我最近开始学习 Swift。当我尝试制作我的第一个 App 时，我对 UIBarButtonItem 感到困惑。如果我将 UIBarButtonItem 初始化放在 viewDidLoad() 函数之外，当我按下 Next 按钮时什么也不会发生。

class ViewController: UIViewController, UITextFieldDelegate {
    let rightBarButton: UIBarButtonItem = UIBarButtonItem(title: "Next", style: .plain, target: self, action: #selector(onClickNext(button:)))

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = .white

        self.navigationItem.rightBarButtonItem = rightBarButton
    }

    func onClickNext(button: UIBarButtonItem) {
        print("should push view controller")
    }
}

但是，当我将initialization 放入viewDidLoad() 函数时，输出区域会输出我在onClickNext(button: ) 函数。

class ViewController: UIViewController, UITextFieldDelegate {
    var rightBarButton: UIBarButtonItem?

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.backgroundColor = .white

        self.rightBarButton = UIBarButtonItem(title: "Next", style: .plain, target: self, action: #selector(onClickNext(button:)))
        self.navigationItem.rightBarButtonItem = rightBarButton
    }

    func onClickNext(button: UIBarButtonItem) {
        print("should push view controller")
    }
}

此外，我发现当我将 initialization 放在 viewDidLoad() 函数之外时，我将 UITextField 添加到 viewController，如果我在按下按钮之前触摸 textfield，rightBarButton 就会工作。

这让我很困惑。机制是什么？

最佳答案

好吧，也许您不知道 ViewController 在内部是如何工作的。

首先，viewDidLoad 是您通常设置或初始化任何 View 或 View 属性 的区域。此方法在 View Controller 对象的生命周期中也只被调用一次。这意味着 self 已经存在。

了解这一点对于理解 let 属性的作用很重要，(来自 Apple)

A constant declaration defines an immutable binding between the constant name and the value of the initializer expression; after the value of a constant is set, it cannot be changed. That said, if a constant is initialized with a class object, the object itself can change, but the binding between the constant name and the object it refers to can’t.

尽管上部区域是您声明变量和常量的地方，通常用于简单的初始化，但它只是告诉 VC 有一个您想要使用的对象并将具有类全局范围的区域，但是当加载 View 层次结构时将添加其余功能(意味着该对象不依赖于自身，例如，将目标添加到按钮时，您指的是自身内部的方法)......这变量或常量称为存储属性

In its simplest form, a stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

最后，您有一个惰性存储属性，它也许可以用于您想要的:

A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.

解决方案:创建一个 lazy var 存储属性或者在 ViewDidLoad 中添加他的属性(当 self 已经存在时)

lazy private var doneButtonItem : UIBarButtonItem = {
    [unowned self] in
    return UIBarButtonItem(title: "Next", style:UIBarButtonItemStyle.Plain, target: self, action: #selector(onClickNext(button:)))
    }()

或

let rightBarButton: UIBarButtonItem?

override func viewDidLoad() {
        super.viewDidLoad()
        rightBarButton = UIBarButtonItem(title: "Next", style: .plain, target: self, action: #selector(onClickNext(button:)))
}

关于ios - Swift，与 UIBarButtonItem 混淆，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/45408498/