UIToolBar在UITabBar之上？

override func viewDidLoad() {
    super.viewDidLoad()

    let toolBar = UIToolbar()
    toolBar.frame = CGRect(x: 0, y: 0, width: view.frame.width, height: 44)
    toolBar.backgroundColor = .white

    let tabBar = self.tabBarController?.tabBar
    view.addSubview(toolBar)
    view.addSubview(tabBar!)

    // 设置UIToolBar约束
    toolBar.translatesAutoresizingMaskIntoConstraints = false
    let constraints = [
        toolBar.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        toolBar.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        toolBar.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor)
    ]
    NSLayoutConstraint.activate(constraints)

    // 设置UITabBar约束
    tabBar?.translatesAutoresizingMaskIntoConstraints = false
    let tabBarConstraints = [
        tabBar!.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        tabBar!.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        tabBar!.topAnchor.constraint(equalTo: toolBar.bottomAnchor),
        tabBar!.bottomAnchor.constraint(equalTo: view.bottomAnchor)
    ]
    NSLayoutConstraint.activate(tabBarConstraints)
}