CAAnimation.on(started:stopped:)

CAAnimation+On.swift

import QuartzCore

private extension CAAnimation {
	final class Delegate: NSObject {
		var startHandlers = Array<StartHandler>()
		var stopHandlers = Array<StopHandler>()

		override func animationDidStart(anim: CAAnimation) {
			startHandlers.forEach { $0(animation: anim) }
		}

		override func animationDidStop(anim: CAAnimation, finished flag: Bool) {
			stopHandlers.forEach { $0(animation: anim, finished: flag) }
		}
	}

	struct AssociatedObjectKey {
		static var on_delegate: Delegate?
	}

	var on_delegate: Delegate {
		var result = objc_getAssociatedObject(self, &AssociatedObjectKey.on_delegate) as? Delegate
		if result == nil {
			result = Delegate()
			delegate = result
			objc_setAssociatedObject(self, &AssociatedObjectKey.on_delegate, result, .OBJC_ASSOCIATION_RETAIN)
		}
		return result!
	}
}

public extension CAAnimation {
	typealias StartHandler = (animation: CAAnimation) -> Void
	typealias StopHandler = (animation: CAAnimation, finished: Bool) -> Void

	func on(started started: StartHandler? = nil, stopped: StopHandler? = nil) -> Self {
		let delegate = on_delegate
		if let handler = started {
			delegate.startHandlers.append(handler)
		}
		if let handler = stopped {
			delegate.stopHandlers.append(handler)
		}
		return self
	}
}