UIScrollView not functioning correctly after Modal view controller displayed in iOS 5.1

Question

I have a strange problem when using a UIScrollView controller combined with iPhone 4 and iOS 5.1.

I have a UIScrollView which has a content size of 640x480 (double screen effectively) and in addition to the swipe to switch between the two "screens" I also permit the user to tap the screen in response to which I call something like...

[scrollView scrollRectToVisible:(CGRectMake 320,0,320,480) animated:YES];

the first 320 would be 0 if the tap occurred whilst the right hand side of the scroll view was displayed (note the scroll view has paging enabled so it can only come to rest either fully left or fully right).

I also have a situation where I sometimes display an additional view controller modally using presentModalViewController over this view controller containing the scroll view.

Everything works perfectly until the modal view controller is presented and subsequently dismissed after which the scrollRectToVisible method will no longer work if animated is set to YES (if I change animated to NO then it works as expected). Note, the tap is still being registered and the scrollRectToVisible being called, it just doesn't do anything when animated is set to YES).

Here's the kicker, this bug only occurs on an iPhone 4 runnings iOS 5.x.
It works perfectly (even after the modal view controller has been displayed) on my: iPhone 3G running 4.x, iPhone 3GS running 3.x, iPod touch (2nd Gen) running 4.x and most surprisingly the simulator running 5.x.

I wondered if this was a bug in the animation system so disabled the animation on the modal view controller presentation and dismiss and this had no effect, problem still occurred on the iPhone 4 with iOS 5.1.

Anyone got any ideas as to what might be causing this and how I might work around it?

Answer 1

I had the same problem. I realized that after modalViewController is dismissed my UIScrollerView shifts downs by 20px, which is the same height as status bar. So, it means when my UIViewController is loaded and UIScrollView is created, UIScrollView thinks there is no status bar, when actually it is there.
So I tried to put in viewDidLoad:

[[UIApplication sharedApplication] setStatusBarHidden:NO];

Now my UIScrollView always stays under status bar, with Y position 20px. It never shifts down.

Answer 2

Finally tracked this down. what a pig...

I'm embedding a view from a view controller as a subview of another view controllers view. So my scroll view contains a view which also has an associated view controller.

Prior to iOS 5.x the methods viewWillAppear, viewWillDisappear, viewDidAppear and viewWillDisappear are never called on the sub views view controllers, only the main view controller. Already knowing this I set up my main view controller to manually call the sub views view controllers methods when these events happen.

However it appears that in iOS 5.x this issue has been "fixed" so where I was manually passing the call to viewWillAppear to my sub view controller I no longer need do this under 5.x as the method automatically gets called under 5.x - as a result it's now being called twice under 5.x but still only once when running on a 4.x or earlier device.

As a result, under 5.x my NSTimer used to call my updateUI method is being created twice, but because in viewDidDisappear I only destroy the timer if it is non nil it only gets destroyed once - therefore I'm leaking NSTimers under 5.x through double allocation where I'm not under 4.x.

As a result of multiple NSTimers hanging around all repeatedly calling my updateUI method is that the constant updating of the UI is killing the animation system and so the animation for the scrollView fails when running on an actual device. I guess it continued working OK on the simulator running 5.x as the CPU in the Mac is more than capable of handling the extra workload and still performing the animations correctly.

A simple check in my viewWillAppear method to ensure the NSTimer hasn't already been created has fixed the problem and has kept compatibility with 4.x and earlier.

I frustratingly run up against these kinds of issues every time Apple update their iOS by major versions... The morale of this story is don't assume that long standing classes still exhibit the same behaviour under different revisions of the OS.

Answer 3

I finally managed to get this working on an iPhone4 running 5.1. Ensuring the bounces horizontally property for the scroll view is set fixed the problem, though why having this unchecked should cause the problem in the first place is beyond me - I'm pretty certain this is a bug in iOS.