UIScrollView with centered UIImageView, like Photo

Question

I would like to have scroll view with an image content view. The image is actually map which is much bigger than the screen. The map should be initially in the center of the scroll view, like photos in Photos app when you turn iPhone to landscape orientation.

I did not manage to have the map in the center with correct zooming and scrolling at the same time. Provided that the map image starts from the top of the screen (in portrait orientation), the code looks something like:

- (void)loadView {
    mapView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"map.jpg"]];
    CGFloat mapHeight = MAP_HEIGHT * SCREEN_WIDTH / MAP_WIDTH;
    mapView.frame = CGRectMake(0, 0, SCREEN_WIDTH, mapHeight);
    scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
    scrollView.delegate = self;
    scrollView.contentSize = mapView.frame.size;
    scrollView.maximumZoomScale = MAP_WIDTH / SCREEN_WIDTH;
    scrollView.minimumZoomScale = 1;
    [scrollView addSubview:mapView];
    self.view = scrollView;
}

When I move the image frame to the center, the image grows only from the top of its frame down. I tried to play around with mapView transform, with dynamically changing frame of the imageView. Nothing works for me so far.

Answer 1

Apple has released the 2010 WWDC session videos to all members of the iphone developer program. One of the topics discussed is how they created the photos app!!! They build a very similar app step by step and have made all the code available for free.

It does not use private api either. I can't put any of the code here because of the non disclosure agreement, but here is a link to the sample code download. You will probably need to login to gain access.

http://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/wa/getSoftware?code=y&source=x&bundleID=20645

And, here is a link to the iTunes WWDC page:

http://insideapple.apple.com/redir/cbx-cgi.do?v=2&la=en&lc=&a=kGSol9sgPHP%2BtlWtLp%2BEP%2FnxnZarjWJglPBZRHd3oDbACudP51JNGS8KlsFgxZto9X%2BTsnqSbeUSWX0doe%2Fzv%2FN5XV55%2FomsyfRgFBysOnIVggO%2Fn2p%2BiweDK%2F%2FmsIXj

Answer 2

Okay, I think I've found a pretty good solution to this problem. The trick is to constantly readjust the imageView's frame. I find this works much better than constantly adjusting the contentInsets or contentOffSets. I had to add a bit of extra code to accommodate both portrait and landscape images.

Here's the code:

- (void) scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale {

CGSize screenSize = [[self view] bounds].size;

if (myScrollView.zoomScale <= initialZoom +0.01) //This resolves a problem with the code not working correctly when zooming all the way out.
{
    imageView.frame = [[self view] bounds];
    [myScrollView setZoomScale:myScrollView.zoomScale +0.01];
}

if (myScrollView.zoomScale > initialZoom)
{
    if (CGImageGetWidth(temporaryImage.CGImage) > CGImageGetHeight(temporaryImage.CGImage)) //If the image is wider than tall, do the following...
    {
            if (screenSize.height >= CGImageGetHeight(temporaryImage.CGImage) * [myScrollView zoomScale]) //If the height of the screen is greater than the zoomed height of the image do the following...
            {
                    imageView.frame = CGRectMake(0, 0, 320*(myScrollView.zoomScale), 368);
            }
            if (screenSize.height < CGImageGetHeight(temporaryImage.CGImage) * [myScrollView zoomScale]) //If the height of the screen is less than the zoomed height of the image do the following...
            {
                    imageView.frame = CGRectMake(0, 0, 320*(myScrollView.zoomScale), CGImageGetHeight(temporaryImage.CGImage) * [myScrollView zoomScale]);
            }
    }
    if (CGImageGetWidth(temporaryImage.CGImage) < CGImageGetHeight(temporaryImage.CGImage)) //If the image is taller than wide, do the following...
    {
            CGFloat portraitHeight;
            if (CGImageGetHeight(temporaryImage.CGImage) * [myScrollView zoomScale] < 368)
            { portraitHeight = 368;}
            else {portraitHeight = CGImageGetHeight(temporaryImage.CGImage) * [myScrollView zoomScale];}

            if (screenSize.width >= CGImageGetWidth(temporaryImage.CGImage) * [myScrollView zoomScale]) //If the width of the screen is greater than the zoomed width of the image do the following...
            {
                    imageView.frame = CGRectMake(0, 0, 320, portraitHeight);
            }
            if (screenSize.width < CGImageGetWidth (temporaryImage.CGImage) * [myScrollView zoomScale]) //If the width of the screen is less than the zoomed width of the image do the following...
            {
                    imageView.frame = CGRectMake(0, 0, CGImageGetWidth(temporaryImage.CGImage) * [myScrollView zoomScale], portraitHeight);
            }
    }
    [myScrollView setZoomScale:myScrollView.zoomScale -0.01];
}

Answer 3

This worked for me:

- (void) scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale {
CGFloat tempx = view.center.x-160;
CGFloat tempy = view.center.y-160;
myScrollViewOffset = CGPointMake(tempx,tempy);

}

where 160 is half the width/height of your UIScrollView.

Then later I set the contentoffset to the one captured here.

Answer 4

This code should work on most versions of iOS (and has been tested to work on 3.1 upwards).

It's based on the Apple WWDC code mentioned in Jonah's answer.

Add the below to your subclass of UIScrollView, and replace tileContainerView with the view containing your image or tiles:

- (void)layoutSubviews {
    [super layoutSubviews];

    // center the image as it becomes smaller than the size of the screen
    CGSize boundsSize = self.bounds.size;
    CGRect frameToCenter = tileContainerView.frame;

    // center horizontally
    if (frameToCenter.size.width < boundsSize.width)
        frameToCenter.origin.x = (boundsSize.width - frameToCenter.size.width) / 2;
    else
        frameToCenter.origin.x = 0;

    // center vertically
    if (frameToCenter.size.height < boundsSize.height)
        frameToCenter.origin.y = (boundsSize.height - frameToCenter.size.height) / 2;
    else
        frameToCenter.origin.y = 0;

    tileContainerView.frame = frameToCenter;
}

Answer 5

I suspect that you need to set the UIScrollView's contentOffset.