Cache

Table of Contents

• Description
• Key features
• Usage
  • Storage
  • Configuration
  • Sync APIs
  • Async APIs
  • Expiry date
• Observations
  • Storage observations
  • Key observations
• Handling JSON response
• What about images?
• Installation
• Author
• Contributing
• License

Description

Cache doesn't claim to be unique in this area, but it's not another monster library that gives you a god's power. It does nothing but caching, but it does it well. It offers a good public API with out-of-box implementations and great customization possibilities. Cache utilizes Codable in Swift 4 to perform serialization.

Read the story here Open Source Stories: From Cachable to Generic Storage in Cache

Key features

• Work with Swift 4 Codable. Anything conforming to Codable will be saved and loaded easily by Storage.
• Hybrid with memory and disk storage.
• Many options via DiskConfig and MemoryConfig.
• Support expiry and clean up of expired objects.
• Thread safe. Operations can be accessed from any queue.
• Sync by default. Also support Async APIs.
• Extensive unit test coverage and great documentation.
• iOS, tvOS and macOS support.

Usage

Storage

Cache is built based on Chain-of-responsibility pattern, in which there are many processing objects, each knows how to do 1 task and delegates to the next one, so can you compose Storages the way you like.

For now the following Storage are supported

• MemoryStorage: save object to memory.
• DiskStorage: save object to disk.
• HybridStorage: save object to memory and disk, so you get persistented object on disk, while fast access with in memory objects.
• SyncStorage: blocking APIs, all read and write operations are scheduled in a serial queue, all sync manner.
• AsyncStorage: non-blocking APIs, operations are scheduled in an internal queue for serial processing. No read and write should happen at the same time.

Although you can use those Storage at your discretion, you don't have to. Because we also provide a convenient Storage which uses HybridStorage under the hood, while exposes sync and async APIs through SyncStorage and AsyncStorage.

All you need to do is to specify the configuration you want with DiskConfig and MemoryConfig. The default configurations are good to go, but you can customise a lot.

let diskConfig = DiskConfig(name: "Floppy")
let memoryConfig = MemoryConfig(expiry: .never, countLimit: 10, totalCostLimit: 10)

let storage = try? Storage(
  diskConfig: diskConfig,
  memoryConfig: memoryConfig,
  transformer: TransformerFactory.forCodable(ofType: User.self) // Storage<String, User>
)

Generic, Type safety and Transformer

All Storage now are generic by default, so you can get a type safety experience. Once you create a Storage, it has a type constraint that you don't need to specify type for each operation afterwards.

If you want to change the type, Cache offers transform functions, look for Transformer and TransformerFactory for built-in transformers.

let storage: Storage<String, User> = ...
storage.setObject(superman, forKey: "user")

let imageStorage = storage.transformImage() // Storage<String, UIImage>
imageStorage.setObject(image, forKey: "image")

let stringStorage = storage.transformCodable(ofType: String.self) // Storage<String, String>
stringStorage.setObject("hello world", forKey: "string")

Each transformation allows you to work with a specific type, however the underlying caching mechanism remains the same, you're working with the same Storage, just with different type annotation. You can also create different Storage for each type if you want.

Transformer is necessary because the need of serialising and deserialising objects to and from Data for disk persistency. Cache provides default Transformer for Data, Codable and UIImage/NSImage

Codable types

Storage supports any objects that conform to Codable protocol. You can make your own things conform to Codable so that can be saved and loaded from Storage.

The supported types are

• Primitives like Int, Float, String, Bool, ...
• Array of primitives like [Int], [Float], [Double], ...
• Set of primitives like Set<String>, Set<Int>, ...
• Simply dictionary like [String: Int], [String: String], ...
• Date
• URL
• Data

Error handling

Error handling is done via try catch. Storage throws errors in terms of StorageError.

public enum StorageError: Error {
  /// Object can not be found
  case notFound
  /// Object is found, but casting to requested type failed
  case typeNotMatch
  /// The file attributes are malformed
  case malformedFileAttributes
  /// Can't perform Decode
  case decodingFailed
  /// Can't perform Encode
  case encodingFailed
  /// The storage has been deallocated
  case deallocated
  /// Fail to perform transformation to or from Data
  case transformerFail
}

