得到“致命错误：索引超出范围”：在 swiftui 的列表项中显示索引

Question

更新：错误：类型'_'没有成员'1'，如果在list-foreach中放置一个if闭包（if !self.showMarkedOnly || name.marked {}），为什么？

代码版本 4：

struct Name: Identifiable, Hashable {
    var id: String = UUID().uuidString
    var name: String
    var marked: Bool
    init(_ name: String, marked: Bool = false) { self.name = name; self.marked = marked }
}

struct TestView: View {
    @State private var list: [Name] = [Name("test1"), Name("test2"), Name("test3", marked: true), Name("test4"), Name("test5", marked: true), Name("test6"), Name("test7"), Name("test8")]
    @State private var showMarkedOnly = false

    var body: some View {
        VStack{
            Toggle(isOn: $showMarkedOnly) { Text("show marked only") }
            List {
                ForEach(Array(zip(0..., list)), id: \.1.id) { index, name in
//                    if !self.showMarkedOnly || name.marked {
                        HStack {
                            Text("\(index)").foregroundColor(name.marked ? .red : .gray)
                            Spacer()
                            Text("\(name.name)")
                        }
                        .background(Color.gray.opacity(0.001))
                        .onTapGesture {
                            self.list.remove(at: index)
                        }
//                    }
                }
            }
        }
    }
}

==========

更新：我发现代码版本 2 的问题，我必须为 ForEach 提供一个 id。并更新了代码版本 2。

我找到了一种显示索引的优雅方式，它避免了 self.list[index]。但是我发现在一些复杂的代码中出现了错误“Type '_' has no member '1'”。

代码版本 3：

var body: some View {
        List {
            ForEach(Array(zip(0..., list)), id: \.1.id) { index, name in // a error occurs in some complex code: "Type '_' has no member '1'"
                HStack {
                    Text("\(index)")
                    Spacer()
                    Text("\(name.name)")
                }
                .background(Color.gray.opacity(0.001))
                .onTapGesture {
                    self.list.remove(at: index)
                }
            }
        }
    }

---

我显示一个列表，然后删除我点击的项目。这是代码版本 1，它工作正常。当我使用索引（代码版本 2）将索引添加到列表项时，点击后出现“致命错误：索引超出范围”。

代码版本 1：

struct Name: Identifiable, Hashable {
    var id: String = UUID().uuidString
    var name: String
    init(_ name: String) { self.name = name }
}

struct TestView: View {
    @State private var list: [Name] = [Name("test1"), Name("test2"), Name("test3"), Name("test4"), Name("test5"), Name("test6"), Name("test7"), Name("test8")]

    var body: some View {
        List {
            ForEach(list) { name in
                HStack {
                    Text("\(0)")
                    Spacer()
                    Text("\(name.name)")
                }
                .background(Color.gray.opacity(0.001))
                .onTapGesture {
                    self.list = self.list.filter { $0 != name }
                }
            }
        }
    }
}

代码版本 2：

struct TestView: View {
    @State private var list: [Name] = [Name("test1"), Name("test2"), Name("test3"), Name("test4"), Name("test5"), Name("test6"), Name("test7"), Name("test8")]

    var body: some View {
        List {
            //ForEach(list.indices) { index in
            ForEach(list.indices, id: \.self) { index in
                HStack {
                    Text("\(index)")
                    Spacer()
                    Text("\(self.list[index].name)")
                }
                .background(Color.gray.opacity(0.001))
                .onTapGesture {
                    self.list.remove(at: index)
                }
            }
        }
    }
}

Answer 1

@State 是属性包装器，它将强制定义它的视图重新计算其主体。

在您的情况下，如果您删除索引处的项目，

HStack {
    Text("\(index)")
    Spacer()
    Text("\(self.list[index].name)")
}
.background(Color.gray.opacity(0.001))
.onTapGesture {
     self.list.remove(at: index)
 }

HStack 中的文本

Text("\(self.list[index].name)")

崩溃，只是因为 list[index] 不再存在。

使用

ForEach(list.indices, id:\.self) { index in ... }

代替

ForEach(list.indices) { index in ... }

将强制 SwiftUI 重新创建 TestView（请参阅 ForEach 构造函数中的 id:\.self）

SwiftUI 将在使用包裹在 @State 属性包装器中的属性的新值时制作新的 TestView 副本。

更新

请不要更新您的问题...

