mask layer的遮罩层

 //渐变层
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.frame = CGRectMake(0, 100, kWidth, kWidth);
    gradientLayer.colors = @[(__bridge id)[[UIColor redColor]colorWithAlphaComponent:0.4] .CGColor,
                             (__bridge id)[UIColor clearColor].CGColor];
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint  = CGPointMake(1, 0);
    
    [self.view.layer addSublayer:gradientLayer];
    
    
    //
    UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, 200, 200)];
    CAShapeLayer *layer = [CAShapeLayer layer];
    layer.frame = self.view.bounds;
    layer.lineWidth = 5;
    layer.strokeColor = [UIColor redColor].CGColor;
    layer.fillColor = [UIColor redColor].CGColor;
    layer.path = path.CGPath;
    layer.lineCap = @"round";
//    [gradientLayer addSublayer:layer];
    
    gradientLayer.mask = layer;