With Swift 2.0 and this talk from Apple, protocol oriented programming has now arrived on the Cocoa scene. Although protocols allow for elegant decoupling of interface from implementation, a little extra glue (in the form of Dependecy Injection) cements the ideal of never having to know if your duck is actually a duck
Apodidae is the taxonomic family of birds known as swifts in english. Is your swift a sooty swift or a lesser swallow tail swift? You don't care! that's why you're using dependency injection, obviously...
Have a protocol that needs filling? add Injectable to your implementation:
protocol Swift {
func airspeedVelocity() -> Int
}
struct WhiteThroatedNeedleTail: Swift, Injectable {
static func dependencies -> [Any.Type] {
return []
}
init(dependencies: DependencyContainer) {}
func airspeedVelocity() -> Int {
return 169
}
}and register with the registrar:
import Apodidae
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, UISplitViewControllerDelegate {
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
// init application
...
registerDependencies()
}
func registerDependencies() -> Void {
let registrar = Registrar.instance
// NOTE: Implementation MUST implement the Injectable protocol
// NOTE: Currently registration is not type safe - you could register a nonconforming class, which will fail at runtime
registrar.register(Swift.self, WhiteThroatedNeedleTail.self)
}
}Instances of your protocol can then be grabbed from the registrar:
struct Thingy {
let swift = Registrar.instance.obtain(Swift.self) as! Swift
func howFastAreMyBirds() -> Int {
return self.swift.airspeedVelocity()
}
}But what if my dependencies have dependencies?? Apodidae will resolve the chained dependencies before providing you the implementation:
protocol BirdCage: Injectable {
func howFastAreMyBirds() -> Int
}
struct Thingy: BirdCage {
let swift: Swift
static func dependencies -> [Any] {
return [
Swift.self // defines the type dependency on a swift
]
}
init(dependencies: DependencyContainer) {
// Dependencies are returned in the same order you specified
self.swift = dependencies.extract(Swift.self)
}
func howFastAreMyBirds() -> Int {
return self.swift.airspeedVelocity()
}
}