KVO in Swift

KeyValueObserver.swift

class KeyValueObserver: NSObject {
    typealias KeyValueObservingCallback = (change: [NSObject : AnyObject]) -> Void
    
    private let object: NSObject
    private let keyPath: String
    private let callback: KeyValueObservingCallback
    private var kvoContext = 0
    
    init(object: NSObject, keyPath: String, options: NSKeyValueObservingOptions, callback: KeyValueObservingCallback) {
        self.object = object
        self.keyPath = keyPath
        self.callback = callback
        super.init()
        object.addObserver(self, forKeyPath: keyPath, options: options, context: &kvoContext)
    }
    
    deinit {
        object.removeObserver(self, forKeyPath: keyPath, context: &kvoContext)
    }
    
    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        if context == &kvoContext {
            self.callback(change: change)
        }
        else {
            super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
        }
    }
}

Sample.swift

class Foo {
    private var observer: KeyValueObserver?
    private let bar = Bar()
    
    init() {
        self.observer = KeyValueObserver(object: bar, keyPath: "string", options: .New) {
            [unowned self] change in
            println("bar changed: \(self.bar.string)")
        }
        bar.string = "world"
    }
    
    deinit {
        self.observer = nil
    }
}

class Bar: NSObject {
    dynamic var string = "hello"
}

nice1, thanks.