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    self.backgroundPanel.layer.maskedCorners =  kCALayerMinXMinYCorner | kCALayerMaxXMinYCorner;
    self.backgroundPanel.layer.cornerRadius = 20;


方式一：通过 layer 设置圆角
 // 创建图片框
    UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    // 添加图片
    imageView.image = [UIImage imageNamed:@"104.jpg"];
    // 使用 layer 设置圆角
    imageView.layer.cornerRadius = 50;
    imageView.layer.masksToBounds = YES;
    [self.view addSubview:imageView];

方式二：使用贝塞尔曲线UIBezierPath和Core Graphics框架画出一个圆角

// 创建图片框
    UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    // 添加图片
    imageView.image = [UIImage imageNamed:@"104.jpg"];
    //开始对imageView进行画图
    UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, NO, 1.0);
    //使用贝塞尔曲线画出一个圆形图
    [[UIBezierPath bezierPathWithRoundedRect:imageView.bounds cornerRadius:imageView.frame.size.width] addClip];
    [imageView drawRect:imageView.bounds];

    imageView.image = UIGraphicsGetImageFromCurrentImageContext();
    //结束画图
    UIGraphicsEndImageContext();
    [self.view addSubview:imageView];

方式三：使用CAShapeLayer和UIBezierPath设置圆角 : -- 这种方式最好，内存的消耗最少，而且渲染快速

// 创建图片框
    UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    // 添加图片
    imageView.image = [UIImage imageNamed:@"104.jpg"];
    UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:imageView.bounds byRoundingCorners:UIRectCornerAllCorners cornerRadii:imageView.bounds.size];
    CAShapeLayer *maskLayer = [[CAShapeLayer alloc]init];
    //设置大小
    maskLayer.frame = imageView.bounds;
    //设置图形样子
    maskLayer.path = maskPath.CGPath;
    imageView.layer.mask = maskLayer;
    [self.view addSubview:imageView];