problem with zoom in UIScrollView on iOS 4.0

Question

Hi,

This is a general question to a specific problem.

I am using a UIScrollView in an app that displays photographs. on iOS < 4.0, zooming works great. the same app running on iOS 4.0.x has problems zooming. specifically, if the image does not fill the view (and black bands appear at top/bottom), the first zoom is jerky and garbage data is shown on the bottom of the screen.

the source code to analyze is way to complex and spread out to adequately share here. Can anyone suggest any areas to look at that might cause this strange behavior?

thanks! Mark

edit: here's the code from the double tap handler (borrowed from the tapDetectingImageView sample code):

- (void)tapDetectingImageView:(TapDetectingImageView *)view gotDoubleTapAtPoint:(CGPoint)tapPoint {
    // double tap zooms in
    float newScale = [self zoomScale] * ZOOM_STEP;
    CGRect zoomRect = [self zoomRectForScale:newScale withCenter:tapPoint];
    [self zoomToRect:zoomRect animated:YES];
}

I can force the weirdness if I change the animated parameter in the call to zoomToRect. when animated is NO, my image becomes 2 images superimposed one on top of the other. the bottom image is the original zoom level, the top image is the new zoom level. if I swipe the screen to pan, the image is refreshed. It's almost as if a call to layoutSubviews or DrawRect is not getting called.

Answer 1

This may or may not be related, but the way that UIScrollView dealt with scale factors changed in an undocumented way in iPhone OS 3.2+.

Previously, if you used -scrollViewDidEndZooming:withView:atScale: to re-render your content by applying the identity transform to the view and then redrawing your image sharply at the new scale factor, UIScrollView would ignore this and keep handing you absolute scale factors based on the initial size of the view.

On iPhone OS 3.2+, UIScrollView now gives you a relative scale factor based on the last time you reset the transform of the content view to be the identity transform. This can lead to significant scaling differences between the various OS versions.