[iOS]对UIView选择性倒角

- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"倒角";
    /*
    UIView部分倒圆角
    typedef NS_OPTIONS(NSUInteger, UIRectCorner) {
        UIRectCornerTopLeft     = 1 << 0,
        UIRectCornerTopRight    = 1 << 1,
        UIRectCornerBottomLeft  = 1 << 2,
        UIRectCornerBottomRight = 1 << 3,
        UIRectCornerAllCorners  = ~0UL
    };*/
    CGRect oldRect = _downView.bounds;
    oldRect.size.width = [UIScreen mainScreen].bounds.size.width;
    
    UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:oldRect byRoundingCorners:UIRectCornerTopLeft | UIRectCornerTopRight cornerRadii:CGSizeMake(20, 20)];
    
    CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
    maskLayer.path = maskPath.CGPath;
    maskLayer.frame = oldRect;
    _downView.layer.mask = maskLayer;
}