list - SwiftUI，以编程方式选择列表中的一行

Question

我想以编程方式在 List 中设置选定的行。
在我下面的示例中，通过 2 个按钮。

struct ContentView: View {

  @State private var selection = 2

  var body: some View {
    VStack {
      List() {
      //List(selection: $selection)  {.  // does not compile
        Text("Line 0").tag(1)
        Text("Line 1").tag(1)
        Text("Line 2").tag(2)
        Text("Line 3").tag(3)
        Text("Line 4").tag(4)
        Text("Line 5").tag(5)
      }
      .listStyle(SidebarListStyle())
      Text("Selected Item :\(self.selection)")
      HStack {
        Button(action: {if (self.selection < 5 ) { self.selection += 1 }} ) {Text("⬇︎")}
        Button(action: {if (self.selection > 0 ) { self.selection -= 1 }} ) {Text("⬆︎")}
      }
    }
    .frame(maxWidth: .infinity, maxHeight: .infinity)
  }
}

试图使列表可以像这样选择:

List(selection: $selection)  

不编译。
编译器提示: Unable to infer complex closure return type; add explicit type to disambiguate

Answer 1

选择类型必须是可选的。找到下面的固定代码。

struct TestListSelectionOnAction: View {

  @State private var selection: Int? = 2 // optional !!

  var body: some View {
    VStack {
      List(selection: $selection)  {
        Text("Line 0").tag(0)
        Text("Line 1").tag(1)
        Text("Line 2").tag(2)
        Text("Line 3").tag(3)
        Text("Line 4").tag(4)
        Text("Line 5").tag(5)
      }
      .listStyle(SidebarListStyle())
      Text("Selected Item :\(self.selection ?? -1)")
      HStack {
        Button(action: {
            if (self.selection! < 5 ) { self.selection! += 1 }} ) {Text("⬇︎")}
        Button(action: {
            if (self.selection! > 0 ) { self.selection! -= 1 }} ) {Text("⬆︎")}
      }
    }
    .frame(maxWidth: .infinity, maxHeight: .infinity)
  }
}