maskView 配合 CAGradientLayer 的使用

   UIImageView *imG = [[UIImageView alloc]initWithFrame:CGRectMake(0, 64, 200, 300)];

    imG.image = [UIImage imageNamed:@"1.jpg"];

    [self.view addSubview:imG];

    

    /* - - - - - - CAGradientLayer - - - - - - */

    CAGradientLayer *layer = [[CAGradientLayer alloc] init];

    layer.frame = imG.bounds;

    layer.colors = @[(__bridge id)[UIColor clearColor].CGColor,

                     (__bridge id)[UIColor redColor].CGColor,

                     (__bridge id)[UIColor clearColor].CGColor

                     ];

    layer.locations = @[@0.25,@0.5,@0.75];

    layer.startPoint = CGPointMake(0, 0);

    layer.endPoint = CGPointMake(1, 1);

    

    /* - - - - - - 添加CAGradientLayer - - - - - - */

    UIView *containerView = [[UIView alloc] initWithFrame:imG.bounds];

    [containerView.layer addSublayer:layer];

    

    imG.maskView = containerView;