swift 实现一个简单的观察者模式

在 Swift 中，一个简单的观察者模式，可以被描述如下：

protocol Observable {
    associatedtype T: Observer
    mutating func attach(observer: T)
}

protocol Observer {
    associatedtype State
    func notify(_ state: State)
}

struct AnyObservable<T: Observer>: Observable{

    var state: T.State {
        didSet {
            notifyStateChange()
        }
    }

    var observers: [T] = []

    init(_ state: T.State) {
        self.state = state
    }

    mutating func attach(observer: T) {
        observers.append(observer)
        observer.notify(state)
    }

    private func notifyStateChange() {
        for observer in observers {
            observer.notify(state)
        }
    }
}

struct AnyObserver<S>: Observer {

    private let name: String

    init(name: String) {
        self.name = name
    }

    func notify(_ state: S) {
        print("\(name)'s state updated to \(state)")
    }
}

var observable = AnyObservable<AnyObserver<String>>("hello")
let observer = AnyObserver<String>(name: "My Observer")
observable.attach(observer: observer)
observable.state = "world"

// "My Observer's state updated: hello"
// "My Observer's state updated: world"