@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@property (nonatomic, weak) UIView *clipView;

- (UIView *)clipView
{
  //如果clipView为被创建，就创建
  if (_clipView == nil)
  {
    UIView *view = [[UIView alloc] init];
    _clipView = view;
    //设置clipView的背景色和透明度
    view.backgroundColor = [UIColor blackColor];
    view.alpha = 0.5;
    //将clipView添加到控制器的view上，此时的clipView不会显示（未设置其frame）
    [self.view addSubview:_clipView];
  }
  return _clipView;
}

/**创建手势**/
UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
  /**
  *每当pan手势的位置发生变化，就会调用pan:方法，并将手势作为参数传递
  */
/**添加手势**/
[self.view addGestureRecognizer:pan];

@property (nonatomic, assign) CGPoint startPoint;

- (void)pan:(UIPanGestureRecognizer *)pan
{
  CGPoint endPoint = CGPointZero;

  if (pan.state == UIGestureRecognizerStateBegan)
  {
    /**开始点击时，记录手势的起点**/
    self.startPoint = [pan locationInView:self.view];
  }
  else if(pan.state == UIGestureRecognizerStateChanged)
  {
    /**当手势移动时，动态改变终点的值，并计算起点与终点之间的矩形区域**/
    endPoint = [pan locationInView:self.view];
    //计算矩形区域的宽高
    CGFloat w = endPoint.x - self.startPoint.x;
    CGFloat h = endPoint.y - self.startPoint.y;
    //计算矩形区域的frame
    CGRect clipRect = CGRectMake(self.startPoint.x, self.startPoint.y, w, h);
    //设置剪切区域的frame
    self.clipView.frame = clipRect;
  }
  else if(pan.state == UIGestureRecognizerStateEnded)
  {
    /**若手势停止，将剪切区域的图片内容绘制到图形上下文中**/
    //开启位图上下文
    UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, 0);
    //创建大小等于剪切区域大小的封闭路径
    UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.clipView.frame];
    //设置超出的内容不显示，
    [path addClip];
    //获取绘图上下文
    CGContextRef context = UIGraphicsGetCurrentContext();
    //将图片渲染的上下文中
    [self.imageView.layer renderInContext:context];
    //获取上下文中的图片
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    //关闭位图上下文
    UIGraphicsEndImageContext();
    //移除剪切区域视图控件，并清空
    [self.clipView removeFromSuperview];
    self.clipView = nil;
    //将图片显示到imageView上
    self.imageView.image = image;
    //通过alertView提示用户，是否将图片保存至相册
    UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"保存图片" message:@"是否将图片保存至相册？" delegate:self cancelButtonTitle:@"否" otherButtonTitles:@"是", nil];
    [alertView show];
}
}

- (void)alertView:(nonnull UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
  //若点击了“是”，则保存图片
  if (buttonIndex == 1)
  {
    UIImageWriteToSavedPhotosAlbum(self.imageView.image, nil, nil, nil);
    /**
    * 该方法可以设置保存完毕调用的方法，此处未进行设置
    */
  }
}