Issue setting CABasicAnimation delegate in Swift 3?

Question

I'm getting the following error

Cannot assign value of type 'StarButton' to type 'CAAnimationDelegate?'

on the last line of this CABasicAnimation block:

 let fillCircle = CABasicAnimation(keyPath: "opacity")
 fillCircle.toValue = 0
 fillCircle.duration = 0.3
 fillCircle.setValue(notFavoriteKey, forKey: starKey)
 fillCircle.delegate = self // this is where the error is thrown

self is a custom UIButton class. This wasn't an issue in previous versions of Swift... any suggestions on a solution?

UPDATE

Here is a downloadable link to the source file for the StarButton class for best reference:

https://www.dropbox.com/s/gvc2sky05f4p3au/StarButton.swift?dl=0

Answer 1

As @Doc mentioned in the comments above, the solution is to replace

class StarButton: UIButton { 

with

class StarButton: UIButton, CAAnimationDelegate {

It would seem that the CAAnimationDelegate protocol is no longer implied and must be specified explicitly.