There can be errors because of disk problem or type mismatch when loading from storage, so if want to handle errors, you need to do try catch

do {
  let storage = try Storage(diskConfig: diskConfig, memoryConfig: memoryConfig)
} catch {
  print(error)
}

Configuration

Here is how you can play with many configuration options

let diskConfig = DiskConfig(
  // The name of disk storage, this will be used as folder name within directory
  name: "Floppy",
  // Expiry date that will be applied by default for every added object
  // if it's not overridden in the `setObject(forKey:expiry:)` method
  expiry: .date(Date().addingTimeInterval(2*3600)),
  // Maximum size of the disk cache storage (in bytes)
  maxSize: 10000,
  // Where to store the disk cache. If nil, it is placed in `cachesDirectory` directory.
  directory: try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask,
    appropriateFor: nil, create: true).appendingPathComponent("MyPreferences"),
  // Data protection is used to store files in an encrypted format on disk and to decrypt them on demand
  protectionType: .complete
)

let memoryConfig = MemoryConfig(
  // Expiry date that will be applied by default for every added object
  // if it's not overridden in the `setObject(forKey:expiry:)` method
  expiry: .date(Date().addingTimeInterval(2*60)),
  /// The maximum number of objects in memory the cache should hold
  countLimit: 50,
  /// The maximum total cost that the cache can hold before it starts evicting objects
  totalCostLimit: 0
)

On iOS, tvOS we can also specify protectionType on DiskConfig to add a level of security to files stored on disk by your app in the app’s container. For more information, see FileProtectionType

Sync APIs

Storage is sync by default and is thread safe, you can access it from any queues. All Sync functions are constrained by StorageAware protocol.

// Save to storage
try? storage.setObject(10, forKey: "score")
try? storage.setObject("Oslo", forKey: "my favorite city", expiry: .never)
try? storage.setObject(["alert", "sounds", "badge"], forKey: "notifications")
try? storage.setObject(data, forKey: "a bunch of bytes")
try? storage.setObject(authorizeURL, forKey: "authorization URL")

// Load from storage
let score = try? storage.object(forKey: "score")
let favoriteCharacter = try? storage.object(forKey: "my favorite city")

// Check if an object exists
let hasFavoriteCharacter = try? storage.objectExists(forKey: "my favorite city")

// Remove an object in storage
try? storage.removeObject(forKey: "my favorite city")

// Remove all objects
try? storage.removeAll()

// Remove expired objects
try? storage.removeExpiredObjects()

Entry

There is time you want to get object together with its expiry information and meta data. You can use Entry

let entry = try? storage.entry(forKey: "my favorite city")
print(entry?.object)
print(entry?.expiry)
print(entry?.meta)

meta may contain file information if the object was fetched from disk storage.

Custom Codable

Codable works for simple dictionary like [String: Int], [String: String], ... It does not work for [String: Any] as Any is not Codable conformance, it will raise fatal error at runtime. So when you get json from backend responses, you need to convert that to your custom Codable objects and save to Storage instead.

struct User: Codable {
  let firstName: String
  let lastName: String
}

let user = User(fistName: "John", lastName: "Snow")
try? storage.setObject(user, forKey: "character")

Async APIs

In async fashion, you deal with Result instead of try catch because the result is delivered at a later time, in order to not block the current calling queue. In the completion block, you either have value or error.

You access Async APIs via storage.async, it is also thread safe, and you can use Sync and Async APIs in any order you want. All Async functions are constrained by AsyncStorageAware protocol.

storage.async.setObject("Oslo", forKey: "my favorite city") { result in
  switch result {
    case .success:
      print("saved successfully")
    case .failure(let error):
      print(error)
  }
}

storage.async.object(forKey: "my favorite city") { result in
  switch result {
    case .success(let city):
      print("my favorite city is \(city)")
    case .failure(let error):
      print(error)
  }
}

storage.async.objectExists(forKey: "my favorite city") { result in
  if case .success(let exists) = result, exists {
    print("I have a favorite city")
  }
}

storage.async.removeAll() { result in
  switch result {
    case .success:
      print("removal completes")
    case .failure(let error):
      print(error)
  }
}

storage.async.removeExpiredObjects() { result in
  switch result {
    case .success:
      print("removal completes")
    case .failure(let error):
      print(error)
  }
}

