以Button為例
建立Button就不寫了

設定圓角通常用layer.cornerRadius，也就是像下面這樣
button.layer.masksToBounds = YES;
button.layer.cornerRadius = 4.;

但是UI也經常變化的，例如要設定成某個角為圓角，layer.cornerRadius就摸瞎了，畢竟這個屬性是通用全形的。
這裡就需要使用UIBezierPath去設定，具體如下：

typedef NS_OPTIONS(NSUInteger, UIRectCorner) {
    UIRectCornerTopLeft     = 1
 
 << 0,
    UIRectCornerTopRight    = 1 << 1,
    UIRectCornerBottomLeft  = 1 << 2,
    UIRectCornerBottomRight = 1 << 3,
    UIRectCornerAllCorners  = ~0UL
};

UIBezierPath *bezierPath = [UIBezierPath bezierPathWithRoundedRect:button.bounds byRoundingCorners:(UIRectCornerTopLeft | UIRectCornerTopRight) cornerRadii:CGSizeMake(4
 
, 4)];
            CAShapeLayer *shapeLayer = [[CAShapeLayer alloc] init];
            shapeLayer.frame = button.bounds;
            shapeLayer.path = bezierPath.CGPath;
            button.layer.mask = shapeLayer;

引數：UIRectCorner：意思就是表面的意思
typedef NS_OPTIONS(NSUInteger, UIRectCorner) {
UIRectCornerTopLeft = 1 << 0,
UIRectCornerTopRight = 1 << 1,
UIRectCornerBottomLeft = 1 << 2,
UIRectCornerBottomRight = 1 << 3,
UIRectCornerAllCorners = ~0UL
};