如何更新 Observable 对象中的数组元素

Question

抱歉，如果我的问题很愚蠢，我是编程的初学者。我有一个导航 Link 到从我的视图模型数组生成的列表中的详细视图。在详细视图中，我希望能够改变点击元素的属性之一，但我似乎无法弄清楚如何做到这一点。我觉得我解释得不是很好，所以这是代码。

// model
struct Activity: Identifiable {
    var id = UUID()
    var name: String
    var completeDescription: String
    var completions: Int = 0
}

// view model
class ActivityViewModel: ObservableObject {
    @Published var activities: [Activity] = []
}

// view
struct ActivityView: View {
    @StateObject var viewModel = ActivityViewModel()
    @State private var showingAddEditActivityView = false
    var body: some View {
        NavigationView {
            VStack {
                List {
                    ForEach(viewModel.activities, id: \.id) {
                        activity in
                        NavigationLink(destination: ActivityDetailView(activity: activity, viewModel: self.viewModel)) {
                            HStack {
                                VStack {
                                    Text(activity.name)
                                    Text(activity.miniDescription)
                                }
                                Text("\(activity.completions)")
                            }
                        }
                    }
                }
            }
            .navigationBarItems(trailing: Button("Add new"){
                self.showingAddEditActivityView.toggle()
            })
            .navigationTitle(Text("Activity List"))
        }
        .sheet(isPresented: $showingAddEditActivityView) {
            AddEditActivityView(copyViewModel: self.viewModel)
        }
    }
}

// detail view
struct ActivityDetailView: View {
    @State var activity: Activity
    @ObservedObject var viewModel: ActivityViewModel
    
    var body: some View {
        VStack {
            Text("Number of times completed: \(activity.completions)")
            Button("Increment completion count"){
                activity.completions += 1
                updateCompletionCount()
            }
            Text("\(activity.completeDescription)")
        }
    }
    func updateCompletionCount() {
         var tempActivity = viewModel.activities.first{ activity in activity.id == self.activity.id
         }!
        tempActivity.completions += 1
    }
}

// Add new activity view (doesn't have anything to do with question)

struct AddEditActivityView: View {
    @ObservedObject var copyViewModel : ActivityViewModel
    @State private var activityName: String = ""
    @State private var description: String = ""
    var body: some View {
        VStack {
            TextField("Enter an activity", text: $activityName)
            TextField("Enter an activity description", text: $description)
            Button("Save"){
                // I want this to be outside of my view
                saveActivity()
            }
        }
    }
    func saveActivity() {
        copyViewModel.activities.append(Activity(name: self.activityName, completeDescription: self.description))
        print(copyViewModel.activities)
    }
}

在详细视图中，我正在尝试更新特定 activity 的完成计数，并让它更新我的视图模型。我上面尝试的方法可能没有意义，显然不起作用。我只是把它留下来展示我的尝试。

感谢任何帮助或见解。

Answer 1

您正在更改并递增 tempActivity 的值，因此它不会影响主数组或数据源。

您可以在视图模型中添加一个更新函数并从视图中调用。

视图模型负责此更新。

class ActivityViewModel: ObservableObject {
    @Published var activities: [Activity] = []
    
    func updateCompletionCount(for id: UUID) {
        if let index = activities.firstIndex(where: {[=10=].id == id}) {
            self.activities[index].completions += 1
        }
    }
}

struct ActivityDetailView: View {
    var activity: Activity
    var viewModel: ActivityViewModel
    
    var body: some View {
        VStack {
            Text("Number of times completed: \(activity.completions)")
            Button("Increment completion count"){
                updateCompletionCount()
            }
            Text("\(activity.completeDescription)")
        }
    }
    func updateCompletionCount() {
        self.viewModel.updateCompletionCount(for: activity.id)
    }
}

如果没有进一步的操作，则不需要@State 或@ObservedObject 来查看详细信息。

Answer 2

问题出在这里：

struct ActivityDetailView: View {
    @State var activity: Activity
    ...

这需要是 @Binding 才能将更改反映回父视图。也没有必要传入整个 viewModel - 一旦你有了 @Binding，你就可以摆脱它。

// detail view
struct ActivityDetailView: View {
    @Binding var activity: Activity /// here!
    
    var body: some View {
        VStack {
            Text("Number of times completed: \(activity.completions)")
            Button("Increment completion count"){
                activity.completions += 1
            }
            Text("\(activity.completeDescription)")
        }
    }
}

但是如何获得绑定呢？如果你使用 iOS 15，你可以直接循环 $viewModel.activities:

/// here!
ForEach($viewModel.activities, id: \.id) { $activity in
    NavigationLink(destination: ActivityDetailView(activity: $activity)) {
        HStack {
            VStack {
                Text(activity.name)
                Text(activity.miniDescription)
            }
            Text("\(activity.completions)")
        }
    }
}

对于 iOS 14 或以下，您需要循环索引。但它有效。

/// from 
ForEach(Array(zip(viewModel.activities.indices, viewModel.activities)), id: \.1.id) { (index, activity)  in
    NavigationLink(destination: ActivityDetailView(activity: $viewModel.activities[index])) {
        HStack {
            VStack {
                Text(activity.name)
                Text(activity.miniDescription)
            }
            Text("\(activity.completions)")
        }
    }
}

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