iOS5: UIScrollView dispaying and scrolling differently from iOS4

问题

This is a curious one.

I have an IBOutlet UIScrollView playScrollView whose height is exactly 1/3 of it's contentSize's height. The app is in landscape. I call this code...

[playScrollView scrollRectToVisible:CGRectMake(0.0f, page * PLAY_VIEW_PAGE_HEIGHT,
                                              480.0, PLAY_VIEW_PAGE_HEIGHT)
                           animated:animated];

... (the int page ranges from 0 to 2) to start on page 1 (displaying the middle third) then go up or down as needed when the user presses buttons.

This works fine for iOS4 both device and simulator, and has been live on the app store for months with no problems. Even iOS5 devices are fine with existing builds, it was only when the app was recompiled for iOS5 that it stopped working correctly on iOS5 devices.

Since updating to XCode 4.2, This doesn't work for iOS5. It goes one page too low, showing the bottom page when it should show the middle. I can get the code to work for iOS5 (device and simulator) by changing page to (page-1)...

[playScrollView scrollRectToVisible:CGRectMake(0.0f, (page-1) * PLAY_VIEW_PAGE_HEIGHT,
                                              480.0, PLAY_VIEW_PAGE_HEIGHT)
                           animated:animated];

...but of course this breaks iOS4, which works fine with the old code, but gets stuck one page too high with this new code. iOS4 and iOS5 are exactly PLAY_VIEW_PAGE_HEIGHT out-of-step (288 pixels, a third of the height of playScrollView). The same thing happens if I use setContentOffset: instead.

One other curious thing, probably the key to this. If I don't do the scrollRectToVisible at all, then iOS4 sits at the top of playScrollView, wheras iOS5 shows the middle third, (ie PLAY_VIEW_PAGE_HEIGHT pixels down).

I could detect the iOS and use different code for each, but that's a horrible kludge. If it's an iOS5 bug and they fix it in a future release, that would break the live app.

Has anyone any ideas, or noticed anything similar? Thanks.

来源：https://stackoverflow.com/questions/8145404/ios5-uiscrollview-dispaying-and-scrolling-differently-from-ios4