CALayer setMask建立遮罩

//會讓Mask圖的黑色部份被顯示，白色部份則會透明
        //取得透明度遮罩的影像與大小
        UIImage *maskImage = [UIImage imageNamed:@"alphaMask.png"];
        CGRect rect = CGRectMake(0.0, 0.0, maskImage.size.width, maskImage.size.height);
       
        //製作一個透明遮罩的Layer
        CALayer *maskLayer = [[CALayer layer]retain];
        maskLayer.bounds = rect;
        maskLayer.contents = (id)maskImage.CGImage;
       
        //將imageView中的影像做透明度的遮罩
        [[middleViewController.view layer] setMask:maskLayer];
       
        [maskLayer release];