初始化问题之前使用的另一个变量“__”

Question

我在这里搜索过。我已经看到并认为这正在影响它

Swift 中的类初始化是一个两阶段的过程。在第一 阶段，每个存储的属性都由类分配一个初始值 介绍了它。一旦每个存储属性的初始状态 已经确定，第二阶段开始，每节课都给 有机会在 新实例被认为可以使用。

当我运行以下代码时，我收到错误 Variable 'self.string' used before being initialized。

但它适用于我不会更改为 var 和初始化的每个变量。这是怎么回事？

这是错误：

下面的行不通。

struct IntTextField<T: Numeric & LosslessStringConvertible>: View {
    @Binding var number: T?
    @State private var string: String
    private let isDecimalAllowed: Bool

    private let title: LocalizedStringKey
    private let onEditingChanged: (Bool) -> Void
    private let onCommit: () -> Void

    public init(_ titleKey: LocalizedStringKey, number: Binding<T?>, isDecimalAllowed: Bool, onEditingChanged: @escaping (Bool) -> Void = { _ in }, onCommit: @escaping () -> Void = {}) {
        self._number = number
        self.string = "Why doesn't this work????"

        self.isDecimalAllowed = isDecimalAllowed

        self.title = titleKey
        self.onEditingChanged = onEditingChanged
        self.onCommit = onCommit
    }

    var body: some View {
        TextField(title, text: $string, onEditingChanged: onEditingChanged, onCommit: onCommit)
            .numericText(text: $string, number: $number, isDecimalAllowed: isDecimalAllowed)
            .modifier(KeyboardModifier(isDecimalAllowed: isDecimalAllowed))
    }
}

以下工作（我必须将它们全部更改为var并将它们初始化为某些东西），为什么我必须这样做？

struct IntTextField<T: Numeric & LosslessStringConvertible>: View {
    @Binding var number: T?
    // Everything below is changed to var and initialized
    @State private var string: String = ""
    private var isDecimalAllowed: Bool = false

    private var title: LocalizedStringKey = ""
    private var onEditingChanged: (Bool) -> Void = { _ in }
    private var onCommit: () -> Void = {}
    // To here

    init(_ titleKey: LocalizedStringKey, number: Binding<T?>, isDecimalAllowed: Bool, onEditingChanged: @escaping (Bool) -> Void = { _ in }, onCommit: @escaping () -> Void = {}) {
        self._number = number
        self.string = "Why does this work????"

        self.isDecimalAllowed = isDecimalAllowed

        self.title = titleKey
        self.onEditingChanged = onEditingChanged
        self.onCommit = onCommit
    }

    var body: some View {
        TextField(title, text: $string, onEditingChanged: onEditingChanged, onCommit: onCommit)
            .numericText(text: $string, number: $number, isDecimalAllowed: isDecimalAllowed)
            .modifier(KeyboardModifier(isDecimalAllowed: isDecimalAllowed))
    }
}

Answer 1

由于其@State 属性包装器，该变量与正常情况略有不同。要在此处初始化它而不会出现该错误，您需要这样做：

self._string = State(initialValue: "Why doesn't this work????")

_ 允许您在没有属性包装器的情况下访问 string。事实上，您已经使用Binding 以类似的策略在上面的行中执行此操作。

在您的第二个示例中，string 已经有一个默认值 ""，因此编译器并不担心它尚未设置。