iOS7截取当前显示的视图
UIView的Category UISnapshotting
@interface UIView (UISnapshotting)
- (UIView *)snapshotViewAfterScreenUpdates:(BOOL)afterUpdates;
- (UIView *)resizableSnapshotViewFromRect:(CGRect)rect afterScreenUpdates:(BOOL)afterUpdates withCapInsets:(UIEdgeInsets)capInsets;
- (BOOL)drawViewHierarchyInRect:(CGRect)rect afterScreenUpdates:(BOOL)afterUpdates;
@end