ios - 如何在 NavigationLink 中为目标视图重用视图模型实例

Question

在下面提供的代码中，我遇到了DetailViewModel正在重新创建的问题。发生这种情况是因为ContentView更新，它也重新创建了所有的NavigationLinks 和目的地。因此，内部的状态DetailViewModel被重置。

这是一些示例代码：

import SwiftUI
import Combine

struct ContentView: View {

    let items = ["Item A", "Item B", "Item C"]

    @State var contentViewUpdater = 0

    var body: some View {
        NavigationView {
            VStack {
                Button("Update ContentView: \(contentViewUpdater)") {
                    self.contentViewUpdater += 1
                }
                List(items, id: \.self) { item in
                    // How to prevent DetailViewModel from recreating after this ContentView receives an update?
                    NavigationLink(destination: DetailView(model: DetailViewModel(item: item))) {
                        Text(item)
                    }
                }
            }
        }
    }
}

final class DetailViewModel: ObservableObject {
    let item: String
    @Published var counter = 0

    init(item: String) {
        self.item = item
    }
}

struct DetailView: View {
    @ObservedObject var model: DetailViewModel

    var body: some View {
        VStack {
            Text("Counter for \(model.item): \(model.counter)")
            Button("Increase counter") {
                self.model.counter += 1
            }
        }
    }
}

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

这是该问题的屏幕录像。如果更新，DetailViewModel.countervar 会重置。ContentView

DetailViewModel当父视图更新时，如何防止状态重置？

标签： iosswiftswiftui

Answer 1

抱歉，我的代码改编自您的代码，因为我还没有更新到最新的测试版，但这对我有用。我使用了React 中“提升状态”的概念，并将模型数据移动到主视图本身。

从游乐场：

import SwiftUI
import PlaygroundSupport


final class ItemViewModel : BindableObject {
    let willChange = PassthroughSubject<Void, Never>()

    var name: String {
        willSet { willChange.send() }
    }
    var counter: Int = 0 {
        willSet { willChange.send() }
    }

    init(name: String) {
        self.name = name
    }
}


struct ContentView : View {
    let items = [
        ItemViewModel(name: "Item A"),
        ItemViewModel(name: "Item B"),
        ItemViewModel(name: "Item C")
    ]

    @State var contentViewUpdater = 0

    var body: some View {
        NavigationView {
            VStack {
                Button("Update ContentView: \(contentViewUpdater)") {
                    self.contentViewUpdater += 1
                }
                List(items) { model in
                    NavigationLink(destination: DetailView(model: model)) {
                        Text(model.name)
                    }
                }
            }
        }
    }
}


struct DetailView : View {
    @ObjectBinding var model: ItemViewModel

    var body: some View {
        let name = model.name
        let counter = model.counter
        return VStack {
            Text("Counter for \(name): \(counter)")
            Button("Increase counter") {
                self.model.counter += 1
            }
        }
    }
}


PlaygroundPage.current.liveView = UIHostingController(rootView: ContentView())
PlaygroundPage.current.needsIndefiniteExecution = true