Swift-like KVO

KeyValueObserver.swift

import Foundation

/// Helper object for more Swift-like KVO. To create, call NSObject.addObserverForKeyPath(_:handler:).
class KeyValueObserver: NSObject {
	
	private weak var observing: NSObject?
	private var keyPath: String
	private var handler: () -> Void
	
	private init(observing: NSObject, keyPath: String, handler: () -> Void) {
		self.observing = observing
		self.keyPath = keyPath
		self.handler = handler
		super.init()
		observing.addObserver(self, forKeyPath: keyPath, options: [], context: nil)
	}
	
	deinit {
		remove()
	}

	/// Manually stop observing.
	func remove() {
		observing?.removeObserver(self, forKeyPath: keyPath)
		observing = nil
	}
	
	override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
		assert(keyPath == self.keyPath)
		handler()
	}
}

extension NSObject {
	
	/// Observe keyPath on the receiver for the lifetime of the returned KeyValueObserver object, or until remove() is called upon it.
	func addObserverForKeyPath(keyPath: String, handler: () -> Void) -> KeyValueObserver {
		let observer = KeyValueObserver(observing: self, keyPath: keyPath, handler: handler)
		return observer
	}
	
}