- (IBAction)pan:(UIPanGestureRecognizer *)pan{
    //獲取當(dāng)前手指所在的點(diǎn).
    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;
        //確定遮蓋的區(qū)域位置
        CGRect rect = CGRectMake(self.startP.x, self.startP.y, offsetX, offsetY);
        //每次移動(dòng)時(shí),調(diào)用遮蓋的Frame.
        self.coverView.frame = rect;
        
        
    }else if(pan.state == UIGestureRecognizerStateEnded){
        
        
        //1.開啟一個(gè)跟原始圖片相同大小的圖片上下文.
        UIGraphicsBeginImageContextWithOptions(self.imageV.bounds.size, NO, 0);
        //把self.cover.frame 設(shè)為裁剪區(qū)域
        UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.coverView.frame];
        [path addClip];
        //獲取當(dāng)前上下文.
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        //把UIImageView當(dāng)中的圖片給繪制上下文當(dāng)中.
        [self.imageV.layer renderInContext:ctx];
        //從上下文當(dāng)中生成一張新的圖片
        UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
        //關(guān)閉上下文.
        UIGraphicsEndImageContext();
        //給圖片重新賦值
        self.imageV.image = newImage;
        //移除遮蓋
        [self.coverView removeFromSuperview];
        
    }
    
    
}