如何在 SwiftUI 的 List 中设置和使用参数“selection”

Question

我了解了 SwiftUI，但在 SwiftUI 中理解 List 有困难。

列表定义如下。

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
public struct List<SelectionValue, Content> : View where SelectionValue : Hashable, Content : View {

    /// Creates a List that supports multiple selection.
    ///
    /// - Parameter selection: A binding to a set that identifies the selected
    ///   rows.
    ///
    /// - See Also: `View.selectionValue` which gives an identifier to the rows.
    ///
    /// - Note: On iOS and tvOS, you must explicitly put the `List` into Edit
    ///   Mode for the selection to apply.
    @available(watchOS, unavailable)
    public init(selection: Binding<Set<SelectionValue>>?, @ViewBuilder content: () -> Content)

    /// Creates a List that supports optional single selection.
    ///
    /// - Parameter selection: A binding to the optionally selected row.
    ///
    /// - See Also: `View.selectionValue` which gives an identifier to the rows.
    ///
    /// - Note: On iOS and tvOS, you must explicitly put the `List` into Edit
    ///   Mode for the selection to apply.
    @available(watchOS, unavailable)
    public init(selection: Binding<SelectionValue?>?, @ViewBuilder content: () -> Content)

    :
    :

}

那么我的问题是，我怎样才能拥有支持多选/单选的列表？ 我会知道如何设置Binding<Set<SelectionValue>>? 和Binding<Set<SelectionValue>>? 的参数。

我已经阅读了How does one enable selections in SwiftUI's List，并且得到了这段代码。此代码确实支持多选。

var demoData = ["Phil Swanson", "Karen Gibbons", "Grant Kilman", "Wanda Green"]

struct ContentView: View {
    @State var selectKeeper = Set<String>()

    var body: some View {
        NavigationView {
            List(demoData, id: \.self, selection: $selectKeeper){ name in
                Text(name)
            }
            .navigationBarItems(trailing: EditButton())
            .navigationBarTitle(Text("Selection Demo \(selectKeeper.count)"))
        }
    }
}

但我仍然无法理解如何设置参数“选择”并设置类型。如何更改为单选列表？什么是Set<String>()...？

有没有人解释得通俗易懂？ 我会举个简单的例子...

非常感谢，老师！感谢您阅读我的问题！

Answer 1

如何更改为单选列表？

@State var selectKeeper: String? = nil // << default, no selection

什么是 Set()...？

所选项目的容器，在您的情况下是来自demoData的字符串

如何设置参数“选择”并设置类型

一个变体在.onAppear 中，如下所示

List(demoData, id: \.self, selection: $selectKeeper){ name in
    Text(name)
}
.onAppear {
   self.selectKeeper = [demoData[0]]
}

选择的类型通过状态变量的类型来检测，如果设置为多选，如果是可选的，则单选。