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
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
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.
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.