Core Animation is not working with "alpha" value

Question

Before this code, my movie pic alpha is set to 0,

CABasicAnimation* fadein= [CABasicAnimation animationWithKeyPath:@"alpha"];
    [fadein setToValue:[NSNumber numberWithFloat:1.0]];
    [fadein setDuration:0.5];
    [[moviepic layer]addAnimation:fadein forKey:@"alpha"];

Nothing happened, if I set alpha to 0.5 beforehand instead, the alpha remains at 0.5 and not animating to 1.

I've seen a code using UIView beginAnimations: around, but I'm teaching core animation so I wondered why CABasicAnimation can't do simple task like this?

Answer 1

[CABasicAnimation animationWithKeyPath:@"opacity"];

UIView exposes this as alpha where as CALayer exposes this as opacity.

Answer 2

For Swift:

let opacity = CABasicAnimation(keyPath: "opacity")
opacity.fromValue = fromValue
opacity.toValue = toValue
opacity.duration = duration
opacity.beginTime = CACurrentMediaTime() + beginTime  //If a delay is needed

view.layer.add(opacity, forKey: nil)

If you want to keep the final alpha value, you have to set the current view controller as the delegate of the opacity animation:

opacity.delegate = self

And, in the delegate function animationDidStop, you should do:

extension ViewController: CAAnimationDelegate {

    func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {   
        view.alpha = toValue
    }
}

Answer 3

@ohho answers the posted question. Mine will be a bit more generic. For a list what can and how be animated with CABasicAnimation please refer to Apple's documentation