Swift 4 UINavigationController导航栏添加按钮UIBarButtonItem无效-蒲公英云

Swift 4 UINavigationController导航栏添加按钮UIBarButtonItem无效

设置UINavigationController的rootViewController属性后，root页面的上方会自动补充一个导航栏（实际上是Navigation Item），如下：
在这里插入图片描述
如果是使用StoryBoard，我们可以直接拖拽UIBarButtonItem加入按钮。

如果我们使用的是代码控制页面跳转，怎么在跳转后的页面上添加UIBarButtonItem呢？可以如下面这样写：

/// MyViewController.swift
override func viewDidLoad() { 
    let backBtn = UIBarButtonItem(image: UIImage(named: "back"), 
                                style: .plain, 
                                target: self, 
                                action: #selector(back))
    // 注意不要多此一举写成，self.navigationController?.navigationItem.leftBarButtonItem = btn1
    self.navigationItem.leftBarButtonItem = btn1
}

如果有多个UIBarButtonItem：

self.navigationItem.leftBarButtonItems = [btn1, btn2, btn3]

实际上，swift提供了8种设置item的方式：

@available(iOS 5.0, *)
    open var leftBarButtonItems: [UIBarButtonItem]?
    @available(iOS 5.0, *)
    open var rightBarButtonItems: [UIBarButtonItem]?
    @available(iOS 5.0, *)
    open func setLeftBarButtonItems(_ items: [UIBarButtonItem]?, animated: Bool)
    @available(iOS 5.0, *)
    open func setRightBarButtonItems(_ items: [UIBarButtonItem]?, animated: Bool)
    open var leftBarButtonItem: UIBarButtonItem?
    open var rightBarButtonItem: UIBarButtonItem?
    open func setLeftBarButton(_ item: UIBarButtonItem?, animated: Bool)
    open func setRightBarButton(_ item: UIBarButtonItem?, animated: Bool)

踩坑

直接设置UINavigationController的navigationItem是无效的，比如下面这样写是不行的：

// 自定义的root页面控制器
let root = MyUIViewController()
// 导航控制器
let navController = UINavigationController(rootViewController: root)
// 设置无效，必须在root控制器里设置
navController.navigationItem.leftBarButtonItem = backBtn
parentViewController?.present(navController, 
                                animated: true, 
                                completion: nil)

为什么可以直接在root指向的Controller内部设置navigationItem的按钮呢？

实际上我们在root看到的navigationItem是所有UIViewController都具有的属性，是Swift为了支持导航栏而增加的属性，我们跳到self.navigationItem的源文件看一下就了解了：

extension UIViewController { 
    // Created on-demand so that a view controller may customize its navigation appearance.
    open var navigationItem: UINavigationItem {  get } 
    // If YES, then when this view controller is pushed into a controller hierarchy with 
    // a bottom bar (like a tab bar), the bottom bar will slide out. Default is NO.
    open var hidesBottomBarWhenPushed: Bool 
    // If this view controller has been pushed onto a navigation controller, return it.
    open var navigationController: UINavigationController? {  get } 
}