CAAnimation extension with starting/completion callback blocks

CAAnimation+blockCallback.swift

//
//  CAAnimation+BlockCallback.swift
//

import UIKit

//
// Modified from http://onedayitwillmake.com/blog/2016/06/caanimation-completion-callback-block/
// Updated for Swift 4 syntax
// All credit to the original author (Mario Gonzalez)
//
// USAGE:
//
//  var animation = CABasicAnimation(keyPath: "strokeEnd")
//  animation.duration = duration
//  animation.fromValue = 0.0
//  animation.toValue = 1.0
//  animation.startBlock { startingAnimation, status in
//    print("Starting...")
//  }
//  animation.completionBlock { completingAnimation, status in
//    print("Completed!")
//  }

public typealias CAAnimationBlockCallback = (CAAnimation, Bool) -> ();

public class CAAnimationBlockCallbackDelegate: NSObject, CAAnimationDelegate {
   var onStartCallback: CAAnimationBlockCallback?
   var onCompleteCallback: CAAnimationBlockCallback?

   public func animationDidStart(_ anim: CAAnimation) {
      if let startHandler = onStartCallback {
         startHandler(anim, true)
      }
   }

   public func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {
      if let completionHandler = onCompleteCallback {
         completionHandler(anim, flag);
      }
   }
}

public extension CAAnimation {
   // See if there is already a CAAnimationDelegate handling this animation
   // If there is, add onStart to it, if not create one
   func startBlock(callback: @escaping CAAnimationBlockCallback) {
      if let myDelegate = self.delegate as? CAAnimationBlockCallbackDelegate {
         myDelegate.onStartCallback = callback;
      } else {
         let callbackDelegate = CAAnimationBlockCallbackDelegate()
         callbackDelegate.onStartCallback = callback
         self.delegate = callbackDelegate
      }
   }

   // See if there is already a CAAnimationDelegate handling this animation
   // If there is, add onCompletion to it, if not create one
   func completionBlock(callback: @escaping CAAnimationBlockCallback) {
      if let myDelegate = self.delegate as? CAAnimationBlockCallbackDelegate {
         myDelegate.onCompleteCallback = callback
      } else {
         let callbackDelegate = CAAnimationBlockCallbackDelegate()
         callbackDelegate.onCompleteCallback = callback
         self.delegate = callbackDelegate
      }
   }
}