双击放大是 iPhone 的一个基本操作，第三方程序里引入这一功能的话，主要是在 scrollview 呈现一张图片或者 PDF 页面时，双击可以放大，主要代码如下

- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale

{     NSLog(@"%s", _cmd);          CGFloat zs = scrollView.zoomScale;     zs = MAX(zs, 0.1);     zs = MIN(zs, 5.0);              [UIView beginAnimations:nil context:NULL];     [UIView setAnimationDuration:0.3];             scrollView.zoomScale = zs;         [UIView commitAnimations]; } #pragma mark - #pragma mark === UITouch Delegate === #pragma mark - - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {     //NSLog(@"%s", _cmd);          UITouch *touch = [touches anyObject];          if ([touch tapCount] == 2)     {         //NSLog(@"double click");                  CGFloat zs = self.zoomScale;         zs = (zs == 1.0) ? 2.0 : 1.0;                  [UIView beginAnimations:nil context:NULL];         [UIView setAnimationDuration:0.3];                     self.zoomScale = zs;             [UIView commitAnimations];     } }