else if migrationTypes.count > 0 {
        // ... will migrate just once. Show simple activity indicator
    }
    else {
        // ... Do nothing
    }
    dataStack.addStorage(storage, completion: { /* ... */ })
}
catch {
    // ... either inspection of the store failed, or if no mapping model was found/inferred
}

requiredMigrationsForStorage(_:) returns an array of MigrationTypes, where each item in the array may be either of the following values:

case lightweight(sourceVersion: String, destinationVersion: String)
case heavyweight(sourceVersion: String, destinationVersion: String)

Each MigrationType indicates the migration type for each step in the MigrationChain. Use these information as fit for your app.

Custom migrations

CoreStore offers several ways to declare migration mappings:

• CustomSchemaMappingProvider: A mapping provider that infers mapping initially, but also accepts custom mappings for specified entities. This was added to support custom migrations with CoreStoreObjects as well, but may also be used with NSManagedObjects.
• XcodeSchemaMappingProvider: A mapping provider which loads entity mappings from .xcmappingmodel files in a specified Bundle.
• InferredSchemaMappingProvider: The default mapping provider which tries to infer model migration between two DynamicSchema versions either by searching all .xcmappingmodel files from Bundle.allBundles, or by relying on lightweight migration if possible.

These mapping providers conform to SchemaMappingProvider and can be passed to SQLiteStore's initializer:

let dataStack = DataStack(migrationChain: ["MyAppModel", "MyAppModelV2", "MyAppModelV3", "MyAppModelV4"])
_ = try dataStack.addStorage(
    SQLiteStore(
        fileName: "MyStore.sqlite",
        migrationMappingProviders: [
            XcodeSchemaMappingProvider(from: "V1", to: "V2", mappingModelBundle: Bundle.main),
            CustomSchemaMappingProvider(from: "V2", to: "V3", entityMappings: [.deleteEntity("Person") ])
        ]
    ),
    completion: { (result) -> Void in
        // ...
    }
)

For version migrations present in the DataStack's MigrationChain but not handled by any of the SQLiteStore's migrationMappingProviders array, CoreStore will automatically try to use InferredSchemaMappingProvider as fallback. Finally if the InferredSchemaMappingProvider could not resolve any mapping, the migration will fail and the DataStack.addStorage(...) method will report the failure.

For CustomSchemaMappingProvider, more granular updates are supported through the dynamic objects UnsafeSourceObject and UnsafeDestinationObject. The example below allows the migration to conditionally ignore some objects:

let person_v2_to_v3_mapping = CustomSchemaMappingProvider(
    from: "V2",
    to: "V3",
    entityMappings: [
        .transformEntity(
            sourceEntity: "Person",
            destinationEntity: "Person",
            transformer: { (sourceObject: UnsafeSourceObject, createDestinationObject: () -> UnsafeDestinationObject) in
                
                if (sourceObject["isVeryOldAccount"] as! Bool?) == true {
                    return // this account is too old, don't migrate 
                }
                // migrate the rest
                let destinationObject = createDestinationObject()
                destinationObject.enumerateAttributes { (attribute, sourceAttribute) in
                
                if let sourceAttribute = sourceAttribute {
                    destinationObject[attribute] = sourceObject[sourceAttribute]
                }
            }
        ) 
    ]
)
SQLiteStore(
    fileName: "MyStore.sqlite",
    migrationMappingProviders: [person_v2_to_v3_mapping]
)

The UnsafeSourceObject is a read-only proxy for an object existing in the source model version. The UnsafeDestinationObject is a read-write object that is inserted (optionally) to the destination model version. Both classes' properties are accessed through key-value-coding.

Saving and processing transactions

To ensure deterministic state for objects in the read-only NSManagedObjectContext, CoreStore does not expose API's for updating and saving directly from the main context (or any other context for that matter.) Instead, you spawn transactions from DataStack instances:

let dataStack = self.dataStack
dataStack.perform(
    asynchronous: { (transaction) -> Void in
        // make changes
    },
    completion: { (result) -> Void in
        // ...
    }
)

Transaction closures automatically save changes once the closures completes. To cancel and rollback a transaction, throw a CoreStoreError.userCancelled from inside the closure by calling try transaction.cancel():

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        // ...
        if shouldCancel {
            try transaction.cancel()
        }
        // ...
    },
    completion: { (result) -> Void in
        if case .failure(.userCancelled) = result {
            // ... cancelled
        }
    }
)

⚠️Important: Never use try? or try! on a transaction.cancel() call. Always use try. Using try? will swallow the cancellation and the transaction will proceed to save as normal. Using try! will crash the app as transaction.cancel() will always throw an error.

The examples above use perform(asynchronous:...), but there are actually 3 types of transactions at your disposal: asynchronous, synchronous, and unsafe.

Transaction types

Asynchronous transactions

are spawned from perform(asynchronous:...). This method returns immediately and executes its closure from a background serial queue. The return value for the closure is declared as a generic type, so any value returned from the closure can be passed to the completion result:

dataStack.perform(
    asynchronous: { (transaction) -> Bool in
        // make changes
        return transaction.hasChanges
    },
    completion: { (result) -> Void in
        switch result {
        case .success(let hasChanges): print("success! Has changes? \(hasChanges)")
        case .failure(let error): print(error)
        }
    }
)

The success and failure can also be declared as separate handlers:

dataStack.perform(
    asynchronous: { (transaction) -> Int in
        // make changes
        return transaction.delete(objects)
    },
    success: { (numberOfDeletedObjects: Int) -> Void in
        print("success! Deleted \(numberOfDeletedObjects) objects")
    },
    failure: { (error) -> Void in
        print(error)
    }
)

⚠️Be careful when returning NSManagedObjects or CoreStoreObjects from the transaction closure. Those instances are for the transaction's use only. See Passing objects safely.

Transactions created from perform(asynchronous:...) are instances of AsynchronousDataTransaction.

Synchronous transactions

are created from perform(synchronous:...). While the syntax is similar to its asynchronous counterpart, perform(synchronous:...) waits for its transaction block to complete before returning:

let hasChanges = dataStack.perform(
    synchronous: { (transaction) -> Bool in
        // make changes
        return transaction.hasChanges
    }
)

transaction above is a SynchronousDataTransaction instance.

Since perform(synchronous:...) technically blocks two queues (the caller's queue and the transaction's background queue), it is considered less safe as it's more prone to deadlock. Take special care that the closure does not block on any other external queues.

By default, perform(synchronous:...) will wait for observers such as ListMonitors to be notified before the method returns. This may cause deadlocks, especially if you are calling this from the main thread. To reduce this risk, you may try to set the waitForAllObservers: parameter to false. Doing so tells the SynchronousDataTransaction to block only until it completes saving. It will not wait for other context's to receive those changes. This reduces deadlock risk but may have surprising side-effects:

dataStack.perform(
    synchronous: { (transaction) in
        let person = transaction.create(Into<Person>())
        person.name = "John"
    },
    waitForAllObservers: false
)
let newPerson = dataStack.fetchOne(From<Person>.where(\.name == "John"))
// newPerson may be nil!
// The DataStack may have not yet received the update notification.