Swift Concurrency

do {
  try await storage.async.setObject("Oslo", forKey: "my favorite city")
  print("saved successfully")
} catch {
  print(error)
}

do {
  let city = try await storage.async.object(forKey: "my favorite city")
  print("my favorite city is \(city)")
} catch {
  print(error)
}

do {
  let exists = try await storage.async.objectExists(forKey: "my favorite city")
  if exists {
    print("I have a favorite city")
  }
} catch {}

do {
  try await storage.async.remoeAll()
  print("removal completes")
} catch {
  print(error)
}

do {
  try await storage.async.removeExpiredObjects()
  print("removal completes")
} catch {
  print(error)
}

Expiry date

By default, all saved objects have the same expiry as the expiry you specify in DiskConfig or MemoryConfig. You can overwrite this for a specific object by specifying expiry for setObject

// Default expiry date from configuration will be applied to the item
try? storage.setObject("This is a string", forKey: "string")

// A given expiry date will be applied to the item
try? storage.setObject(
  "This is a string",
  forKey: "string",
  expiry: .date(Date().addingTimeInterval(2 * 3600))
)

// Clear expired objects
storage.removeExpiredObjects()

Observations

Storage allows you to observe changes in the cache layer, both on a store and a key levels. The API lets you pass any object as an observer, while also passing an observation closure. The observation closure will be removed automatically when the weakly captured observer has been deallocated.

Storage observations

// Add observer
let token = storage.addStorageObserver(self) { observer, storage, change in
  switch change {
  case .add(let key):
    print("Added \(key)")
  case .remove(let key):
    print("Removed \(key)")
  case .removeAll:
    print("Removed all")
  case .removeExpired:
    print("Removed expired")
  }
}

// Remove observer
token.cancel()

// Remove all observers
storage.removeAllStorageObservers()

Key observations

let key = "user1"

let token = storage.addObserver(self, forKey: key) { observer, storage, change in
  switch change {
  case .edit(let before, let after):
    print("Changed object for \(key) from \(String(describing: before)) to \(after)")
  case .remove:
    print("Removed \(key)")
  }
}

// Remove observer by token
token.cancel()

// Remove observer for key
storage.removeObserver(forKey: key)

// Remove all observers
storage.removeAllKeyObservers()

Handling JSON response

Most of the time, our use case is to fetch some json from backend, display it while saving the json to storage for future uses. If you're using libraries like Alamofire or Malibu, you mostly get json in the form of dictionary, string, or data.

Storage can persist String or Data. You can even save json to Storage using JSONArrayWrapper and JSONDictionaryWrapper, but we prefer persisting the strong typed objects, since those are the objects that you will use to display in UI. Furthermore, if the json data can't be converted to strongly typed objects, what's the point of saving it ? 😉

You can use these extensions on JSONDecoder to decode json dictionary, string or data to objects.

let user = JSONDecoder.decode(jsonString, to: User.self)
let cities = JSONDecoder.decode(jsonDictionary, to: [City].self)
let dragons = JSONDecoder.decode(jsonData, to: [Dragon].self)

This is how you perform object converting and saving with Alamofire

Alamofire.request("https://gameofthrones.org/mostFavoriteCharacter").responseString { response in
  do {
    let user = try JSONDecoder.decode(response.result.success, to: User.self)
    try storage.setObject(user, forKey: "most favorite character")
  } catch {
    print(error)
  }
}

What about images

If you want to load image into UIImageView or NSImageView, then we also have a nice gift for you. It's called Imaginary and uses Cache under the hood to make your life easier when it comes to working with remote images.

Installation

Cocoapods

Cache is available through CocoaPods. To install it or update it, use the following line to your Podfile:

pod 'Cache', :git => 'https://github.com/hyperoslo/Cache.git'

Carthage

Cache is also available through Carthage. To install just write into your Cartfile:

github "hyperoslo/Cache"

You also need to add SwiftHash.framework in your copy-frameworks script.

Author

• Hyper made this with ❤️
• Inline MD5 implementation from SwiftHash

Contributing

We would love you to contribute to Cache, check the CONTRIBUTING file for more info.

License

Cache is available under the MIT license. See the LICENSE file for more info.