如何在 SwiftUI 的列表中启用选择

Question

我正在尝试使用 SwiftUI 创建一个简单的多选列表。我无法使它工作。

List 接受第二个参数，它是一个 SelectionManager，所以我尝试创建一个具体的实现。但是，它永远不会被调用，并且行永远不会突出显示。

import SwiftUI

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

struct SelectKeeper : SelectionManager{
    var selections = Set<UUID>()

    mutating func select(_ value: UUID) {
        selections.insert(value)
    }

    mutating func deselect(_ value: UUID) {
        selections.remove(value)
    }

    func isSelected(_ value: UUID) -> Bool {
        return selections.contains(value)
    }

    typealias SelectionValue = UUID

}

struct SelectionDemo : View {
    @State var selectKeeper = SelectKeeper()

    var body: some View {
        NavigationView {
            List(demoData.identified(by: \.self)){ name in
                Text(name)
            }
                .navigationBarTitle(Text("Selection Demo"))
        }
    }
}

#if DEBUG
struct SelectionDemo_Previews : PreviewProvider {
    static var previews: some View {
        SelectionDemo()
    }
}
#endif

代码运行良好，但行不突出显示，并且永远不会调用 SelectionManager 代码。

Answer 1

编辑模式

如上一个答案中所述，您可以在编辑模式下添加它。这意味着用户必须在某些时候按下编辑按钮来选择行。如果您想为列表设置视图状态和编辑状态，这将非常有用。

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

struct SelectionDemo : 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)"))
        }
    }
}

恒定编辑模式

您也可以简单地始终打开编辑模式。 SwiftUI 具有环境修饰符，允许您手动控制任何环境变量。在这种情况下，我们要控制editMode 变量。

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

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

    var body: some View {
        NavigationView {
            List(demoData, id: \.self, selection: $selectKeeper){ name in
                Text(name)
            }
// the next line is the modifier
            .environment(\.editMode, .constant(EditMode.active))
            .navigationBarTitle(Text("Selection Demo \(selectKeeper.count)"))
        }
    }
}

Answer 2

而不是使用编辑模式，我只是根据模型更新行，并按照https://stackoverflow.com/a/57023746/1271826 的建议在点击行时在模型中切换一个布尔值。也许是这样的：

struct MultipleSelectionRow<RowContent: SelectableRow>: View {
    var content: Binding<RowContent>

    var body: some View {
        Button(action: {
            self.content.value.isSelected.toggle()
        }) {
            HStack {
                Text(content.value.text)
                Spacer()
                Image(systemName: content.value.isSelected ? "checkmark.circle.fill" : "circle")
            }
        }
    }
}

在哪里

protocol SelectableRow {
    var text: String { get }
    var isSelected: Bool { get set }
}

然后您可以执行以下操作：

struct Person: Hashable, Identifiable, SelectableRow {
    let id = UUID().uuidString
    let text: String
    var isSelected: Bool = false
}

struct ContentView : View {
    @State var people: [Person] = [
        Person(text: "Mo"),
        Person(text: "Larry"),
        Person(text: "Curly")
    ]

    var body: some View {
        List {
            ForEach($people.identified(by: \.id)) { person in
                MultipleSelectionRow(content: person)
            }
        }
    }
}

产量：

Answer 3

根据您的需要，有两种方法可以做到这一点：

如果您想在“编辑模式”中执行此操作：

您必须先在列表中启用“编辑模式”，然后才能进行选择。来自List的界面：

    /// Creates an instance.
    ///
    /// - Parameter selection: A selection manager that identifies the selected row(s).
    ///
    /// - 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<Selection>?, content: () -> Content)

您可以通过在某处的视图中添加EditButton 来做到这一点。之后，你只需要为实现SelectionManager的东西绑定一个var（你不需要在这里滚动你自己的：D）

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

struct SelectionDemo : View {
    @State var selectKeeper = Set<String>()
    
    var body: some View {
        NavigationView {
            List(demoData.identified(by: \.self), selection: $selectKeeper){ name in
                Text(name)
            }
            .navigationBarItems(trailing: EditButton())
            .navigationBarTitle(Text("Selection Demo \(selectKeeper.count)"))
        }
    }
}

这种方法如下所示：

如果您不想使用“编辑模式”：

此时，我们将不得不推出自己的产品。 注意：这个实现有一个错误，这意味着只有Text 会导致选择发生。可以使用Button 执行此操作，但由于 Beta 2 中的更改删除了borderlessButtonStyle()，它看起来很傻，而且我还没有找到解决方法。

struct Person: Identifiable, Hashable {
    let id = UUID()
    let name: String
}

var demoData = [Person(name: "Phil Swanson"), Person(name: "Karen Gibbons"), Person(name: "Grant Kilman"), Person(name: "Wanda Green")]

struct SelectKeeper : SelectionManager{
    var selections = Set<UUID>()
    
    mutating func select(_ value: UUID) {
        selections.insert(value)
    }
    
    mutating func deselect(_ value: UUID) {
        selections.remove(value)
    }
    
    func isSelected(_ value: UUID) -> Bool {
        return selections.contains(value)
    }
    
    typealias SelectionValue = UUID
    
}

struct SelectionDemo : View {
    @State var selectKeeper = Set<UUID>()
    
    var body: some View {
        NavigationView {
            List(demoData) { person in
                SelectableRow(person: person, selectedItems: self.$selectKeeper)
            }
            .navigationBarTitle(Text("Selection Demo \(selectKeeper.count)"))
        }
    }
}

struct SelectableRow: View {
    var person: Person
    
    @Binding var selectedItems: Set<UUID>
    var isSelected: Bool {
        selectedItems.contains(person.id)
    }
    
    var body: some View {
        GeometryReader { geo in
            HStack {
                Text(self.person.name).frame(width: geo.size.width, height: geo.size.height, alignment: .leading)
            }.background(self.isSelected ? Color.gray : Color.clear)
            .tapAction {
                if self.isSelected {
                    self.selectedItems.remove(self.person.id)
                } else {
                    self.selectedItems.insert(self.person.id)
                }
            }
        }
    }
}