ios4 - In iOS 4.0, why does UIScrollView zoomToRect:animated: not trigger the scrollViewDidScroll or scrollViewDidZoom delegates while animating?

Question

I need to closely monitor the scale of the scroll view so that I can update the content view's elements (a subview managing multiple CALayers) according to the scroll view's animated zoom.

On iOS 3.1, everything works as expected, I use zoomToRect:animated: and the scrollViewDidScroll: message of the UIScrollViewDelegate gets called repeatedly while the animation takes place, letting me update the subview elements according to actual zoom.

The same code on iOS 4.0 does not exibit the same behavior. When I call zoomToRect:animated:, the delegates (scrollViewDidScroll: and scrollViewDidZoom) only get called once, which makes my sub elements desinchronized until the animation is finished. In fact, the sub elements immediately jump and then get caught up by the zoom animation until everything is in the correct place. It's as if the animation is not picking up the modifications on the subviews CALayers.

I have tried animating manually with animation blocks, but the situation is the same, no progressive callback calls. I have also tried KVO, but it is not clear to me how I would tap into a UIScrollView-managed animation.

Is there a workaround on iOS 4 that would allow me to push the scale value to my subviews as the UIScrollView scale is animated?

Answer 1

Brutal but works.

Define some properties:

@property (nonatomic, strong) UIView *zoomedView;    
@property (nonatomic, strong) CADisplayLink *displayLink;
@property (nonatomic) CGFloat currentScale;

Inform UIScrollView which UIView is going to be zoomed:

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
     return self.zoomedView;
}

Register for a CADisplayLink ticks. It runs Core Animation as well, so it will be kicked on same intervals:

self.displayLink  = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkTick)];
[self.displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];

Check zoomedView's presentationLayer for current values.

- (void)displayLinkTick
{
    CALayer *zoomedLayer = self.zoomedView.layer.presentationLayer;
    CGFloat scale = zoomedLayer.transform.m11;

    if (scale != self.currentScale) {
         self.currentScale = scale; // update property

         // the scale has changed!
    }
}