Core Animation problem on iPhone

Question

I'm new to iPhone development, and doing some experimentation with Core Animation. I've run into a small problem regarding the duration of the animation I'm attempting.

Basically, Ive got a view with two subviews, and I'm trying to animate their opacity so that one fades in while the other fades out. Problem is, instead of a gradual fade in/out, the subviews simply switch instantly to/from full/zero opacity. I've tried to adjust the animation duration with CATransaction with no noticable effect. It's also not specific to animating opacity - animating position shows the same problem.

The code I'm using (inside a method of the superview) follows:

CALayer* oldLayer = ((UIView*) [[self subviews] objectAtIndex:0]).layer;
CALayer* newLayer = ((UIView*) [[self subviews] objectAtIndex:1]).layer;

[CATransaction begin];
[CATransaction setAnimationDuration:1.0f];
oldLayer.opacity = 0.0;
newLayer.opacity = 1.0;
[CATransaction commit];

Does anyone have an idea what the problem might be?

Answer 1

Noah's solution is the cleanest way to do what you want, but the reason why you're not seeing the animation is that implicit animations are disabled for CALayers that back UIViews. Because nothing is animating, the CATransaction is unable to set the duration for anything.

If you really wanted to perform this animation on the layer, you'd need to set up a manual CABasicAnimation to do this:

CABasicAnimation *opacityAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
opacityAnimation.removedOnCompletion = NO;
opacityAnimation.fillMode = kCAFillModeForwards;

[oldLayer addAnimation:opacityAnimation forKey:@"opacity"];
[newLayer addAnimation:opacityAnimation forKey:@"opacity"];

[CATransaction begin];
[CATransaction setAnimationDuration:1.0f];
oldLayer.opacity = 0.0;
newLayer.opacity = 1.0;
[CATransaction commit];

(Note that I'm not sure if you can re-use the opacity animation here. You might need to create separate instances for each layer.)

Answer 2

Is there a reason you're using Core Animation over the generic UIView animation wrapper? It's pretty straightforward to do something like this with

UIView *oldView = [[self subviews] objectAtIndex:0];
UIView *newView = [[self subviews] objectAtIndex:1];

[UIView beginAnimations:@"swapViews" context:nil];
[UIView setAnimationDuration:1];
    oldView.alpha = 0;
    newView.alpha = 1;
[UIView commitAnimations];

Answer 3

Do you need to use an explicit CATransaction here?

The docs here imply that you don't.