Center content of UIScrollView when smaller

Question

I have a UIImageView inside a UIScrollView which I use for zooming and scrolling. If the image / content of the scroll view is bigger than the scroll view, everything works fine. However, when the image becomes smaller than the scroll view, it sticks to the top left corner of the scroll view. I would like to keep it centered, like the Photos app.

Any ideas or examples about keeping the content of the UIScrollView centered when it's smaller?

I am working with iPhone 3.0.

The following code almost works. The image returns to the top left corner if I pinch it after reaching the minimum zoom level.

- (void)loadView {
    [super loadView];

    // set up main scroll view
    imageScrollView = [[UIScrollView alloc] initWithFrame:[[self view] bounds]];
    [imageScrollView setBackgroundColor:[UIColor blackColor]];
    [imageScrollView setDelegate:self];
    [imageScrollView setBouncesZoom:YES];
    [[self view] addSubview:imageScrollView];

    UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"WeCanDoIt.png"]];
    [imageView setTag:ZOOM_VIEW_TAG];
    [imageScrollView setContentSize:[imageView frame].size];
    [imageScrollView addSubview:imageView];

    CGSize imageSize = imageView.image.size;
    [imageView release];

    CGSize maxSize = imageScrollView.frame.size;
    CGFloat widthRatio = maxSize.width / imageSize.width;
    CGFloat heightRatio = maxSize.height / imageSize.height;
    CGFloat initialZoom = (widthRatio > heightRatio) ? heightRatio : widthRatio;

    [imageScrollView setMinimumZoomScale:initialZoom];
    [imageScrollView setZoomScale:1];

    float topInset = (maxSize.height - imageSize.height) / 2.0;
    float sideInset = (maxSize.width - imageSize.width) / 2.0;
    if (topInset < 0.0) topInset = 0.0;
    if (sideInset < 0.0) sideInset = 0.0;
    [imageScrollView setContentInset:UIEdgeInsetsMake(topInset, sideInset, -topInset, -sideInset)];
}

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {
    return [imageScrollView viewWithTag:ZOOM_VIEW_TAG];
}

/************************************** NOTE **************************************/
/* The following delegate method works around a known bug in zoomToRect:animated: */
/* In the next release after 3.0 this workaround will no longer be necessary      */
/**********************************************************************************/
- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale {
    [scrollView setZoomScale:scale+0.01 animated:NO];
    [scrollView setZoomScale:scale animated:NO];
    // END Bug workaround

    CGSize maxSize = imageScrollView.frame.size;
    CGSize viewSize = view.frame.size;
    float topInset = (maxSize.height - viewSize.height) / 2.0;
    float sideInset = (maxSize.width - viewSize.width) / 2.0;
    if (topInset < 0.0) topInset = 0.0;
    if (sideInset < 0.0) sideInset = 0.0;
    [imageScrollView setContentInset:UIEdgeInsetsMake(topInset, sideInset, -topInset, -sideInset)];
}

Answer 1

I've got very simple solution! All you need is to update the center of your subview (imageview) while zooming in the ScrollViewDelegate. If zoomed image is smaller than scrollview then adjust subview.center else center is (0,0).

- (void)scrollViewDidZoom:(UIScrollView *)scrollView 
{
    UIView *subView = [scrollView.subviews objectAtIndex:0];

    CGFloat offsetX = MAX((scrollView.bounds.size.width - scrollView.contentSize.width) * 0.5, 0.0);
    CGFloat offsetY = MAX((scrollView.bounds.size.height - scrollView.contentSize.height) * 0.5, 0.0);

    subView.center = CGPointMake(scrollView.contentSize.width * 0.5 + offsetX, 
                                 scrollView.contentSize.height * 0.5 + offsetY);
}