Observer Design Pattern in Swift

swift_design_patterns_observer.swift

// Observer pattern in Swift
// Author: Jean-Marc Boullianne

class Subject<T> {
    
    var val:T {
        didSet {
            notifyObservers()
        }
    }
    var observers:[Observer<T>]
    init(val: T) {
        self.val = val
        self.observers = []
    }
    
    func attach(observer: Observer<T>) {
        observers.append(observer)
    }
    
    func notifyObservers() {
        for observer in observers {
            observer.callback(self.val)
        }
    }
    
}

class Observer<T> {
    var callback:((T)->())
    
    init(callback: @escaping (T) -> ()) {
        self.callback = callback
    }
}

var x = Subject<Int>(val: 0)

var observer1 = Observer<Int>(callback:  { (val) in
    print("Observer 1:", val)
})

var observer2 = Observer<Int>(callback:  { (val) in
    print("Observer 2:", val)
})

x.attach(observer: observer1)
x.attach(observer: observer2)

x.val = 10 // Will trigger calls to both callbacks above !
           // --------------------------------------------
           // 'Observer 1: 10'
           // 'Observer 2: 10'

x.val = 22 // --------------------------------------------
           // 'Observer 1: 22'
           // 'Observer 2: 22'