override func viewDidLoad() {
  super.viewDidLoad()
  // 替换 tabbar
  let _tabBar = TabBar()
  // 监听按钮点击事件
  _tabBar.handleBtnReleaseClick(self, action: #selector(btnReleaseClick))
  setValue(_tabBar, forKey: "tabBar")
}

// MARK: 删除占位“按钮”，添加自定义按钮
override func layoutSubviews() {
  super.layoutSubviews()
  btnRelease.backgroundColor = UIColor.white
  // 过滤掉 _UIBarBackground 类型的对象，只保留 UITabBarButton 类型的对象
  let tabbarButtonList = iews.filter{NSStringFromClass(type(of: $0)) == abBarButton"}
  // 取出需要删除的“按钮”（经测试，知道中间那个加号按钮的下标是2）
  let tmpView = tabbarButtonList[safe: 2]
  // 把默认生成的从父容器中移除
  tmpView?.removeFromSuperview()
  let x = (InnerConst.ScreenW - btnRelease.w) * 0.5
  let y = InnerConst.ButtonOffsetY
  btnRelease.origin = CGPoint(x: x, y: y)
  addSubview(btnRelease)
}

// MARK: 处理默认情况下，按钮超出 TabBar 以外区域不能点击问题
override func hitTest(_ point: CGPoint, with event: UIEvent?) -View? {
  if isHidden == false {
    let newPoint = convert(point, to: btnRelease)
    if btnRelease.point(inside: newPoint, with: event) {
      return btnRelease
    }
    return super.hitTest(point, with: event)
  }
  return super.hitTest(point, with: event)
}

func handleBtnReleaseClick(_ target: AnyObject?, action: Selector) {
  btnRelease.addTarget(target, action: action, for: .touchUpInside)
}