- (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 {
  // 如果控件不允许与用用户交互,那么返回nil
  if (!self.userInteractionEnabled || [self isHidden] || self.alpha <= 0.01) {
    return nil;
  }

  //判断当前视图是否在点击范围内
  if ([self pointInside:point withEvent:event]) {
    //遍历当前对象的子视图(倒序)
    __block UIView *hit = nil;
    [self.subviews enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
      //坐标转换，把当前坐标系上的点转换成子控件坐标系上的点
      CGPoint convertPoint = [self convertPoint:point toView:obj];
      //调用子视图的hitTest方法，判断自己的子控件是不是最适合的View
      hit = [obj hitTest:convertPoint withEvent:event];
      //如果找到了就停止遍历
      if (hit) *stop = YES;
    }];

    //返回当前的视图对象
    return hit?hit:self;
  }else {
    return nil;
  }
}

// 该方法判断触摸点是否在控件身上，是则返回YES，否则返回NO，point参数必须是方法调用者的坐标系
- (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;
  
  //判断是否在圆形区域内
  double dis = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
  if (dis <= self.frame.size.width / 2) {
    return YES;
  }
  else{
    return NO;
  }
}