KVO-sample.swift

class Foo: NSObject {
    private let bar = Bar()
    private var kvoContext = 0
    
    override init() {
        super.init()
        bar.addObserver(self, forKeyPath: "string", options: .New, context: &kvoContext)
        bar.string = "world"
    }
    
    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        if context == &kvoContext {
            println("bar changed: \(bar.string)")
        }
        else {
            super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
        }
    }
    
    deinit {
        bar.removeObserver(self, forKeyPath: "string", context: &kvoContext)
    }
}

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