Center content of UIScrollView when smaller

Question

I have an UIImageView inside a UIScrollView which I use for zooming and scrolling. If the image/content of the scroll view if 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 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

You could watch the contentSize property of the UIScrollView (using key-value observing or similar), and automatically adjust the contentInset whenever the contentSize changes to be less than the size of the scroll view.

Answer 2

The way I've done this is to add an extra view into the hierarchy:

UIScrollView -> UIView -> UIImageView

Give your UIView the same aspect ratio as your UIScrollView, and centre your UIImageView into that.

Answer 3

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 4

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 5

Hi Jonah, I tried your latest solution. However, what is temporaryImage ? I tried putting temporaryImage = imageView.image ; however, once I zoom, the image disappears.

Thanks, Pannag

Answer 6

Here's the current way I'm making this work. It's better but still not perfect. Try setting myScrollView.bouncesZoom = YES; to fix the problem with the view not centering when at minZoomScale.

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGSize screenSize = [[self view] bounds].size;//[[UIScreen mainScreen] bounds].size;//
CGSize photoSize = [yourImage size];
CGFloat topInset = (screenSize.height - photoSize.height * [myScrollView zoomScale]) / 2.0;
CGFloat sideInset = (screenSize.width - photoSize.width * [myScrollView zoomScale]) / 2.0;

if (topInset < 0.0)
{ topInset = 0.0; }
if (sideInset < 0.0)
{ sideInset = 0.0; } 
[myScrollView setContentInset:UIEdgeInsetsMake(topInset, sideInset, -topInset, -sideInset)];
ApplicationDelegate *appDelegate = (ApplicationDelegate *)[[UIApplication sharedApplication] delegate];

CGFloat scrollViewHeight; //Used later to calculate the height of the scrollView
if (appDelegate.navigationController.navigationBar.hidden == YES) //If the NavBar is Hidden, set scrollViewHeight to 480
{ scrollViewHeight = 480; }
if (appDelegate.navigationController.navigationBar.hidden == NO) //If the NavBar not Hidden, set scrollViewHeight to 360
{ scrollViewHeight = 368; }

imageView.frame = CGRectMake(0, 0, CGImageGetWidth(yourImage)* [myScrollView zoomScale], CGImageGetHeight(yourImage)* [myScrollView zoomScale]);

[imageView setContentMode:UIViewContentModeCenter];
}

Also, I do the following to prevent the image from sticking a the side after zooming out.

- (void) scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale {
myScrollView.frame = CGRectMake(0, 0, 320, 420);
 //put the correct parameters for your scroll view width and height above
}

Answer 7

HI,

Ok, this solution is working for me. I have a subclass of UIScrollView with a reference to the UIImageView it is displaying. Whenever the UIScrollView zooms, the contentSize property is adjusted. It is in the setter that I scale the UIImageView appropriately and also adjust its center position.

-(void) setContentSize:(CGSize) size{
CGSize lSelfSize = self.frame.size;
CGPoint mid;
if(self.zoomScale >= self.minimumZoomScale){
    CGSize lImageSize = cachedImageView.initialSize;
    float newHeight = lImageSize.height * self.zoomScale;

    if (newHeight < lSelfSize.height ) {
        newHeight = lSelfSize.height;
    }
    size.height = newHeight;

    float newWidth = lImageSize.width * self.zoomScale;
    if (newWidth < lSelfSize.width ) {
        newWidth = lSelfSize.width;
    }
    size.width = newWidth;
    mid = CGPointMake(size.width/2, size.height/2);

}
else {
    mid = CGPointMake(lSelfSize.width/2, lSelfSize.height/2);
}

cachedImageView.center = mid;
[super  setContentSize:size];
[self printLocations];
NSLog(@"zoom %f setting size %f x %f",self.zoomScale,size.width,size.height);
}

Evertime I set the image on the UIScrollView I resize it. The UIScrollView in the scrollview is also a custom class I created.

-(void) resetSize{
    if (!scrollView){//scroll view is view containing imageview
        return;
    }

    CGSize lSize = scrollView.frame.size;

    CGSize lSelfSize = self.image.size; 
    float lWidth = lSize.width/lSelfSize.width;
    float lHeight = lSize.height/lSelfSize.height;

    // choose minimum scale so image width fits screen
    float factor  = (lWidth<lHeight)?lWidth:lHeight;

    initialSize.height = lSelfSize.height  * factor;
    initialSize.width = lSelfSize.width  * factor;

    [scrollView setContentSize:lSize];
    [scrollView setContentOffset:CGPointZero];
    scrollView.userInteractionEnabled = YES;
}

With these two methods I am able to have a view that behaves just like the photos app.

Answer 8

Okay, I've been fighting this for the past two days on and off and having finally come to a pretty reliable (so far...) solution I thought I should share it and save others some pain. :) If you do find a problem with this solution please shout!

I've basically gone through what everyone else has: searching StackOverflow, the Apple Developer Forums, looked at the code for three20, ScrollingMadness, ScrollTestSuite, etc. I've tried enlarging the UIImageView frame, playing with the UIScrollView's offset and/or insets from the ViewController, etc. but nothing worked great (as everyone else has found out too).

After sleeping on it, I tried a couple of alternative angles:

1. Subclassing the UIImageView so it alters it's own size dynamically - this didn't work well at all.
2. Subclassing the UIScrollView so it alters it's own contentOffset dynamically - this is the one that seems to be a winner for me.

With this subclassing UIScrollView method I'm overriding the contentOffset mutator so it isn't setting {0,0} when the image is scaled smaller than the viewport - instead it's setting the offset such that the image will be kept centred in the viewport. So far, it always seems to work. I've checked it with wide, tall, tiny & large images and doesn't have the "works but pinch at minimum zoom breaks it" issue.

I've uploaded an example project to github that uses this solution, you can find it here: http://github.com/nyoron/NYOBetterZoom

Answer 9

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 *)aScrollView {
    CGFloat offsetX = (scrollView.bounds.size.width > scrollView.contentSize.width)? 
                      (scrollView.bounds.size.width - scrollView.contentSize.width) * 0.5 : 0.0;
    CGFloat offsetY = (scrollView.bounds.size.height > scrollView.contentSize.height)? 
                      (scrollView.bounds.size.height - scrollView.contentSize.height) * 0.5 : 0.0;
    mySubView.center = CGPointMake(scrollView.contentSize.width * 0.5 + offsetX, 
                                   scrollView.contentSize.height * 0.5 + offsetY);
}

Answer 10

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 11

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 for the photoscoller.

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;
}