Due to this complicated nature of synchronous transactions, if your app has very heavy transaction throughput it is highly recommended to use asynchronous transactions instead.

Unsafe transactions

are special in that they do not enclose updates within a closure:

let transaction = dataStack.beginUnsafe()
// make changes
downloadJSONWithCompletion({ (json) -> Void in

    // make other changes
    transaction.commit()
})
downloadAnotherJSONWithCompletion({ (json) -> Void in

    // make some other changes
    transaction.commit()
})

This allows for non-contiguous updates. Do note that this flexibility comes with a price: you are now responsible for managing concurrency for the transaction. As uncle Ben said, "with great power comes great race conditions."

As the above example also shows, with unsafe transactions commit() can be called multiple times.

You've seen how to create transactions, but we have yet to see how to make creates, updates, and deletes. The 3 types of transactions above are all subclasses of BaseDataTransaction, which implements the methods shown below.

Creating objects

The create(...) method accepts an Into clause which specifies the entity for the object you want to create:

let person = transaction.create(Into<MyPersonEntity>())

While the syntax is straightforward, CoreStore does not just naively insert a new object. This single line does the following:

• Checks that the entity type exists in any of the transaction's parent persistent store
• If the entity belongs to only one persistent store, a new object is inserted into that store and returned from create(...)
• If the entity does not belong to any store, an assertion failure will be raised. This is a programmer error and should never occur in production code.
• If the entity belongs to multiple stores, an assertion failure will be raised. This is also a programmer error and should never occur in production code. Normally, with Core Data you can insert an object in this state but saving the NSManagedObjectContext will always fail. CoreStore checks this for you at creation time when it makes sense (not during save).

If the entity exists in multiple configurations, you need to provide the configuration name for the destination persistent store:

let person = transaction.create(Into<MyPersonEntity>("Config1"))

or if the persistent store is the auto-generated "Default" configuration, specify nil:

let person = transaction.create(Into<MyPersonEntity>(nil))

Note that if you do explicitly specify the configuration name, CoreStore will only try to insert the created object to that particular store and will fail if that store is not found; it will not fall back to any other configuration that the entity belongs to.

Updating objects

After creating an object from the transaction, you can simply update its properties as normal:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let person = transaction.create(Into<MyPersonEntity>())
        person.name = "John Smith"
        person.age = 30
    },
    completion: { _ in }
)

To update an existing object, fetch the object's instance from the transaction:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let person = try transaction.fetchOne(
            From<MyPersonEntity>()
                .where(\.name == "Jane Smith")
        )
        person.age = person.age + 1
    },
    completion: { _ in }
)

(For more about fetching, see Fetching and querying)

Do not update an instance that was not created/fetched from the transaction. If you have a reference to the object already, use the transaction's edit(...) method to get an editable proxy instance for that object:

let jane: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        // WRONG: jane.age = jane.age + 1
        // RIGHT:
        let jane = transaction.edit(jane)! // using the same variable name protects us from misusing the non-transaction instance
        jane.age = jane.age + 1
    },
    completion: { _ in }
)

This is also true when updating an object's relationships. Make sure that the object assigned to the relationship is also created/fetched from the transaction:

let jane: MyPersonEntity = // ...
let john: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        // WRONG: jane.friends = [john]
        // RIGHT:
        let jane = transaction.edit(jane)!
        let john = transaction.edit(john)!
        jane.friends = NSSet(array: [john])
    },
    completion: { _ in }
)

Deleting objects

Deleting an object is simpler because you can tell a transaction to delete an object directly without fetching an editable proxy (CoreStore does that for you):

let john: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        transaction.delete(john)
    },
    completion: { _ in }
)

or several objects at once:

let john: MyPersonEntity = // ...
let jane: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        try transaction.delete(john, jane)
        // try transaction.delete([john, jane]) is also allowed
    },
    completion: { _ in }
)

If you do not have references yet to the objects to be deleted, transactions have a deleteAll(...) method you can pass a query to:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        try transaction.deleteAll(
            From<MyPersonEntity>()
                .where(\.age > 30)
        )
    },
    completion: { _ in }
)

Passing objects safely

Always remember that the DataStack and individual transactions manage different NSManagedObjectContexts so you cannot just use objects between them. That's why transactions have an edit(...) method:

let jane: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let jane = transaction.edit(jane)!
        jane.age = jane.age + 1
    },
    completion: { _ in }
)

But CoreStore, DataStack and BaseDataTransaction have a very flexible fetchExisting(...) method that you can pass instances back and forth with:

let jane: MyPersonEntity = // ...

dataStack.perform(
    asynchronous: { (transaction) -> MyPersonEntity in
        let jane = transaction.fetchExisting(jane)! // instance for transaction
        jane.age = jane.age + 1
        return jane
    },
    success: { (transactionJane) in
        let jane = dataStack.fetchExisting(transactionJane)! // instance for DataStack
        print(jane.age)
    },
    failure: { (error) in
        // ...
    }
)

fetchExisting(...) also works with multiple NSManagedObjects, CoreStoreObjects, or with NSManagedObjectIDs:

var peopleIDs: [NSManagedObjectID] = // ...

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let jane = try transaction.fetchOne(
            From<MyPersonEntity>()
                .where(\.name == "Jane Smith")
        )
        jane.friends = NSSet(array: transaction.fetchExisting(peopleIDs)!)
        // ...
    },
    completion: { _ in }
)

Importing data

Some times, if not most of the time, the data that we save to Core Data comes from external sources such as web servers or external files. If you have a JSON dictionary for example, you may be extracting values as such:

let json: [String: Any] = // ...
person.name = json["name"] as? NSString
person.age = json["age"] as? NSNumber
// ...

If you have many attributes, you don't want to keep repeating this mapping everytime you want to import data. CoreStore lets you write the data mapping code just once, and all you have to do is call importObject(...) or importUniqueObject(...) through BaseDataTransaction subclasses:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let json: [String: Any] = // ...
        try! transaction.importObject(
            Into<MyPersonEntity>(),
            source: json
        )
    },
    completion: { _ in }
)

To support data import for an entity, implement either ImportableObject or ImportableUniqueObject on the NSManagedObject or CoreStoreObject subclass:

• ImportableObject: Use this protocol if the object have no inherent uniqueness and new objects should always be added when calling importObject(...).
• ImportableUniqueObject: Use this protocol to specify a unique ID for an object that will be used to distinguish whether a new object should be created or if an existing object should be updated when calling importUniqueObject(...).

Both protocols require implementers to specify an ImportSource which can be set to any type that the object can extract data from:

typealias ImportSource = NSDictionary

typealias ImportSource = [String: Any]

typealias ImportSource = NSData

You can even use external types from popular 3rd-party JSON libraries, or just simple tuples or primitives.

