UIScrollView image/photo viewer with paging enable

Question

OK, I think it's time to make an official place on the internet for this problem: How to make a UIScrollView photoviewer with paging and zooming. Welcome my fellow UIScrollView hackers.

I have a UIScrollView with paging enabled, and I'm displaying UIImageViews like the built-in photos app. (Does this sound familiar yet?)

I found the following project on github:

http://wiki.github.com/andreyvit/ScrollingMadness

Which shows how to implement zooming in a scroll view while paging is enabled. If anyone else tries this out, I actually had to remove the UIScrollView subclass and use the native class otherwise it doesn't work. I think it's because of changes in the 3.0 SDK relating to how the scroll view intercepts touch events.

So the the idea is to remove all the other views when you start zooming, and move the current view to (0, 0) in the scrollview, updating the contentsize etc. Then when you zoom back to 1.0f it adds the other views back and puts things all back in order.

Anyway, that project works perfectly in the simulator, but on the device there is some nasty movement of the view you are resizing, which looks like it's caused by the fact we are changing the contentsize/offset etc. for the view being resized. You have to do this view moving otherwise you can pan left through the whitespace left by the other views.

I found one interesting note in the "Known Issues" of the 3.0 SDK release notes:

UIScrollView: After zooming, content inset is ignored and content is left in the wrong position.

This kind of sounds like what is happening here. After zooming in, the view will shift offscreen because you have changed the offset etc.

I've spent hours on this already and I'm slowing coming to the sad realization that this just isn't going to work.

Three20's photo viewer is out of the question: it's too heavy weight and there is too much unnecessary UI and other behaviour.

The built in Photo app seems to do some magic. If you zoom in on an image and pan to the far edges, the current photo moves independently of the photo next to it which isn't what you get when trying this with a standard UIScrollView.

I've seen discussion about nesting the UIScrollView's but I really don't want to go there.

Has anybody managed this with the standard UIScrollView (and works in the 2.2 and 3.0 SDK)? I don't fancy rolling my own zoom + bounce + pan + paging code.

Answer 1

i write a code for that , and can be as reference

1. load current view scrollview and imageview .. and for the screen next to the current view , only imageview

2. remove all view when current page load to save memory , so good for many photo project

3. use tag to differentiate different scrollview

_xxxxxxx

the download link click here

Answer 2

I did some playing around with the native Photos app, and I think I can say with confidence they are using a single UIScrollView. The giveaway is this: zoom in on an image, and pull to the left or right. You will see the next or previous photo. If you tug hard enough, it will even page to the next photo at 1.0f zoom. Flip back and the previously zoomed photo will be back to 1.0f zoom as well.

Obivously I didn't write Photos.app, but I'll take a wild guess at how they did it:

• A single UIScrollView and a single UIScrollViewDelegate
• Populate the UIScrollView with UIImageView children
• Listen for scrollViewDidScroll:
• Do some math and figure out what page you are currently on
• Listen for viewForZoomingInScrollView:
• Return a different view depending on the page index
• Listen for scrollViewDidEndZooming:withView:atScale: and optionally do some anti-aliasing, etc based on the content

If you decide to try that out, let me know how it works out for you. I'd love to know how you finally end up getting this to work. Even better, post it to github.