SwiftUI：选择列表中的项目后更新数据

Question

我花了两天时间试图解决这个问题，希望有人可以帮助我。很确定这个基本的东西一定是可行的，但我缺少一些基本概念......

在 ma​​cOS 上使用 XCode 11.4，但我怀疑这也适用于 iOS。

为了说明问题，我已将源代码缩减到最低限度。

我在主详细信息视图中有一个项目列表，其中项目列表是从一个来源获取的，一旦用户单击其中一个，详细信息将从第二个来源获取并显示在右侧详细视图。

预期结果

当我点击一个项目时，项目的主体会显示在右侧的详细视图中。

实际结果

1. 当我单击第一项时，正确的正文显示在右侧。但是ItemDetailStore.load()方法被调用了两次！

2. 当我点击第二项和第三项时，详细视图保持不变。

来源

import SwiftUI

struct ContentView: View {
    @State var selectedItem: Item?

    var body: some View {
        NavigationView {
            ItemList(selectedItem: $selectedItem)
            if selectedItem != nil {
                ItemDetail(selectedItem: $selectedItem)
            }
        }
    }
}

struct ItemList: View {
    @Binding var selectedItem: Item?
    var items = [
        Item(itemId: 0, title: "Item #0", body: "Empty."),
        Item(itemId: 1, title: "Item #1", body: "Empty."),
        Item(itemId: 2, title: "Item #2", body: "Empty.")
    ]

    var body: some View {
        List(selection: $selectedItem) {
            ForEach(Array(items.enumerated()), id: \.element) { index, item in
                ItemRow(item: item)
          }
        }
        .listStyle(SidebarListStyle())
    }
}

struct ItemRow: View {
    var item: Item

    var body: some View {
        Text("\(item.title)")
            .padding(12)
    }
}

struct ItemDetail: View {
    @EnvironmentObject var itemDetailStore: ItemDetailStore
    @Binding var selectedItem: Item?

    var body: some View {
        VStack {
            if itemDetailStore.items.count > 0 {
                Text(itemDetailStore.items[0].body)
            }
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .onAppear {
            if self.selectedItem != nil {
                self.itemDetailStore.load(id: self.selectedItem!.itemId)
            }
        }
    }
}

struct Item: Codable, Hashable {
    var itemId: Int
    var title: String
    var body: String
}

class ItemDetailStore: ObservableObject {
    @Published var items = [Item]()

    var itemsList = [
        Item(itemId: 0, title: "Item #0", body: "This is an excellent item #0."),
        Item(itemId: 1, title: "Item #1", body: "This is an excellent item #1."),
        Item(itemId: 2, title: "Item #2", body: "This is an excellent item #2.")
    ]

    func load(id: Int) {
        self.items = [itemsList[id]]
        print("\(self.items[0].title) loaded.")
    }
}

我在 AppDelegate 中注入环境：

let contentView = ContentView().environmentObject(ItemDetailStore())

罪魁祸首似乎是 ItemDetail 中的 onAppear 修饰符，因为它仅在单击列表中的第一项时才被调用。每次 selectedItem 更改时不应该更新视图吗？

非常感谢任何反馈。

Answer 1

看看这个：

import SwiftUI


struct ContentView: View {

    @EnvironmentObject var itemDetailStore: ItemDetailStore

    var body: some View {
        NavigationView {
            ItemList()
            if self.itemDetailStore.selectedItem != nil {
                ItemDetail()
            }
        }
    }
}

struct ItemList: View {

    @EnvironmentObject var itemDetailStore: ItemDetailStore

    var items = [
        Item(itemId: 0, title: "Item #0", body: "Empty."),
        Item(itemId: 1, title: "Item #1", body: "Empty."),
        Item(itemId: 2, title: "Item #2", body: "Empty.")
    ]

    var body: some View {
        List(selection: self.$itemDetailStore.selectedItem) {
            ForEach(Array(items.enumerated()), id: \.element) { index, item in
                ItemRow(item: item)
          }
        }
        .listStyle(SidebarListStyle())
    }
}

struct ItemRow: View {
    var item: Item

    var body: some View {
        Text("\(item.title)")
            .padding(12)
    }
}

struct ItemDetail: View {
    @EnvironmentObject var itemDetailStore: ItemDetailStore

    var body: some View {
        VStack {
            if itemDetailStore.items.count > 0 {
                Text(itemDetailStore.items[0].body)
            }
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .onAppear {

        }
    }
}

struct Item: Codable, Hashable, Identifiable {
    var id = UUID().uuidString

    var itemId: Int
    var title: String
    var body: String
}

class ItemDetailStore: ObservableObject {
    @Published var items = [Item]()
    @Published var selectedItem: Item? {
        didSet {
            if self.selectedItem != nil {
                self.load(id: self.selectedItem!.itemId)
            }
        }
    }

    var itemsList = [
        Item(itemId: 0, title: "Item #0", body: "This is an excellent item #0."),
        Item(itemId: 1, title: "Item #1", body: "This is an excellent item #1."),
        Item(itemId: 2, title: "Item #2", body: "This is an excellent item #2.")
    ]

    func load(id: Int) {
        self.items = [itemsList[id]]
        print("\(self.items[0].title) loaded.")
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environmentObject(ItemDetailStore())
    }
}