ImportableObject

ImportableObject is a very simple protocol:

public protocol ImportableObject: AnyObject {
    typealias ImportSource
    static func shouldInsert(from source: ImportSource, in transaction: BaseDataTransaction) -> Bool
    func didInsert(from source: ImportSource, in transaction: BaseDataTransaction) throws
}

First, set ImportSource to the expected type of the data source:

typealias ImportSource = [String: Any]

This lets us call importObject(_:source:) with any [String: Any] type as the argument to source:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let json: [String: Any] = // ...
        try! transaction.importObject(
            Into<MyPersonEntity>(),
            source: json
        )
        // ...
    },
    completion: { _ in }
)

The actual extraction and assignment of values should be implemented in the didInsert(from:in:) method of the ImportableObject protocol:

func didInsert(from source: ImportSource, in transaction: BaseDataTransaction) throws {
    self.name = source["name"] as? NSString
    self.age = source["age"] as? NSNumber
    // ...
}

Transactions also let you import multiple objects at once using the importObjects(_:sourceArray:) method:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let jsonArray: [[String: Any]] = // ...
        try! transaction.importObjects(
            Into<MyPersonEntity>(),
            sourceArray: jsonArray // make sure this is of type Array<MyPersonEntity.ImportSource>
        )
        // ...
    },
    completion: { _ in }
)

Doing so tells the transaction to iterate through the array of import sources and calls shouldInsert(from:in:) on the ImportableObject to determine which instances should be created. You can do validations and return false from shouldInsert(from:in:) if you want to skip importing from a source and continue on with the other sources in the array.

If on the other hand, your validation in one of the sources failed in such a manner that all other sources should also be rolled back and cancelled, you can throw from within didInsert(from:in:):

func didInsert(from source: ImportSource, in transaction: BaseDataTransaction) throws {
    self.name = source["name"] as? NSString
    self.age = source["age"] as? NSNumber
    // ...
    if self.name == nil {
        throw Errors.InvalidNameError
    }
}

Doing so can let you abandon an invalid transaction immediately:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let jsonArray: [[String: Any]] = // ...

        try transaction.importObjects(
            Into<MyPersonEntity>(),
            sourceArray: jsonArray
        )
    },
    success: {
        // ...
    },
    failure: { (error) in
        switch error {
        case Errors.InvalidNameError: print("Invalid name")
        // ...
        }
    }
)

ImportableUniqueObject

Typically, we don't just keep creating objects every time we import data. Usually we also need to update already existing objects. Implementing the ImportableUniqueObject protocol lets you specify a "unique ID" that transactions can use to search existing objects before creating new ones:

public protocol ImportableUniqueObject: ImportableObject {
    typealias ImportSource
    typealias UniqueIDType: ImportableAttributeType

    static var uniqueIDKeyPath: String { get }
    var uniqueIDValue: UniqueIDType { get set }

    static func shouldInsert(from source: ImportSource, in transaction: BaseDataTransaction) -> Bool
    static func shouldUpdate(from source: ImportSource, in transaction: BaseDataTransaction) -> Bool
    static func uniqueID(from source: ImportSource, in transaction: BaseDataTransaction) throws -> UniqueIDType?
    func didInsert(from source: ImportSource, in transaction: BaseDataTransaction) throws
    func update(from source: ImportSource, in transaction: BaseDataTransaction) throws
}

Notice that it has the same insert methods as ImportableObject, with additional methods for updates and for specifying the unique ID:

class var uniqueIDKeyPath: String {
    return #keyPath(MyPersonEntity.personID) 
}
var uniqueIDValue: Int { 
    get { return self.personID }
    set { self.personID = newValue }
}
class func uniqueID(from source: ImportSource, in transaction: BaseDataTransaction) throws -> Int? {
    return source["id"] as? Int
}

For ImportableUniqueObject, the extraction and assignment of values should be implemented from the update(from:in:) method. The didInsert(from:in:) by default calls update(from:in:), but you can separate the implementation for inserts and updates if needed.

You can then create/update an object by calling a transaction's importUniqueObject(...) method:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let json: [String: Any] = // ...
        try! transaction.importUniqueObject(
            Into<MyPersonEntity>(),
            source: json
        )
        // ...
    },
    completion: { _ in }
)

or multiple objects at once with the importUniqueObjects(...) method:

dataStack.perform(
    asynchronous: { (transaction) -> Void in
        let jsonArray: [[String: AnyObject]] = // ...
        try! transaction.importUniqueObjects(
            Into<MyPersonEntity>(),
            sourceArray: jsonArray
        )
        // ...
    },
    completion: { _ in }
)

As with ImportableObject, you can control whether to skip importing an object by implementing shouldInsert(from:in:) and shouldUpdate(from:in:), or to cancel all objects by throwing an error from the uniqueID(from:in:), didInsert(from:in:) or update(from:in:) methods.

Fetching and Querying

Before we dive in, be aware that CoreStore distinguishes between fetching and querying:

• A fetch executes searches from a specific transaction or data stack. This means fetches can include pending objects (i.e. before a transaction calls on commit().) Use fetches when:
  • results need to be NSManagedObject or CoreStoreObject instances
  • unsaved objects should be included in the search (though fetches can be configured to exclude unsaved ones)
• A query pulls data straight from the persistent store. This means faster searches when computing aggregates such as count, min, max, etc. Use queries when:
  • you need to compute aggregate functions (see below for a list of supported functions)
  • results can be raw values like NSStrings, NSNumbers, Ints, NSDates, an NSDictionary of key-values, or any type that conform to QueryableAttributeType. (See QueryableAttributeType.swift for a list of built-in types)
  • only values for specified attribute keys need to be included in the results
  • unsaved objects should be ignored

From clause

The search conditions for fetches and queries are specified using clauses. All fetches and queries require a From clause that indicates the target entity type:

let people = try dataStack.fetchAll(From<MyPersonEntity>())

people in the example above will be of type [MyPersonEntity]. The From<MyPersonEntity>() clause indicates a fetch to all persistent stores that MyPersonEntity belong to.

If the entity exists in multiple configurations and you need to only search from a particular configuration, indicate in the From clause the configuration name for the destination persistent store:

let people = try dataStack.fetchAll(From<MyPersonEntity>("Config1")) // ignore objects in persistent stores other than the "Config1" configuration

or if the persistent store is the auto-generated "Default" configuration, specify nil:

let person = try dataStack.fetchAll(From<MyPersonEntity>(nil))

Now we know how to use a From clause, let's move on to fetching and querying.

Fetching

There are currently 5 fetch methods you can call from CoreStore, from a DataStack instance, or from a BaseDataTransaction instance. All of the methods below accept the same parameters: a required From clause, and an optional series of Where, OrderBy, and/or Tweak clauses.

• fetchAll(...) - returns an array of all objects that match the criteria.
• fetchOne(...) - returns the first object that match the criteria.
• fetchCount(...) - returns the number of objects that match the criteria.
• fetchObjectIDs(...) - returns an array of NSManagedObjectIDs for all objects that match the criteria.
• fetchObjectID(...) - returns the NSManagedObjectIDs for the first objects that match the criteria.

Each method's purpose is straightforward, but we need to understand how to set the clauses for the fetch.

