iOS_Responder chain 响应链

override func hitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
    print("hitTest: \(self)")
    /// 1. 判断当前view是否可响应
    guard isUserInteractionEnabled else {
        /// 不允许用户交互
        return nil
    }
    guard !isHidden else {
        /// 已隐藏
        return nil
    }
    guard alpha > 0.01 else {
        /// 不透明度小于等于 0.01
        return nil
    }
    
    /// 2. touch的坐标是否在view的frame内
    guard self.point(inside: point, with: event) else {
        return nil
    }

    /// 3. 倒序遍历子视图, 递归调用hitTest
    for subview in subviews.reversed() {
        let subPoint = self.convert(point, to: subview)
        /// 首个非空子视图, 即为 first responder
        if let fitView = subview.hitTest(subPoint, with: event) {
            return fitView
        }
    }
    /// 4. 遍历所有的子视图都没有响应 hit-testing, 则该view为 first responder
    return self
}