How would it be possible to have an iOS app display the user's wallpaper? Apple uses this in places like the tab view in Safari and the keypad in Phone. Setting the transparency of UIWindow proved to no avail...
This is no longer available. Too bad, because it allowed a sweet native looking effect
there are a few things you need to do to show the user wallpaper
1 - set UIApplicationIsOpaque to false in the project plist
2 - In the app delegate load function
self.window.backgroundColor = [UIColor clearColor];
self.window.opaque = NO;
3 - set the UIViewController.view background color to [UIColor clearColor]
This works for us currently. If you try to do a screen capture or actually use the wallpaper it just returns solid black (so privacy concerns don't apply since you can't do anything with the wallpaper).
This functionality is not available to third-party developers due to privacy concerns.
If you could do this in a third-party app, you could capture the background image without asking the user.
来源：https://stackoverflow.com/questions/18924135/ios-7-display-user-wallpaper-as-uiwindow-background