copy修饰的时候set方法是第一行代码，伪代码就是需要做上面那个判断，如果是strong修饰，直接就是最后面代码直接赋值

截屏代码，1.0代表图片最高质量，0是最低质量

png没有压缩质量参数，这个方法默认就是不压缩了，就是最高质量

在c语言当中是不会转换的

第一个参数size乘以最后面的参数，是生成图片的大小

- (IBAction)pan:(UIPanGestureRecognizer *)pan {
     CGPoint curP = [pan locationInView:self.imageView];
    //获取起始点
    if (pan.state == UIGestureRecognizerStateBegan) {
        //获取当前点
        self.startP = curP;
        
    } else if (pan.state == UIGestureRecognizerStateChanged) {
        //获取当前点
        CGFloat X = self.startP.x;
        CGFloat Y = self.startP.y;
        CGFloat W = curP.x - self.startP.x;
        CGFloat H = curP.y - self.startP.y;
   
        CGRect rect =  CGRectMake(X, Y, W, H);
        self.coverVeiw.frame = rect;
    } else if(pan.state == UIGestureRecognizerStateEnded){
        //开启一个位图上下文
        UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, 0.0);

        //UIRectClip(self.coverVeiw.frame);

        UIBezierPath  *path = [UIBezierPath bezierPathWithRect:self.coverVeiw.frame];
        [path addClip];

        //把ImageView的内容渲染上下文当中.
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        [self.imageView.layer renderInContext:ctx];

        UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
        self.imageView.image = newImage;

        UIGraphicsEndImageContext();

        [self.coverVeiw removeFromSuperview];
    }
}

// 图片擦除
- (IBAction)pan:(UIPanGestureRecognizer *)pan {
    //获取当前手指的点
    UIImageView *imageV = (UIImageView *)pan.view;
    
    CGPoint curP = [pan locationInView:imageV];
    CGRect rect = CGRectMake(curP.x - 15, curP.y - 15, 30, 30);
    
    //开启一个位图上下文
    UIGraphicsBeginImageContextWithOptions(imageV.bounds.size, NO, 0.0);
    
    //把ImageView内容渲染到上下文当中
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    [imageV.layer renderInContext:ctx];
    
    //擦除上下文当中某一块区域
    CGContextClearRect(ctx, rect);
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    imageV.image = newImage;
    
    UIGraphicsEndImageContext();
}