Where clause

The Where clause is CoreStore's NSPredicate wrapper. It specifies the search filter to use when fetching (or querying). It implements all initializers that NSPredicate does (except for -predicateWithBlock:, which Core Data does not support):

var people = try dataStack.fetchAll(
    From<MyPersonEntity>(),
    Where<MyPersonEntity>("%K > %d", "age", 30) // string format initializer
)
people = try dataStack.fetchAll(
    From<MyPersonEntity>(),
    Where<MyPersonEntity>(true) // boolean initializer
)

If you do have an existing NSPredicate instance already, you can pass that to Where as well:

let predicate = NSPredicate(...)
var people = dataStack.fetchAll(
    From<MyPersonEntity>(),
    Where<MyPersonEntity>(predicate) // predicate initializer
)

Where clauses are generic types. To avoid verbose repetition of the generic object type, fetch methods support Fetch Chain builders. We can also use Swift's Smart KeyPaths as the Where clause expression:

var people = try dataStack.fetchAll(
    From<MyPersonEntity>()
        .where(\.age > 30) // Type-safe!
)

Where clauses also implement the &&, ||, and ! logic operators, so you can provide logical conditions without writing too much AND, OR, and NOT strings:

var people = try dataStack.fetchAll(
    From<MyPersonEntity>()
        .where(\.age > 30 && \.gender == "M")
)

If you do not provide a Where clause, all objects that belong to the specified From will be returned.

OrderBy clause

The OrderBy clause is CoreStore's NSSortDescriptor wrapper. Use it to specify attribute keys in which to sort the fetch (or query) results with.

var mostValuablePeople = try dataStack.fetchAll(
    From<MyPersonEntity>(),
    OrderBy<MyPersonEntity>(.descending("rating"), .ascending("surname"))
)

As seen above, OrderBy accepts a list of SortKey enumeration values, which can be either .ascending or .descending. As with Where clauses, OrderBy clauses are also generic types. To avoid verbose repetition of the generic object type, fetch methods support Fetch Chain builders. We can also use Swift's Smart KeyPaths as the OrderBy clause expression:

var people = try dataStack.fetchAll(
    From<MyPersonEntity>()
        .orderBy(.descending(\.rating), .ascending(\.surname)) // Type-safe!
)

You can use the + and += operator to append OrderBys together. This is useful when sorting conditionally:

var orderBy = OrderBy<MyPersonEntity>(.descending(\.rating))
if sortFromYoungest {
    orderBy += OrderBy(.ascending(\.age))
}
var mostValuablePeople = try dataStack.fetchAll(
    From<MyPersonEntity>(),
    orderBy
)

Tweak clause

The Tweak clause lets you, uh, tweak the fetch (or query). Tweak exposes the NSFetchRequest in a closure where you can make changes to its properties:

var people = try dataStack.fetchAll(
    From<MyPersonEntity>(),
    Where<MyPersonEntity>("age > %d", 30),
    OrderBy<MyPersonEntity>(.ascending("surname")),
    Tweak { (fetchRequest) -> Void in
        fetchRequest.includesPendingChanges = false
        fetchRequest.returnsObjectsAsFaults = false
        fetchRequest.includesSubentities = false
    }
)

Tweak also supports Fetch Chain builders:

var people = try dataStack.fetchAll(
    From<MyPersonEntity>(),
        .where(\.age > 30)
        .orderBy(.ascending(\.surname))
        .tweak {
            $0.includesPendingChanges = false
            $0.returnsObjectsAsFaults = false
            $0.includesSubentities = false
        }
)

The clauses are evaluated the order they appear in the fetch/query, so you typically need to set Tweak as the last clause. Tweak's closure is executed only just before the fetch occurs, so make sure that any values captured by the closure is not prone to race conditions.

While Tweak lets you micro-configure the NSFetchRequest, note that CoreStore already preconfigured that NSFetchRequest to suitable defaults. Only use Tweak when you know what you are doing!

Querying

One of the functionalities overlooked by other Core Data wrapper libraries is raw properties fetching. If you are familiar with NSDictionaryResultType and -[NSFetchedRequest propertiesToFetch], you probably know how painful it is to setup a query for raw values and aggregate values. CoreStore makes this easy by exposing the 2 methods below:

• queryValue(...) - returns a single raw value for an attribute or for an aggregate value. If there are multiple results, queryValue(...) only returns the first item.
• queryAttributes(...) - returns an array of dictionaries containing attribute keys with their corresponding values.

Both methods above accept the same parameters: a required From clause, a required Select<T> clause, and an optional series of Where, OrderBy, GroupBy, and/or Tweak clauses.

Setting up the From, Where, OrderBy, and Tweak clauses is similar to how you would when fetching. For querying, you also need to know how to use the Select<T> and GroupBy clauses.

Select<T> clause

The Select<T> clause specifies the target attribute/aggregate key, as well as the expected return type:

let johnsAge = try dataStack.queryValue(
    From<MyPersonEntity>(),
    Select<Int>("age"),
    Where<MyPersonEntity>("name == %@", "John Smith")
)

The example above queries the "age" property for the first object that matches the Where condition. johnsAge will be bound to type Int?, as indicated by the Select<Int> generic type. For queryValue(...), types that conform to QueryableAttributeType are allowed as the return type (and therefore as the generic type for Select<T>).

For queryAttributes(...), only NSDictionary is valid for Select, thus you are allowed to omit the generic type:

let allAges = try dataStack.queryAttributes(
    From<MyPersonEntity>(),
    Select("age")
)

query methods also support Query Chain builders. We can also use Swift's Smart KeyPaths to use in the expressions:

let johnsAge = try dataStack.queryValue(
    From<MyPersonEntity>()
        .select(\.age) // binds the result to Int
        .where(\.name == "John Smith")
)

If you only need a value for a particular attribute, you can just specify the key name (like we did with Select<Int>("age")), but several aggregate functions can also be used as parameter to Select:

• .average(...)
• .count(...)
• .maximum(...)
• .minimum(...)
• .sum(...)

let oldestAge = try dataStack.queryValue(
    From<MyPersonEntity>(),
    Select<Int>(.maximum("age"))
)

For queryAttributes(...) which returns an array of dictionaries, you can specify multiple attributes/aggregates to Select:

let personJSON = try dataStack.queryAttributes(
    From<MyPersonEntity>(),
    Select("name", "age")
)

personJSON will then have the value:

[
    [
        "name": "John Smith",
        "age": 30
    ],
    [
        "name": "Jane Doe",
        "age": 22
    ]
]

You can also include an aggregate as well:

let personJSON = try dataStack.queryAttributes(
    From<MyPersonEntity>(),
    Select("name", .count("friends"))
)

which returns:

[
    [
        "name": "John Smith",
        "count(friends)": 42
    ],
    [
        "name": "Jane Doe",
        "count(friends)": 231
    ]
]

The "count(friends)" key name was automatically used by CoreStore, but you can specify your own key alias if you need:

let personJSON = try dataStack.queryAttributes(
    From<MyPersonEntity>(),
    Select("name", .count("friends", as: "friendsCount"))
)

which now returns:

[
    [
        "name": "John Smith",
        "friendsCount": 42
    ],
    [
        "name": "Jane Doe",
        "friendsCount": 231
    ]
]

GroupBy clause

The GroupBy clause lets you group results by a specified attribute/aggregate. This is useful only for queryAttributes(...) since queryValue(...) just returns the first value.

let personJSON = try dataStack.queryAttributes(
    From<MyPersonEntity>(),
    Select("age", .count("age", as: "count")),
    GroupBy("age")
)

