Quartz2d--图片区域截屏

//懒加载遮盖,保存遮盖只创建一次.
-(UIView *)coverView{
    if (_coverView == nil) 
    {
        UIView *view = [[UIView alloc] init];
        view.backgroundColor = [UIColor blackColor];
        view.alpha = 0.7;
        [self.view addSubview:view];
        _coverView = view;
     }
    return _coverView;
}

- (IBAction)pan:(UIPanGestureRecognizer *)pan{
    //获取当前手指所在的点.
    CGPoint curP = [pan locationInView:self.view];

    if(pan.state == UIGestureRecognizerStateBegan){
        
        CGPoint startP = curP;
        self.startP = startP;
        
    }else if(pan.state == UIGestureRecognizerStateChanged){
        
        CGFloat offsetX = curP.x - self.startP.x;
        CGFloat offsetY = curP.y - self.startP.y;
        //确定遮盖的区域位置
        CGRect rect = CGRectMake(self.startP.x, self.startP.y, offsetX, offsetY);
        //每次移动时,调用遮盖的Frame.
        self.coverView.frame = rect;       
    }else if(pan.state == UIGestureRecognizerStateEnded){
        //1.开启一个跟原始图片相同大小的图片上下文.
        UIGraphicsBeginImageContextWithOptions(self.imageV.bounds.size, NO, 0);
        //把self.cover.frame 设为裁剪区域
        UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.coverView.frame];
        [path addClip];
        //获取当前上下文.
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        //把UIImageView当中的图片给绘制上下文当中.
        [self.imageV.layer renderInContext:ctx];
        //从上下文当中生成一张新的图片
        UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
        //关闭上下文.
        UIGraphicsEndImageContext();
        //给图片重新赋值
        self.imageV.image = newImage;
        //移除遮盖
        [self.coverView removeFromSuperview];
        
    }     
}