iOS 快照功能

• (UIImage *)snapshot:(UIView *)view
  {
  UIGraphicsBeginImageContextWithOptions(view.bounds.size, YES, 0);
  [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:YES];
  UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();

  return image;
  }

还有UIView 的 [-snapshotViewAfterScreenUpdates:] and [-resizableSnapshotViewFromRect:afterScreenUpdates:withCapInsets:] 也能截图
UIScreen的[-snapshotViewAfterScreenUpdates:]方法