copy修飾的時候set方法是第一行代碼，偽代碼就是需要做上面那個判斷，如果是strong修飾，直接就是最后面代碼直接賦值

截屏代碼，1.0代表圖片最高質(zhì)量，0是最低質(zhì)量

png沒有壓縮質(zhì)量參數(shù)，這個方法默認就是不壓縮了，就是最高質(zhì)量

在c語言當中是不會轉(zhuǎn)換的

第一個參數(shù)size乘以最后面的參數(shù)，是生成圖片的大小

- (IBAction)pan:(UIPanGestureRecognizer *)pan {
     CGPoint curP = [pan locationInView:self.imageView];
    //獲取起始點
    if (pan.state == UIGestureRecognizerStateBegan) {
        //獲取當前點
        self.startP = curP;
        
    } else if (pan.state == UIGestureRecognizerStateChanged) {
        //獲取當前點
        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){
        //開啟一個位圖上下文
        UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, 0.0);

        //UIRectClip(self.coverVeiw.frame);

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

        //把ImageView的內(nèi)容渲染上下文當中.
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        [self.imageView.layer renderInContext:ctx];

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

        UIGraphicsEndImageContext();

        [self.coverVeiw removeFromSuperview];
    }
}

// 圖片擦除
- (IBAction)pan:(UIPanGestureRecognizer *)pan {
    //獲取當前手指的點
    UIImageView *imageV = (UIImageView *)pan.view;
    
    CGPoint curP = [pan locationInView:imageV];
    CGRect rect = CGRectMake(curP.x - 15, curP.y - 15, 30, 30);
    
    //開啟一個位圖上下文
    UIGraphicsBeginImageContextWithOptions(imageV.bounds.size, NO, 0.0);
    
    //把ImageView內(nèi)容渲染到上下文當中
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    [imageV.layer renderInContext:ctx];
    
    //擦除上下文當中某一塊區(qū)域
    CGContextClearRect(ctx, rect);
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    imageV.image = newImage;
    
    UIGraphicsEndImageContext();
}