iphone - UIView scaling during animation

Question

I've got a custom UIView to show a tiled image.

    - (void)drawRect:(CGRect)rect
    {
        ...
        CGContextRef context = UIGraphicsGetCurrentContext();       
        CGContextClipToRect(context,
             CGRectMake(0.0, 0.0, rect.size.width, rect.size.height));      
        CGContextDrawTiledImage(context, imageRect, imageRef);
        ...
    }

Now I am trying to animate the resize of that view.

[UIView beginAnimations:nil context:nil];
[UIView setAnimationDuration:0.3];

// change frame of view

[UIView commitAnimations];

What I would expect is for the tiled area just to grow, leaving the tile sizes constant. What happens instead is that while the area grows the original content of the view is scaled to the new size. At least during the animation. So the at the beginning and the end of the animation all is good. During the animation the tiles are distorted.

Why is CA trying to scale? How can I prevent it from doing so? What did I miss?

Answer 1

If Core Animation had to call back to your code for every animation frame it would never be as fast as it is, and animation of custom properties has been a long requested feature and FAQ for CA on the Mac.

Using UIViewContentModeRedraw is on the right track, and is also the best you'll get from CA. The problem is from the UIKit point of view the frame only has two values: the value at the beginning of the transition and the value at the end of the transition, and that's what you're seeing. If you look at the Core Animation architecture documents you'll see how CA has a private representation of all layer properties and their values changing over time. That's where the frame interpolation is happening, and you can't be notified of changes to that as they happen.

So the only way is to use an NSTimer (or performSelector:withObject:afterDelay:) to change the view frame over time, the old fashioned way.