- (nullable UIView *)hitTest:(CGPoint)point withEvent:(nullable UIEvent *)event;  // recursively calls -pointInside:withEvent:. point is in the receiver's coordinate system
- (BOOL)pointInside:(CGPoint)point withEvent:(nullable UIEvent *)event;  // default returns YES if point is in bounds

1. alpha <0.01
2. userInteractionEnabled = NO
3. hidden ＝ YES

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {
  // 如果控件不允許與用用戶交互,那么返回nil
  if (!self.userInteractionEnabled || [self isHidden] || self.alpha <= 0.01) {
    return nil;
  }

  //判斷當前視圖是否在點擊范圍內(nèi)
  if ([self pointInside:point withEvent:event]) {
    //遍歷當前對象的子視圖(倒序)
    __block UIView *hit = nil;
    [self.subviews enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
      //坐標轉(zhuǎn)換，把當前坐標系上的點轉(zhuǎn)換成子控件坐標系上的點
      CGPoint convertPoint = [self convertPoint:point toView:obj];
      //調(diào)用子視圖的hitTest方法，判斷自己的子控件是不是最適合的View
      hit = [obj hitTest:convertPoint withEvent:event];
      //如果找到了就停止遍歷
      if (hit) *stop = YES;
    }];

    //返回當前的視圖對象
    return hit?hit:self;
  }else {
    return nil;
  }
}

// 該方法判斷觸摸點是否在控件身上，是則返回YES，否則返回NO，point參數(shù)必須是方法調(diào)用者的坐標系
- (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {  
  CGFloat x1 = point.x;
  CGFloat y1 = point.y;
  
  CGFloat x2 = self.frame.size.width / 2;
  CGFloat y2 = self.frame.size.height / 2;
  
  //判斷是否在圓形區(qū)域內(nèi)
  double dis = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
  if (dis <= self.frame.size.width / 2) {
    return YES;
  }
  else{
    return NO;
  }
}