Redrawing UIScrollView contents after every zoom

Question

I have a UIView inside a UIScrollView. Whenever the UIScrollView zoom changes, I want to redraw the entire UIView at the new zoom level.

In iOS < 3.2, I was doing this by resizing the UIView within the UIScrollView to make it the new size, and then setting the transform back to Identity, so that it wouldn't try to resize it further. However, with iOS >= 3.2, changing the identity also changes the UIScrollView's zoomScale property.

The result is that whenever I zoom (say 2x), I adjust the embedded UIView to be the appropriate size, and redraw it. However now (since I reset the transform to Identity), the UIScrollView thinks its once again at zoomScale 1, rather than zoomScale 2. So if I have my maxZoomScale set at 2, it will still try zooming further, which is wrong.

I thought about using the CATiledLayer, but I don't think this is sufficient for me, since I want to redraw after every zoom, not just at certain zoom thresholds like it tries to do.

Does anyone know how to do the proper redrawing of the UIView on a zoom?

Answer 1

Tom,

Your question is a bit old, but I came up with a solution for this, so I figured I'd put in an answer in case it helps you or anyone else. The basic trick is to reset the scroll view's zoomScale to 1, and then adjust the minimumZoomScale and maximumZoomScale so that the user can still zoom in and out as expected.

In my implementation, I've subclassed UIScrollView and set it to be its own delegate. In my subclass, I implement the two delegate methods you need for zooming (shown below). contentView is a property I added to my UIScrollView subclass that in order to give it the view that actually displays the content.

So, my init method looks something like this (kMinimumZoomScale and kMaximumZoomScale are #define's at the top of the class):

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        self.autoresizesSubviews = YES;
        self.showsVerticalScrollIndicator = YES;
        self.showsHorizontalScrollIndicator = NO;
        self.bouncesZoom = YES;
        self.alwaysBounceVertical = YES;
        self.delegate = self;

        self.minimumZoomScale = kMinimumZoomScale;
        self.maximumZoomScale = kMaximumZoomScale;
    }

    return self;
}

Then I implement the standard UIScrollView delegate methods for zooming. My ContentView class has a property called zoomScale that tells it what scale to use for displaying its content. It uses that in its drawRect method to resize the content as appropriate.

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)aScrollView {
    return contentView;
}


- (void)scrollViewDidEndZooming:(UIScrollView *)aScrollView withView:(UIView *)view atScale:(float)scale {
    CGFloat oldZoomScale = contentView.zoomScale;
    CGSize size = self.bounds.size;

    // Figure out where the scroll view was centered so that we can
    // fix up its offset after adjusting the scaling
    CGPoint contentCenter = self.contentOffset;
    contentCenter.x += size.width / (oldZoomScale * scale) / 2;
    contentCenter.y += size.height / (oldZoomScale * scale) / 2;

    CGFloat newZoomScale = scale * oldZoomScale;
    newZoomScale = MAX(newZoomScale, kMinimumZoomscale);
    newZoomScale = MIN(newZoomScale, kMaximumZoomscale);

    // Set the scroll view's zoom scale back to 1 and adjust its minimum and maximum
    // to allow the expected amount of zooming.
    self.zoomScale = 1.0;
    self.minimumZoomScale = kMinimumZoomScale / newZoomScale;
    self.maximumZoomScale = kMaximumZoomScale / newZoomScale;

    // Tell the contentView about its new scale. My contentView.zoomScale setter method
    // calls setNeedsDisplay, but you could also call it here
    contentView.zoomScale = newZoomScale;

    // My ContentView class overrides sizeThatFits to give its expected size with
    // zoomScale taken into account
    CGRect newContentSize = [contentView sizeThatFits];

    // update the content view's frame and the scroll view's contentSize with the new size
    contentView.frame = CGRectMake(0, 0, newContentSize.width, newContentSize.height);
    self.contentSize = newContentSize;

    // Figure out the new contentOffset so that the contentView doesn't appear to move
    CGPoint newContentOffset = CGPointMake(contentCenter.x - size.width / newZoomScale / 2,
                                           contentCenter.y - size.height / newZoomScale / 2);
    newContentOffset.x = MIN(newContentOffset.x, newContentSize.width - size.width);
    newContentOffset.x = MAX(0, newContentOffset.x);
    newContentOffset.y = MIN(newContentOffset.y, newContentSize.height - .size.height);
    newContentOffset.y = MAX(0, newContentOffset.y);
    [self setContentOffset:newContentOffset animated:NO];
}