UIScrollView with centered UIImageView, like Photos app

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

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

Answer 2

I wish it was that simple. I did some research on the net and found that it is not just my problem, but many people are struggling with the same issue not just on iPhone, but on Apple's desktop Cocoa as well. See following links:

http://www.iphonedevsdk.com/forum/iphone-sdk-development/5740-uiimageview-uiscrollview.html
The described solution is based on the property UIViewContentModeScaleAspectFit of the image, but unfortunately it does not work very well .The image is centered and grows properly, but the bouncing area seems to be much bigger than the picture.

This guy did not get the answer either:
http://discussions.apple.com/thread.jspa?messageID=8322675

And finally, the same problem on Apple's desktop Cocoa:
http://www.cocoadev.com/index.pl?CenteringInsideNSScrollView
I suppose the solution works, but it is based on the NSClipView, which is not on iPhone...

Anybody has some solution working on iPhone?

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 scrollview.

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

Answer 4

Note: this method sort of works. if the image is smaller than the imageView, it will scroll partially off the screen. Not a big deal, but also not as nice as the photos app.

First, it's important to understand that we are dealing with 2 views, the imageview with the image in it, and the scrollview with the imageview in it. So, first set the imageview to the size of the screen:

 [myImageView setFrame:self.view.frame];

Then, center your image in the imageview:

 myImageView.contentMode = UIViewContentModeCenter;

Here's my entire code:

- (void)viewDidLoad {
AppDelegate *appDelegate = (pAppDelegate *)[[UIApplication sharedApplication] delegate];
 [super viewDidLoad];
 NSString *Path = [[NSBundle mainBundle] bundlePath];
 NSString *ImagePath = [Path stringByAppendingPathComponent:(@"data: %@", appDelegate.MainImageName)];
 UIImage *tempImg = [[UIImage alloc] initWithContentsOfFile:ImagePath];
 [imgView setImage:tempImg];

 myScrollView = [[UIScrollView alloc] initWithFrame:[[self view] bounds]];
 [myScrollView addSubview:myImageView];

//Set ScrollView Appearance
 [myScrollView setBackgroundColor:[UIColor blackColor]];
 myScrollView.indicatorStyle = UIScrollViewIndicatorStyleWhite;

//Set Scrolling Prefs
 myScrollView.bounces = YES;
 myScrollView.delegate = self;
 myScrollView.clipsToBounds = YES; // default is NO, we want to restrict drawing within our scrollview
 [myScrollView setCanCancelContentTouches:NO];
 [myScrollView setScrollEnabled:YES];


//Set Zooming Prefs
 myScrollView.maximumZoomScale = 3.0;
 myScrollView.minimumZoomScale = CGImageGetWidth(tempImg.CGImage)/320;
 myScrollView.zoomScale = 1.01; //Added the .01 to enable scrolling immediately upon view load.
 myScrollView.bouncesZoom = YES;


 [myImageView setFrame:self.view.frame];//rect];// .frame.size.height = imageHeight;
 myImageView.contentMode = UIViewContentModeCenter;
 self.view = myScrollView;
 [tempImg release];
 }

Answer 5

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 6

One elegant way to center the content of UISCrollView is this.

Add one observer to the contentSize of your UIScrollView, so this method will be called everytime the content change...

[myScrollView addObserver:delegate 
               forKeyPath:@"contentSize"
                  options:(NSKeyValueObservingOptionNew) 
                  context:NULL];

Now on your observer method:

- (void)observeValueForKeyPath:(NSString *)keyPath   ofObject:(id)object   change:(NSDictionary *)change   context:(void *)context { 

 // Correct Object Class.
 UIScrollView *pointer = object;

 // Calculate Center.
 CGFloat topCorrect = ([pointer bounds].size.height - [pointer viewWithTag:100].bounds.size.height * [pointer zoomScale])  / 2.0 ;
   topCorrect = ( topCorrect < 0.0 ? 0.0 : topCorrect );

 topCorrect = topCorrect - (  pointer.frame.origin.y - imageGallery.frame.origin.y );

 // Apply Correct Center.
 pointer.center = CGPointMake(pointer.center.x,
         pointer.center.y + topCorrect ); }

• You should change the [pointer viewWithTag:100]. Replace by your content view UIView.

  • Also change imageGallery pointing to your window size.

This will correct the center of the content everytime his size change.

NOTE: The only way this content don't works very well is with standard zoom functionality of the UIScrollView.

Answer 7

Hokay, Here is what I'd consider, the solution as in it behaves exactly like apple's photo app. I had been using solutions that used:

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

to recenter but I didn't like that solution because after the zooming was done, it'd bounce then quick 'jump' into the center which was very un-sexy. Turns out if you pretty much do the exact same logic but in this delegate function:

-(void)scrollViewDidZoom:(UIScrollView *)pScrollView

it both starts off centered and when you zoom out it stays centered:

-(void)scrollViewDidZoom:(UIScrollView *)pScrollView {
CGRect innerFrame = imageView.frame;
CGRect scrollerBounds = pScrollView.bounds;

if ( ( innerFrame.size.width < scrollerBounds.size.width ) || ( innerFrame.size.height < scrollerBounds.size.height ) )
{
    CGFloat tempx = imageView.center.x - ( scrollerBounds.size.width / 2 );
    CGFloat tempy = imageView.center.y - ( scrollerBounds.size.height / 2 );
    CGPoint myScrollViewOffset = CGPointMake( tempx, tempy);

    pScrollView.contentOffset = myScrollViewOffset;

}

UIEdgeInsets anEdgeInset = { 0, 0, 0, 0};
if ( scrollerBounds.size.width > innerFrame.size.width )
{
    anEdgeInset.left = (scrollerBounds.size.width - innerFrame.size.width) / 2;
    anEdgeInset.right = -anEdgeInset.left;  // I don't know why this needs to be negative, but that's what works
}
if ( scrollerBounds.size.height > innerFrame.size.height )
{
    anEdgeInset.top = (scrollerBounds.size.height - innerFrame.size.height) / 2;
    anEdgeInset.bottom = -anEdgeInset.top;  // I don't know why this needs to be negative, but that's what works
}
pScrollView.contentInset = anEdgeInset;
}

Where 'imageView' is the imageView you're using.

Answer 8

Here's the best way to solve this problem:

http://blog.proculo.de/archives/180-Paging-enabled-UIScrollView-With-Previews.html

It worked very well for me.

Answer 9

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 10

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;
}