GroupBy clauses are also generic types and support Query Chain builders. We can also use Swift's Smart KeyPaths to use in the expressions:

let personJSON = try dataStack.queryAttributes(
    From<MyPersonEntity>()
        .select(.attribute(\.age), .count(\.age, as: "count"))
        .groupBy(\.age)
)

this returns dictionaries that shows the count for each "age":

[
    [
        "age": 42,
        "count": 1
    ],
    [
        "age": 22,
        "count": 1
    ]
]

Logging and error reporting

One unfortunate thing when using some third-party libraries is that they usually pollute the console with their own logging mechanisms. CoreStore provides its own default logging class, but you can plug-in your own favorite logger by implementing the CoreStoreLogger protocol.

public protocol CoreStoreLogger {
    func log(level level: LogLevel, message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString)
    func log(error error: CoreStoreError, message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString)
    func assert(@autoclosure condition: () -> Bool, @autoclosure message: () -> String, fileName: StaticString, lineNumber: Int, functionName: StaticString)
    func abort(message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString)
}

Implement this protocol with your custom class then pass the instance to CoreStoreDefaults.logger:

CoreStoreDefaults.logger = MyLogger()

Doing so channels all logging calls to your logger.

Note that to keep the call stack information intact, all calls to these methods are NOT thread-managed. Therefore you have to make sure that your logger is thread-safe or you may otherwise have to dispatch your logging implementation to a serial queue.

Take special care when implementing CoreStoreLogger's assert(...) and abort(...) functions:

• assert(...): The behavior between DEBUG and release builds, or -O and -Onone, are all left to the implementers' responsibility. CoreStore calls CoreStoreLogger.assert(...) only for invalid but usually recoverable errors (for example, early validation failures that may cause an error thrown and handled somewhere else)
• abort(...): This method is the last-chance for your app to synchronously log a fatal error within CoreStore. The app will be terminated right after this function is called (CoreStore calls fatalError() internally)

All CoreStore types have very useful (and pretty formatted!) print(...) outputs. A couple of examples, ListMonitor:

CoreStoreError.mappingModelNotFoundError:

These are all implemented with CustomDebugStringConvertible.debugDescription, so they work with lldb's po command as well.

Observing changes and notifications

CoreStore provides type-safe wrappers for observing managed objects:

Observe a single property

To get notifications for single property changes in an object, there are two methods depending on the object's base class.

• For NSManagedObject subclasses: Use the standard KVO method:

let observer = person.observe(\.age, options: [.new]) { (person, change)
    print("Happy \(change.newValue)th birthday!")
}

• For CoreStoreObject subclasses: Call the observe(...) method directly on the property. You'll notice that the API itself is a bit similar to the KVO method:

let observer = person.age.observe(options: [.new]) { (person, change)
    print("Happy \(change.newValue)th birthday!")
}

For both methods, you will need to keep a reference to the returned observer for the duration of the observation.

Observe a single object's updates

Observers of an ObjectPublisher can receive notifications if any of the object's property changes. You can create an ObjectPublisher from the object directly:

let objectPublisher: ObjectPublisher<Person> = person.asPublisher(in: dataStack)

or by indexing a ListPublisher's ListSnapshot:

let listPublisher: ListPublisher<Person> = // ...
// ...
let objectPublisher = listPublisher.snapshot[indexPath]

(See ListPublisher examples below)

To receive notifications, call the ObjectPublisher's addObserve(...) method passing the owner of the callback closure:

objectPublisher.addObserver(self) { [weak self] (objectPublisher) in
    let snapshot: ObjectSnapshot<Person> = objectPublisher.snapshot
    // handle changes
}

Note that the owner instance will not be retained. You may call ObjectPublisher.removeObserver(...) explicitly to stop receiving notifications, but the ObjectPublisher also discontinues sending events to deallocated observers.

The ObjectSnapshot returned from the ObjectPublisher.snapshot property returns a full-copy struct of all properties of the object. This is ideal for managing states as they are thread-safe and are not affected by further changes to the actual object. ObjectPublisher automatically updates its snapshot value to the latest state of the object.

(A reactive-programming variant of this method is explained in detail in the section on ObjectPublisher Combine publishers)

Observe a single object's per-property updates

If you need to track specifically which properties change in an object, implement the ObjectObserver protocol and specify the EntityType:

class MyViewController: UIViewController, ObjectObserver {
    func objectMonitor(monitor: ObjectMonitor<MyPersonEntity>, willUpdateObject object: MyPersonEntity) {
        // ...
    }
    
    func objectMonitor(monitor: ObjectMonitor<MyPersonEntity>, didUpdateObject object: MyPersonEntity, changedPersistentKeys: Set<KeyPathString>) {
        // ...
    }
    
    func objectMonitor(monitor: ObjectMonitor<MyPersonEntity>, didDeleteObject object: MyPersonEntity) {
        // ...
    }
}

We then need to keep an ObjectMonitor instance and register our ObjectObserver as an observer:

let person: MyPersonEntity = // ...
self.monitor = dataStack.monitorObject(person)
self.monitor.addObserver(self)

The controller will then notify our observer whenever the object's attributes change. You can add multiple ObjectObservers to a single ObjectMonitor without any problem. This means you can just share around the ObjectMonitor instance to different screens without problem.

You can get ObjectMonitor's object through its object property. If the object is deleted, the object property will become nil to prevent further access.

While ObjectMonitor exposes removeObserver(...) as well, it only stores weak references of the observers and will safely unregister deallocated observers.

Observe a diffable list

Observers of a ListPublisher can receive notifications whenever its fetched result set changes. You can create a ListPublisher by fetching from the DataStack:

let listPublisher = dataStack.listPublisher(
    From<Person>()
        .sectionBy(\.age") { "Age \($0)" } // sections are optional
        .where(\.title == "Engineer")
        .orderBy(.ascending(\.lastName))
)

To receive notifications, call the ListPublisher's addObserve(...) method passing the owner of the callback closure:

listPublisher.addObserver(self) { [weak self] (listPublisher) in
    let snapshot: ListSnapshot<Person> = listPublisher.snapshot
    // handle changes
}

Note that the owner instance will not be retained. You may call ListPublisher.removeObserver(...) explicitly to stop receiving notifications, but the ListPublisher also discontinues sending events to deallocated observers.

The ListSnapshot returned from the ListPublisher.snapshot property returns a full-copy struct of all sections and NSManagedObject items in the list. This is ideal for managing states as they are thread-safe and are not affected by further changes to the result set. ListPublisher automatically updates its snapshot value to the latest state of the fetch.

