UIView replicate CAAnimation of another view, in real time? - swift

Question

So I've got a background view with a gradient sublayer, animating continuously to change the colors slowly. I'm doing it with a CATransaction, because I need to animate other properties as well:
CATransaction.begin()
gradientLayer.add(colorAnimation, forKey: "colors")
// other animations
CATransaction.setCompletionBlock({
// start animation again, loop forever
}
CATransaction.commit()
Now I want to replicate this gradient animation, let's say, for the title of a button for instance.
Note 1: I can't just "make a hole" in the button, if such a thing is possible, because I might have other opaque views between the button and the background.
Note 2: The gradient position on the button is not important. I don't want the text gradient to replicate the exact colors underneath, but rather to mimic the "mood" of the background.
So when the button is created, I add its gradient sublayer to a list of registered layers, that the background manager will update as well:
func register(layer: CAGradientLayer) {
let pointer = Unmanaged.passUnretained(layer).toOpaque()
registeredLayers.addPointer(pointer)
}
So while it's easy to animate the text gradient at the next iteration of the animation, I would prefer that the button starts animating as soon as it's added, since the animation usually takes a few seconds. How can I copy the background animation, i.e. set the text gradient to the current state of the background animation, and animate it with the right duration left and timing function?

Answer 1

The solution was indeed to use the beginTime property, as suggested by #Shivam Gaur's comment. I implemented it as follows:
// The background layer, with the original animation
var backgroundLayer: CAGradientLayer!
// The animation
var colorAnimation: CABasicAnimation!
// Variable to store animation begin time
var animationBeginTime: CFTimeInterval!
// Registered layers replicating the animation
private var registeredLayers: NSPointerArray = NSPointerArray.weakObjects()
...
// Somewhere in our code, the setup function
func setup() {
colorAnimation = CABasicAnimation(keyPath: "colors")
// do the animation setup here
...
}
...
// Called by an external class when we add a view that should replicate the background animation
func register(layer: CAGradientLayer) {
// Store a pointer to the layer in our array
let pointer = Unmanaged.passUnretained(layer).toOpaque()
registeredLayers.addPointer(pointer)
layer.colors = colorAnimation.toValue as! [Any]?
// HERE'S THE KEY: We compute time elapsed since the beginning of the animation, and start the animation at that time, using 'beginTime'
let timeElapsed = CACurrentMediaTime() - animationBeginTime
colorAnimation.beginTime = -timeElapsed
layer.add(colorAnimation, forKey: "colors")
colorAnimation.beginTime = 0
}
// The function called recursively for an endless animation
func animate() {
// Destination layer
let toLayer = newGradient() // some function to create a new color gradient
toLayer.frame = UIScreen.main.bounds
// Setup animation
colorAnimation.fromValue = backgroundLayer.colors;
colorAnimation.toValue = toLayer.colors;
// Update background layer
backgroundLayer.colors = toLayer.colors
// Update registered layers (iterate is a custom function I declared as an extension of NSPointerArray)
registeredLayers.iterate() { obj in
guard let layer = obj as? CAGradientLayer else { return }
layer.colors = toLayer.colors
}
CATransaction.begin()
CATransaction.setCompletionBlock({
animate()
})
// Add animation to background
backgroundLayer.add(colorAnimation, forKey: "colors")
// Store starting time
animationBeginTime = CACurrentMediaTime();
// Add animation to registered layers
registeredLayers.iterate() { obj in
guard let layer = obj as? CAGradientLayer else { return }
layer.add(colorAnimation, forKey: "colors")
}
CATransaction.commit()
}