iOS UIView 选择性倒角

有些APP中会有卡券，卡券做成了选择性倒角，例如左上，右上倒角。非常美观。看一下iOS的实现：

#import "Masonry.h"

@interface WJWDaojiaoViewController ()
@property (nonatomic, strong) UIView *myView;
@end

@implementation WJWDaojiaoViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"视图倒角";
    [self configUI];
}

- (void)configUI {
    [self.view addSubview:self.myView];
    self.view.backgroundColor = [UIColor redColor];
//    self.myView.backgroundColor = [UIColor redColor];

    [self.myView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(self.view).offset(84 + 10);
        make.left.equalTo(self.view);
        make.right.equalTo(self.view);
        make.height.mas_equalTo(100);
    }];

}

- (void)viewDidLayoutSubviews {
    CGRect oldRect = _myView.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;
    _myView.layer.mask = maskLayer;

}

- (UIView *)myView {
    if (!_myView) {
        _myView = [[UIView alloc] init];
        _myView.backgroundColor = [UIColor blueColor];
    }
    return _myView;
}

效果图：

 需要注意的是，myView用masonry在configUI中布局，直接在布局后设置倒角是不行的。因为布局结束系统并不会立即更新控件的frame,frame是在-viewDidLayoutSubviews()方法执行后确定的，所以设置倒角在这里。
而且把布局方法放在 -viewDidLayoutSubviews() 中也是不行的。想想为什么。