(A reactive-programming variant of this method is explained in detail in the section on ListPublisher Combine publishers)

Unlike ListMonitors (See ListMonitor examples below), a ListPublisher does not track detailed inserts, deletes, and moves. In return, a ListPublisher is a lot more lightweight and are designed to work well with DiffableDataSource.TableViewAdapters and DiffableDataSource.CollectionViewAdapters:

self.dataSource = DiffableDataSource.CollectionViewAdapter<Person>(
    collectionView: self.collectionView,
    dataStack: CoreStoreDefaults.dataStack,
    cellProvider: { (collectionView, indexPath, person) in
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "PersonCell") as! PersonCell
        cell.setPerson(person)
        return cell
    }
)

// ...

listPublisher.addObserver(self) { [weak self] (listPublisher) in
   self?.dataSource?.apply(
       listPublisher.snapshot, animatingDifferences: true
   )
}

Observe detailed list changes

If you need to track each object's inserts, deletes, moves, and updates, implement one of the ListObserver protocols and specify the EntityType:

class MyViewController: UIViewController, ListObserver {
    func listMonitorDidChange(monitor: ListMonitor<MyPersonEntity>) {
        // ...
    }
    
    func listMonitorDidRefetch(monitor: ListMonitor<MyPersonEntity>) {
        // ...
    }
}

Including ListObserver, there are 3 observer protocols you can implement depending on how detailed you need to handle a change notification:

• ListObserver: lets you handle these callback methods:

    func listMonitorWillChange(_ monitor: ListMonitor<MyPersonEntity>)
    func listMonitorDidChange(_ monitor: ListMonitor<MyPersonEntity>)
    func listMonitorWillRefetch(_ monitor: ListMonitor<MyPersonEntity>)
    func listMonitorDidRefetch(_ monitor: ListMonitor<MyPersonEntity>)

listMonitorDidChange(_:) and listMonitorDidRefetch(_:) implementations are both required. listMonitorDidChange(_:) is called whenever the ListMonitor's count, order, or filtered objects change. listMonitorDidRefetch(_:) is called when the ListMonitor.refetch() was executed or if the internal persistent store was changed.

• ListObjectObserver: in addition to ListObserver methods, also lets you handle object inserts, updates, and deletes:

    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didInsertObject object: MyPersonEntity, toIndexPath indexPath: IndexPath)
    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didDeleteObject object: MyPersonEntity, fromIndexPath indexPath: IndexPath)
    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didUpdateObject object: MyPersonEntity, atIndexPath indexPath: IndexPath)
    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didMoveObject object: MyPersonEntity, fromIndexPath: IndexPath, toIndexPath: IndexPath)

• ListSectionObserver: in addition to ListObjectObserver methods, also lets you handle section inserts and deletes:

    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didInsertSection sectionInfo: NSFetchedResultsSectionInfo, toSectionIndex sectionIndex: Int)
    func listMonitor(_ monitor: ListMonitor<MyPersonEntity>, didDeleteSection sectionInfo: NSFetchedResultsSectionInfo, fromSectionIndex sectionIndex: Int)

We then need to create a ListMonitor instance and register our ListObserver as an observer:

self.monitor = dataStack.monitorList(
    From<MyPersonEntity>()
        .where(\.age > 30)
        .orderBy(.ascending(\.name))
        .tweak { $0.fetchBatchSize = 20 }
)
self.monitor.addObserver(self)

Similar to ObjectMonitor, a ListMonitor can also have multiple ListObservers registered to a single ListMonitor.

If you have noticed, the monitorList(...) method accepts Where, OrderBy, and Tweak clauses exactly like a fetch. As the list maintained by ListMonitor needs to have a deterministic order, at least the From and OrderBy clauses are required.

A ListMonitor created from monitorList(...) will maintain a single-section list. You can therefore access its contents with just an index:

let firstPerson = self.monitor[0]

If the list needs to be grouped into sections, create the ListMonitor instance with the monitorSectionedList(...) method and a SectionBy clause:

self.monitor = dataStack.monitorSectionedList(
    From<MyPersonEntity>()
        .sectionBy(\.age)
        .where(\.gender == "M")
        .orderBy(.ascending(\.age), .ascending(\.name))
        .tweak { $0.fetchBatchSize = 20 }
)

A list controller created this way will group the objects by the attribute key indicated by the SectionBy clause. One more thing to remember is that the OrderBy clause should sort the list in such a way that the SectionBy attribute would be sorted together (a requirement shared by NSFetchedResultsController.)

The SectionBy clause can also be passed a closure to transform the section name into a displayable string:

self.monitor = dataStack.monitorSectionedList(
    From<MyPersonEntity>()
        .sectionBy(\.age) { (sectionName) -> String? in
            "\(sectionName) years old"
        }
        .orderBy(.ascending(\.age), .ascending(\.name))
)

This is useful when implementing a UITableViewDelegate's section header:

func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    let sectionInfo = self.monitor.sectionInfoAtIndex(section)
    return sectionInfo.name
}

To access the objects of a sectioned list, use an IndexPath or a tuple:

let indexPath = IndexPath(row: 2, section: 1)
let person1 = self.monitor[indexPath]
let person2 = self.monitor[1, 2]
// person1 and person2 are the same object

Type-safe CoreStoreObjects

Starting CoreStore 4.0, we can now create persisted objects without depending on .xcdatamodeld Core Data files. The new CoreStoreObject subclass replaces NSManagedObject, and specially-typed properties declared on these classes will be synthesized as Core Data attributes.

class Animal: CoreStoreObject {
    @Field.Stored("species")
    var species: String = ""
}

class Dog: Animal {
    @Field.Stored("nickname")
    var nickname: String?
    
    @Field.Relationship("master")
    var master: Person?
}

class Person: CoreStoreObject {
    @Field.Stored("name")
    var name: String = ""
    
    @Field.Relationship("pets", inverse: \Dog.$master)
    var pets: Set<Dog>
}

The property names to be saved to Core Data is specified as the keyPath argument. This lets us refactor our Swift code without affecting the underlying database. For example:

class Person: CoreStoreObject {
    @Field.Stored("name")
    private var internalName: String = ""
    // note property name is independent of the storage key name
}

Here we used the property name internalName and made it private, but the underlying key-path "name" was unchanged so our model will not trigger a data migration.

To tell the DataStack about these types, add all CoreStoreObjects' entities to a CoreStoreSchema:

CoreStoreDefaults.dataStack = DataStack(
    CoreStoreSchema(
        modelVersion: "V1",
        entities: [
            Entity<Animal>("Animal", isAbstract: true),
            Entity<Dog>("Dog"),
            Entity<Person>("Person")
        ]
    )
)
CoreStoreDefaults.dataStack.addStorage(/* ... */)

And that's all CoreStore needs to build the model; we don't need .xcdatamodeld files anymore.

In addition, @Field properties can be used to create type-safe key-path strings

let keyPath = String(keyPath: \Dog.$nickname)

as well as Where and OrderBy clauses

let puppies = try dataStack.fetchAll(
    From<Dog>()
        .where(\.$age < 5)
        .orderBy(.ascending(\.$age))
)

All CoreStore APIs that are usable with NSManagedObjects are also available for CoreStoreObjects. These include ListMonitors, ImportableObjects, fetching, etc.

New @Field Property Wrapper syntax

⚠️Important: @Field properties are only supported for CoreStoreObject subclasses. If you are using NSManagedObjects, you need to keep using @NSManaged for your attributes.

Starting CoreStore 7.1.0, CoreStoreObject properties may be converted to @Field Property Wrappers.

‼️ Please take note of the warnings below before converting or else the model's hash might change.