你的最后一个代码版本 4 一团糟，所以我把它改写成你可以复制 - 粘贴 - 运行的东西

import SwiftUI

struct Name: Identifiable, Hashable {
    var id: String = UUID().uuidString
    var name: String
    var marked: Bool
    init(_ name: String, marked: Bool = false) { self.name = name; self.marked = marked }
}

struct ContentView: View {
    @State private var list: [Name] = [Name("test1"), Name("test2"), Name("test3", marked: true), Name("test4"), Name("test5", marked: true), Name("test6"), Name("test7"), Name("test8")]
    @State private var showMarkedOnly = false

    var body: some View {
        VStack{
            Toggle(isOn: $showMarkedOnly) {
                Text("show marked only")
            }.padding(.horizontal)
            List {
                ForEach(Array(zip(0..., list)).filter({!self.showMarkedOnly || $0.1.marked}), id: \.1.id) { index, name in
                    HStack {
                        Text("\(index)").foregroundColor(name.marked ? .red : .gray)
                        Spacer()
                        Text("\(name.name)")
                    }
                    .background(Color.gray.opacity(0.001))
                    .onTapGesture {
                        self.list.remove(at: index)
                    }
                }
            }
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

它应该看起来像

根据讨论更新

ForEach 不同版本的构造函数在内部使用不同的 ViewBuilder 功能

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
extension ViewBuilder {

    /// Provides support for "if" statements in multi-statement closures, producing an `Optional` view
    /// that is visible only when the `if` condition evaluates `true`.
    public static func buildIf<Content>(_ content: Content?) -> Content? where Content : View

    /// Provides support for "if" statements in multi-statement closures, producing
    /// ConditionalContent for the "then" branch.
    public static func buildEither<TrueContent, FalseContent>(first: TrueContent) -> _ConditionalContent<TrueContent, FalseContent> where TrueContent : View, FalseContent : View

    /// Provides support for "if-else" statements in multi-statement closures, producing
    /// ConditionalContent for the "else" branch.
    public static func buildEither<TrueContent, FalseContent>(second: FalseContent) -> _ConditionalContent<TrueContent, FalseContent> where TrueContent : View, FalseContent : View
}

这是关于“实现细节”，希望它会在下一个版本中记录。SwiftUI 还处于非常早期的开发阶段，我们必须小心。

让我们尝试强制 SwiftUI 遵循我们自己的方式！第一个单独的 RowView

struct RowView: View {
    var showMarkedOnly: Bool
    var index: Int
    var name: Name
    //@ViewBuilder
    var body: some View {
        if !self.showMarkedOnly || name.marked {
            HStack {
                Text(verbatim: index.description).foregroundColor(name.marked ? .red : .gray)
                Spacer()
                Text(verbatim: name.name)
            }
            .background(Color.gray.opacity(0.001))

        }
    }
}

编译器抱怨

Function declares an opaque return type, but has no return statements in its body from which to infer an underlying type

取消注释包裹我们身体的行

struct RowView: View {
    var showMarkedOnly: Bool
    var index: Int
    var name: Name
    @ViewBuilder
    var body: some View {
        if !self.showMarkedOnly || name.marked {
            HStack {
                Text(verbatim: index.description).foregroundColor(name.marked ? .red : .gray)
                Spacer()
                Text(verbatim: name.name)
            }
            .background(Color.gray.opacity(0.001))

        }
    }
}

现在我们可以按照您喜欢的方式使用代码:-)

struct ContentView: View {
    @State private var list: [Name] = [Name("test1"), Name("test2"), Name("test3", marked: true), Name("test4"), Name("test5", marked: true), Name("test6"), Name("test7"), Name("test8")]
    @State private var showMarkedOnly = false

    var body: some View {
        VStack{
            Toggle(isOn: $showMarkedOnly) {
                Text("show marked only")
            }.padding(.horizontal)
            List {
                ForEach(Array(zip(0..., list)), id: \.1.id) { (index, name) in
                    RowView(showMarkedOnly: self.showMarkedOnly, index: index, name: name).onTapGesture {
                    self.list.remove(at: index)
                }
                }
            }
        }
    }
}

最终结果使用 nowbuildIf<Content>构造，所有代码再次运行（结果看起来与上图完全相同）