Start new CAAnimation from current CAAnimation's state

Was just led to a solution via this answer. If one looks in the right part of the docs, you'll find the following:

- (id)presentationLayer

Returns a copy of the layer containing all properties as they were at the start of the current transaction, with any active animations applied.

So if I add this code before I first check the transparent mask location (aka solid level), I grab the current animated position and can switch between fill up and fill down.

CALayer *temp;

if (transMask.animationKeys.count > 0) {
    temp = transMask.presentationLayer;
    transMask.position = temp.position;
}

if (emptyMask.animationKeys.count > 0) {
    temp = emptyMask.presentationLayer;
    emptyMask.position = temp.position;
}

layer.presentation() makes copy of original layer each time you get it, and you need to override init(layer: Any) for every custom CALayer you have.

Another possible way to start from current state is to set animation's beginTime using current animation's beginTime. It allows to start new animation with offset.

In your case it could be (Swift):

if let currentAnimation = slider.animation(forKey: "colorize") {
    let currentTime = CACurrentMediaTime()
    let animationElapsedTime = currentAnimation.beginTime + currentAnimation.duration - currentTime
    a.beginTime = currentTime - animationElapsedTime
}

However this works great for linear timings, but for others it could be difficult to calculate the proper time.