If conversion is too risky, the current Value.Required, Value.Optional, Transformable.Required, Transformable.Optional, Relationship.ToOne, Relationship.ToManyOrdered, and Relationship.ToManyUnordered will all be supported for while so you can opt to use them as is for now.

‼️ This cannot be stressed enough, but please make sure to set your schema's VersionLock before converting!

@Field.Stored

The @Field.Stored property wrapper is used for persisted value types. This is the replacement for "non-transient" Value.Required and Value.Optional properties.

@Field.Stored

class Person: CoreStoreObject {
    

    let title = Value.Required<String>("title", initial: "Mr.")
    let nickname = Value.Optional<String>("nickname")
}

class Person: CoreStoreObject {
    

    @Field.Stored("title")
    var title: String = "Mr."
    

    @Field.Stored("nickname")
    var nickname: String?
}

⚠️ Only Value.Required and Value.Optional that are NOT transient values can be converted to Field.Stored. For transient/computed properties, refer to @Field.Virtual properties in the next section. ⚠️ When converting, make sure that all parameters, including the default values, are exactly the same or else the model's hash might change.

@Field.Virtual

The @Field.Virtual property wrapper is used for unsaved, computed value types. This is the replacement for "transient" Value.Required and Value.Optional properties.

@Field.Virtual

class Animal: CoreStoreObject {
    

    let speciesPlural = Value.Required<String>(
        "speciesPlural",
        transient: true,
        customGetter: Animal.getSpeciesPlural(_:)
    )
    

    let species = Value.Required<String>("species", initial: "")
    

    static func getSpeciesPlural(_ partialObject: PartialObject<Animal>) -> String? {
        let species = partialObject.value(for: { $0.species })
        return species + "s"
    }
}

class Animal: CoreStoreObject {
    

    @Field.Virtual(
        "speciesPlural",
        customGetter: { (object, field) in
            return object.$species.value + "s"
        }
    )
    var speciesPlural: String
    

    @Field.Stored("species")
    var species: String = ""
}

⚠️ Only Value.Required and Value.Optional that ARE transient values can be converted to Field.Virtual. For non-transient properties, refer to @Field.Stored properties in the previous section. ⚠️ When converting, make sure that all parameters, including the default values, are exactly the same or else the model's hash might change.

@Field.Coded

The @Field.Coded property wrapper is used for binary-codable values. This is the new counterpart, not replacement, for Transformable.Required and Transformable.Optional properties. @Field.Coded also supports other encodings such as JSON and custom binary converters.

‼️ The current Transformable.Required and Transformable.Optional mechanism have no safe one-to-one conversion to @Field.Coded. Please use @Field.Coded only for newly added attributes.

@Field.Coded

class Vehicle: CoreStoreObject {
    

    let color = Transformable.Optional<UIColor>("color", initial: .white)
}

class Vehicle: CoreStoreObject {
    

    @Field.Coded("color", coder: FieldCoders.NSCoding.self)
    var color: UIColor? = .white
}

Built-in encoders such as FieldCoders.NSCoding, FieldCoders.Json, and FieldCoders.Plist are available, and custom encoding/decoding is also supported:

class Person: CoreStoreObject {
    
    struct CustomInfo: Codable {
        // ...
    }
    
    @Field.Coded("otherInfo", coder: FieldCoders.Json.self)
    var otherInfo: CustomInfo?
    
    @Field.Coded(
        "photo",
        coder: {
            encode: { $0.toData() },
            decode: { Photo(fromData: $0) }
        }
    )
    var photo: Photo?
}

‼️Important: Any changes in the encoders/decoders are not reflected in the VersionLock, so make sure that the encoder and decoder logic is compatible for all versions of your persistent store.

@Field.Relationship

The @Field.Relationship property wrapper is used for link relationships with other CoreStoreObjects. This is the replacement for Relationship.ToOne, Relationship.ToManyOrdered, and Relationship.ToManyUnordered properties.

The type of relationship is determined by the @Field.Relationship generic type:

• Optional<T> : To-one relationship
• Array<T> : To-many ordered relationship
• Set<T> : To-many unordered relationship

@Field.Stored

class Pet: CoreStoreObject {
    

    let master = Relationship.ToOne<Person>("master")
}
class Person: CoreStoreObject {
    

    let pets: Relationship.ToManyUnordered<Pet>("pets", inverse: \.$master)
}

class Pet: CoreStoreObject {
    

    @Field.Relationship("master")
    var master: Person?
}
class Person: CoreStoreObject {
    

    @Field.Relationship("pets", inverse: \.$master)
    var pets: Set<Pet>
}

⚠️ When converting, make sure that all parameters, including the default values, are exactly the same or else the model's hash might change.

Also note how Relationships are linked statically with the inverse: argument. All relationships are required to have an "inverse" relationship. Unfortunately, due to Swift compiler limitation we can declare the inverse: on only one of the relationship-pair.

@Field usage notes

Accessor syntax

When using key-path utilities, properties using @Field property wrappers need to use the $ syntax:

• Before: From<Person>.where(\.title == "Mr.")
• After: From<Person>.where(\.$title == "Mr.")

This applies to property access using ObjectPublishers and ObjectSnapshots.

• Before: let name = personSnapshot.name
• After: let name = personSnapshot.$name

Default values vs. Initial values

One common mistake when assigning default values to CoreStoreObject properties is to assign it a value and expect it to be evaluated whenever an object is created:

// ❌
class Person: CoreStoreObject {

    @Field.Stored("identifier")
    var identifier: UUID = UUID() // Wrong!
    
    @Field.Stored("createdDate")
    var createdDate: Date = Date() // Wrong!
}

This default value will be evaluated only when the DataStack sets up the schema, and all instances will end up having the same values. This syntax for "default values" are usually used only for actual reasonable constant values, or sentinel values such as "" or 0.

For actual "initial values", @Field.Stored and @Field.Coded now supports dynamic evaluation during object creation via the dynamicInitialValue: argument:

// ✅
class Person: CoreStoreObject {

    @Field.Stored("identifier", dynamicInitialValue: { UUID() })
    var identifier: UUID
    
    @Field.Stored("createdDate", dynamicInitialValue: { Date() })
    var createdDate: Date
}

When using this feature, a "default value" should not be assigned (i.e. no = expression).

VersionLocks

While it is convenient to be able to declare entities only in code, it is worrying that we might accidentally change the CoreStoreObject's properties and break our users' model version history. For this, the CoreStoreSchema allows us to "lock" our properties to a particular configuration. Any changes to that VersionLock will raise an assertion failure during the CoreStoreSchema initialization, so you can then look for the commit which changed the VersionLock hash.

