关于KVO context argument:

由于swift没有了指针类型 

let myContext = UnsafePointer<()>()

observee.addObserver(observer, forKeyPath: …, options: nil, context: myContext)


override func observeValueForKeyPath(keyPath: String!, ofObject object: AnyObject!, change: [NSObject : AnyObject]!, context: UnsafePointer<()>) {
    if context == myContext {
        …
    } else {
        super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
    }
}