iOS 利用mask layer 使view中扣掉一块露出下边的view

http://blog.csdn.net/Dwarven/article/details/42492199

#define SCREEN_WIDTH [[UIScreen mainScreen] bounds].size.width
#define SCREEN_HEIGHT [[UIScreen mainScreen] bounds].size.height

- (void)addMask{
    UIButton * _maskButton = [[UIButton alloc] init];
    [_maskButton setFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
    [_maskButton setBackgroundColor:[UIColor colorWithWhite:0 alpha:0.7]];
    [self.view addSubview:_maskButton];

    //create path
    UIBezierPath *path = [UIBezierPath bezierPathWithRect:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];

    // MARK: circlePath
    [path appendPath:[UIBezierPath bezierPathWithArcCenter:CGPointMake(SCREEN_WIDTH / 2, 200) radius:100 startAngle:0 endAngle:2*M_PI clockwise:NO]];

    // MARK: roundRectanglePath
    [path appendPath:[[UIBezierPath bezierPathWithRoundedRect:CGRectMake(20, 400, SCREEN_WIDTH - 22 * 20, 100) cornerRadius:15] bezierPathByReversingPath]];

    CAShapeLayer *shapeLayer = [CAShapeLayer layer];

    shapeLayer.path = path.CGPath;

    [_maskButton.layer setMask:shapeLayer];
}