To use VersionLocks, create the CoreStoreSchema, run the app, and look for a similar log message that is automatically printed to the console:

Copy this dictionary value and use it as the versionLock: argument of the CoreStoreSchema initializer:

CoreStoreSchema(
    modelVersion: "V1",
    entities: [
        Entity<Animal>("Animal", isAbstract: true),
        Entity<Dog>("Dog"),
        Entity<Person>("Person"),
    ],
    versionLock: [
        "Animal": [0x1b59d511019695cf, 0xdeb97e86c5eff179, 0x1cfd80745646cb3, 0x4ff99416175b5b9a],
        "Dog": [0xe3f0afeb109b283a, 0x29998d292938eb61, 0x6aab788333cfc2a3, 0x492ff1d295910ea7],
        "Person": [0x66d8bbfd8b21561f, 0xcecec69ecae3570f, 0xc4b73d71256214ef, 0x89b99bfe3e013e8b]
    ]
)

You can also get this hash after the DataStack has been fully set up by printing to the console:

print(CoreStoreDefaults.dataStack.modelSchema.printCoreStoreSchema())

Once the version lock is set, any changes in the properties or to the model will trigger an assertion failure similar to this:

Reactive Programming

RxSwift

RxSwift utilities are available through the RxCoreStore external module.

Combine

Combine publishers are available from the DataStack, ListPublisher, and ObjectPublisher's .reactive namespace property.

DataStack.reactive

Adding a storage through DataStack.reactive.addStorage(_:) returns a publisher that reports a MigrationProgress enum value. The .migrating value is only emitted if the storage goes through a migration. Refer to the Setting up section for details on the storage setup process itself.

dataStack.reactive
    .addStorage(
        SQLiteStore(fileName: "core_data.sqlite")
    )
    .sink(
        receiveCompletion: { result in
            // ...
        },
        receiveValue: { (progress) in
            print("\(round(progress.fractionCompleted * 100)) %") // 0.0 ~ 1.0
            switch progress {
            case .migrating(let storage, let nsProgress):
                // ...
            case .finished(let storage, let migrationRequired):
                // ...
            }
        }
    )
    .store(in: &cancellables)

Transactions are also available as publishers through DataStack.reactive.perform(_:), which returns a Combine Future that emits any type returned from the closure parameter:

dataStack.reactive
    .perform(
        asynchronous: { (transaction) -> (inserted: Set<NSManagedObject>, deleted: Set<NSManagedObject>) in

            // ...
            return (
                transaction.insertedObjects(),
                transaction.deletedObjects()
            )
        }
    )
    .sink(
        receiveCompletion: { result in
            // ...
        },
        receiveValue: { value in
            let inserted = dataStack.fetchExisting(value0.inserted)
            let deleted = dataStack.fetchExisting(value0.deleted)
            // ...
        }
    )
    .store(in: &cancellables)

For importing convenience, ImportableObject and ImportableUniqueObjects can be imported directly through DataStack.reactive.import[Unique]Object(_:source:) and DataStack.reactive.import[Unique]Objects(_:sourceArray:) without having to create a transaction block. In this case the publisher emits objects that are already usable directly from the main queue:

dataStack.reactive
    .importUniqueObjects(
        Into<Person>(),
        sourceArray: [
            ["name": "John"],
            ["name": "Bob"],
            ["name": "Joe"]
        ]
    )
    .sink(
        receiveCompletion: { result in
            // ...
        },
        receiveValue: { (people) in
            XCTAssertEqual(people?.count, 3)
            // ...
        }
    )
    .store(in: &cancellables)

ListPublisher.reactive

ListPublishers can be used to emit ListSnapshots through Combine using ListPublisher.reactive.snapshot(emitInitialValue:). The snapshot values are emitted in the main queue:

listPublisher.reactive
    .snapshot(emitInitialValue: true)
    .sink(
        receiveCompletion: { result in
            // ...
        },
        receiveValue: { (listSnapshot) in
            dataSource.apply(
                listSnapshot,
                animatingDifferences: true
            )
        }
    )
    .store(in: &cancellables)

ObjectPublisher.reactive

ObjectPublishers can be used to emit ObjectSnapshots through Combine using ObjectPublisher.reactive.snapshot(emitInitialValue:). The snapshot values are emitted in the main queue:

objectPublisher.reactive
    .snapshot(emitInitialValue: true)
    .sink(
        receiveCompletion: { result in
            // ...
        },
        receiveValue: { (objectSnapshot) in
            tableViewCell.setObject(objectSnapshot)
        }
    )
    .store(in: &tableViewCell.cancellables)

SwiftUI Utilities

Observing list and object changes in SwiftUI can be done through a couple of approaches. One is by creating views that autoupdates their contents, or by declaring property wrappers that trigger view updates. Both approaches are implemented almost the same internally, but this lets you be flexible depending on the structure of your custom Views.

SwiftUI Views

CoreStore provides View containers that automatically update their contents when data changes.

ListReader

A ListReader observes changes to a ListPublisher and creates its content views dynamically. The builder closure receives a ListSnapshot value that can be used to create the contents:

let people: ListPublisher<Person>

var body: some View {
   List {
       ListReader(self.people) { listSnapshot in
           ForEach(objectIn: listSnapshot) { person in
               // ...
           }
       }
   }
   .animation(.default)
}

As shown above, a typical use case is to use it together with CoreStore's ForEach extensions.

A KeyPath can also be optionally provided to extract specific properties of the ListSnapshot:

let people: ListPublisher<Person>

var body: some View {
    ListReader(self.people, keyPath: \.count) { count in
        Text("Number of members: \(count)")
    }
}

ObjectReader

An ObjectReader observes changes to an ObjectPublisher and creates its content views dynamically. The builder closure receives an ObjectSnapshot value that can be used to create the contents:

let person: ObjectPublisher<Person>

var body: some View {
   ObjectReader(self.person) { objectSnapshot in
       // ...
   }
   .animation(.default)
}

A KeyPath can also be optionally provided to extract specific properties of the ObjectSnapshot:

let person: ObjectPublisher<Person>

var body: some View {
    ObjectReader(self.person, keyPath: \.fullName) { fullName in
        Text("Name: \(fullName)")
    }
}

By default, an ObjectReader does not create its views wheen the object observed is deleted from the store. In those cases, the placeholder: argument can be used to provide a custom View to display when the object is deleted:

let person: ObjectPublisher<Person>

var body: some View {
   ObjectReader(
       self.person,
       content: { objectSnapshot in
           // ...
       },
       placeholder: { Text("Record not found") }
   )
}

SwiftUI Property Wrappers

As an alternative to ListReader and ObjectReader, CoreStore also provides property wrappers that trigger view updates when the data changes.

ListState

A @ListState property exposes a ListSnapshot value that automatically updates to the latest changes.

@ListState
var people: ListSnapshot<Person>

init(listPublisher: ListPublisher<Person>) {
   self._people = .init(listPublisher)
}

var body: some View {
   List {
       ForEach(objectIn: self.people) { objectSnapshot in
           // ...
       }
   }
   .animation(.default)
}

As shown above, a typical use case is to use it together with CoreStore's ForEach extensions.

If a ListPublisher instance is not available yet, the fetch can be done inline by providing the fetch clauses and the DataStack instance. By doing so the property can be declared without an initial value:

@ListState(
    From<Person>()
        .sectionBy(\.age)
        .where(\.isMember == true)
        .orderBy(.ascending(\.lastName))
)
var people: ListSnapshot<Person>

var body: some View {
    List {
        ForEach(sectionIn: self.people) { section in
            Section(header: Text(section.sectionID)) {
                ForEach(objectIn: section) { person in
                    // ...
                }
            }
        }
    }
    .animation(.default)
}