Observe property change of class instance - ios

Question

Given an instance of a class, how can I observe a property change?
For e.g., I'm building an SDK that initializes a host app's chat view to provide more functionality with a simple inplementation that looks like:
sdk.initialize(chatView)
In that initializing function, I need to track the host app's chat-view's hidden property so that the SDK's view matches.

Answer 1

A simple KVO example for observing hidden:
class SDKViewController : UIViewController {
private var context = 0
private var observingView: UIView?
func initialize(view: UIView) {
removeObservations()
observingView = view
// start observing changes to hidden property of UIView
observingView?.addObserver(self, forKeyPath: "hidden", options: [.New], context: &context)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
if let newValue = change?[NSKeyValueChangeNewKey] as? Bool where context == &self.context {
print("hidden changed: \(newValue)")
}
}
// this is called by deinit
// it should also be called if they can deregister the view from your SDK
func removeObservations() {
if let view = observingView {
view.removeObserver(self, forKeyPath: "hidden")
observingView = nil
}
}
deinit {
removeObservations()
}
}
This is making some assumptions about your configuration, but if you allow initialization of many views, you can adjust easily.
Also, a lot of this is more concise if you use KVOController by Facebook, which is not in Swift.
Edit: Just to note, hidden does work with KVO.
Edit #2: Updated YourSDKClass to SDKViewController (NSObject -> UIViewController)

Answer 2

Here is an example using protocols
protocol MyClassDelegate:class {
func myClassValueDidChange(newValue:Int)
}
class MyClass {
weak var delegate:MyClassDelegate?
var value = 0 {
didSet {
delegate?.myClassValueDidChange(value)
}
}
}
class ViewController:UIViewController,MyClassDelegate {
let myClass = MyClass()
override func viewDidLoad() {
super.viewDidLoad()
myClass.delegate = self
}
func myClassValueDidChange(newValue:Int) {
//Do something
}
}

Answer 3

You can use Key Value Observing (KVO) to monitor changes to general properties on classes, which includes the hidden property on UIView instances. This is done using addObserver:forKeyPath:options:context: defined in the NSKeyValueObserving protocol.
Note that you can also hide a view by removing it from its superview or by setting its alpha to zero.