问题
I know it's not recommended to change any view appearance when the application is moved to background state, but for my specific task I cannot postpone this activity and I really like to complete it in a couple of seconds after the user hits the home button.
That's what I do: I load a webpage in a UIWebView and after loading completion I change the contentOffset and zoomScale in order to make a specific screenshot of that webpage. When the app is in active application state everything works fine.
Now I wanted to add some more multitasking capabilities. I encapsulated the above described webpage loading and rendering activity in a protected background task (iOS Task Completion), in order to complete it even when the user hits the home button.
I have noticed that the page loading and the rendering of the webpage screenshot (with the renderInContext: method) works just fine in background, BUT it seems that the webview (i.e. the encapsulated UIScrollView instance) does not react on setContentOffset: and setZoomScale: messages as soon as the app is in background..
So I'm asking you if you know any alternatives to change the content offset / zoomscale? (modifications directly on CALayer?) Or any method to "force" the scrollview to perform the content changes even in background.
PS: On the iOS simulator everything works fine even in application background state, but on real devices it does not.
EDIT: I have created a simple demo project that demonstrates the issue. You can download it here (please read the instructions in the zip file): http://dl.dropbox.com/u/3556271/BackgroundBugDemo.zip
I appreciate every hint or tip. Thanks!
来源：https://stackoverflow.com/questions/15604276/change-zoomscale-and-contentoffset-of-uiscrollview-in-uiapplicationstatebackgrou