iOS: UIScrollView zooming programmatically not working

Question

I have a pageable UIScrollView which contains different kind of informations like UITables but also zoomable images. Therefore I set up a pageable main-ScrollView and as subviews I added zoomable image-ScrollViews with the images as content. Works everything fine, just I fail to set the smaller current zoom scale of the imageScrollViews.

UIImageView *imageView = [[UIImageView alloc] initWithImage:Image];

//storing a link to the imageView
[imagelinkArray addObject:imageView];

CGRect ScrollViewImageRect;
ScrollViewImageRect = CGRectMake((self.scrollView.frame.size.width) * i, 0, 320, self.scrollView.frame.size.height);

float widthfactor = ScrollViewImageRect.size.width / imageView.frame.size.width;
float heightfactor = ScrollViewImageRect.size.height / imageView.frame.size.height;
float zoomscale = MIN(widthfactor, heightfactor);

UIScrollView *imageScrollView = [[UIScrollView alloc] initWithFrame:ScrollViewImageRect];
imageScrollView.contentSize = CGSizeMake(imageView.frame.size.width, imageView.frame.size.height);

imageScrollView.delegate = self;

[imageScrollView setMinimumZoomScale:zoomscale];
[imageScrollView setMaximumZoomScale:1.5];

[imageScrollView addSubview:imageView];

//doesn't work:
[imageScrollView setZoomScale:0.5 animated:YES];

[self.scrollView addSubview:imageScrollView];

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return [imagelinkArray objectAtIndex:page];
}

The main-ScrollView and the image-ScrollViews are drawn perfectly and it's possible to zoom and page properly. The minimum zoom factor is calculated also correct. When I zoom out I can zoom until the image limits are reached. However the first time I page to the image-ScrollView it's current zoom scale is always 1.0 while it should be the minimum scale.

Wherever I set the zoom scale in the code above it doesn't work:

[changeScrollView setZoomScale:changeScrollView.minimumZoomScale animated:YES];

If I log the current zoom scale i always get 1.0;

The only thing which works is changing the zoom scale in the - (void)scrollViewDidScroll: method, which of course doesn't help a lot since zooming also calls it which resets the zoom immediately. But at least I could figure out, that the code somehow works. I have the feeling a UIScrollView doesn't zoom when it's not visible on the screen right now. How can I fix this?

Update: Okay. In the meantime I figured out that the problem most likely comes from my base layout of "sub-viewing" ScrollViews into another ScrollView. When I zoom one of the images and log the current zoom factor of the ScrollViews they are all the same (main ScrollView as well as ALL sub-ScrollViews). What could be the reason for it or how could I solve it with a different layout?

Answer 1

In the meantime I figured out that the code below can under certain circumstances make problems. Sometimes when I zoom in and out from an image the main ScrollView can't be scrolled anymore, so there is no way to get to the next image, except of triggering the pageControl or except of zooming into the image and then scrolling to its borders so the main ScrollView jumps to the next page.

Means the problem only occurs when the image is fully zoomed out (.scale = .minimumscale). I don't understand exactly when and why it happens. Logging viewDidScroll returns nothing in this situations.

Anybody experienced similar problems?

Answer 2

As user Rivera pointed out the return value of viewForZoomingInScrollView: was off course not right. I always returned the current visible page shown in the scrollView. Which means setting the zoom in viewDidAppear doesn't change anything.

Here is the correct code: I made a function which will always resets both images left and right of the current visible scroll page:

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self updateImageSize];
}


- (void)updateImageSize
{
    updatePage = updatePage + 1;

    if (updatePage < imagescrolllinkArray.count) {
        UIScrollView *changeScrollView = (UIScrollView*)[imagescrolllinkArray objectAtIndex:updatePage];
        [changeScrollView setZoomScale:changeScrollView.minimumZoomScale];
    }

    updatePage = updatePage - 2;

    if (updatePage > -1) {
        UIScrollView *changeScrollView = (UIScrollView*)[imagescrolllinkArray objectAtIndex:updatePage];
        [changeScrollView setZoomScale:changeScrollView.minimumZoomScale];
    }

    //reset to current view
    updatePage = updatePage + 1;
}


- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return [imagelinkArray objectAtIndex:updatePage];
}

and in viewDidAppear I added the following to update the first image before any dragging.

if (i == 1) {
    updatePage = 0;
    [imageScrollView setZoomScale:imageScrollView.minimumZoomScale];
}

Answer 3

I think viewForZoomingInScrollView: may not be getting called by the scrollview. Try to add a breakpoint and check what you're returning there.

Answer 4

Your code doesn't work because you havent set minimumZoomScale ,which has default value of 1.0 . Since you are trying to set 0.5 which is below the default value, it wouldn't work.

Add following line just after the line which sets maximumZoomScale.

[imageScrollView setMinimumZoomScale:0.25];