SwiftUI 中可选数据类型的选择器? [英] Picker for optional data type in SwiftUI?

问题描述

通常我可以在 SwiftUI 中显示这样的项目列表:

Normally I can display a list of items like this in SwiftUI:

enum Fruit {
    case apple
    case orange
    case banana
}

struct FruitView: View {

    @State private var fruit = Fruit.apple

    var body: some View {
        Picker(selection: $fruit, label: Text("Fruit")) {
            ForEach(Fruit.allCases) { fruit in
                Text(fruit.rawValue).tag(fruit)
            }
        }
    }
}

这很完美，让我可以选择我想要的任何水果.但是，如果我想将 fruit 切换为可以为空(又名可选)，则会导致问题:

This works perfectly, allowing me to select whichever fruit I want. If I want to switch fruit to be nullable (aka an optional), though, it causes problems:

struct FruitView: View {

    @State private var fruit: Fruit?

    var body: some View {
        Picker(selection: $fruit, label: Text("Fruit")) {
            ForEach(Fruit.allCases) { fruit in
                Text(fruit.rawValue).tag(fruit)
            }
        }
    }
}

选择的水果名称不再显示在第一屏上，无论我选择什么选择项，它都不会更新水果值.

The selected fruit name is no longer displayed on the first screen, and no matter what selection item I choose, it doesn't update the fruit value.

如何将 Picker 与可选类型一起使用?

How do I use Picker with an optional type?

推荐答案

标签必须与正在包装的绑定完全匹配的数据类型.在这种情况下，提供给 tag 的数据类型是 Fruit 但 $fruit.wrappedValue 的数据类型是 Fruit?>.您可以通过在 tag 方法中转换数据类型来解决此问题:

The tag must match the exact data type as the binding is wrapping. In this case the data type provided to tag is Fruit but the data type of $fruit.wrappedValue is Fruit?. You can fix this by casting the datatype in the tag method:

struct FruitView: View {

    @State private var fruit: Fruit?

    var body: some View {
        Picker(selection: $fruit, label: Text("Fruit")) {
            ForEach(Fruit.allCases) { fruit in
                Text(fruit.rawValue).tag(fruit as Fruit?)
            }
        }
    }
}

奖励:如果您想要 nil 的自定义文本(而不是空白)，并希望允许用户选择 nil(注意:这里要么全部要么全无)，您可以为 nil 包含一个项目:

Bonus: If you want custom text for nil (instead of just blank), and want the user to be allowed to select nil (Note: it's either all or nothing here), you can include an item for nil:

struct FruitView: View {

    @State private var fruit: Fruit?

    var body: some View {
        Picker(selection: $fruit, label: Text("Fruit")) {
            Text("No fruit").tag(nil as Fruit?)
            ForEach(Fruit.allCases) { fruit in
                Text(fruit.rawValue).tag(fruit as Fruit?)
            }
        }
    }
}

不要忘记转换 nil 值.

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