iPad端NavigationSplitView后台返回丢失列表选中状态问题

问题描述 #

在iPad设备上，使用NavigationSplitView实现的列表，按下Home键回到桌面再返回应用后，之前选中的列表项状态会丢失；相同场景下使用NavigationView则能正常保留选中状态，且该问题仅出现在iPad，iPhone无此现象。

有问题的NavigationSplitView代码 #

import SwiftUI
 
struct NavigationSplitViewExample: View {
     private var alphabets = ["A","B","C"]
        @State private var alphabetChosen: String?
 
        var body: some View {
            NavigationSplitView(columnVisibility: .constant(NavigationSplitViewVisibility.all)) {
                List(alphabets, id:\.self,   selection: $alphabetChosen) { alphabets in
                    Text(alphabets)
                }
            } detail: {
                Text(alphabetChosen ?? "0")
                     
            }
        }
}

正常保留状态的NavigationView代码 #

struct ContentView: View {
     private var alphabets = ["A","B","C"]
      @State private var alphabetChosen: String?

   let columns = Array(repeating: GridItem(.adaptive(minimum: 94, maximum: 120)), count: 3)
   
   var body: some View {
       NavigationView {
           List(alphabets, id:\.self) {   alphabet in
               NavigationLink(destination: Text("chosen \(alphabet)")) {
                                  Text(alphabet)
                              }
               }
           }
       .navigationBarTitle(Text("Alphabets"))
       .navigationViewStyle(StackNavigationViewStyle())
           }
}

问题原因 #

NavigationSplitView在iPad上的状态管理逻辑和NavigationView存在差异：当应用进入后台时，@State修饰的选中状态变量会被系统重置。NavigationSplitView的列选中状态依赖于自身的绑定变量，而后台恢复时系统没有自动保留这个@State变量的值；反观NavigationView，它通过NavigationLink维护的导航栈自动保留了选中对应的页面状态。

解决方案 #

方案1：使用@AppStorage持久化状态 #

@AppStorage会将状态存储到用户默认配置中，跨应用后台恢复、甚至重启都能保留选中状态：

import SwiftUI
 
struct NavigationSplitViewExample: View {
    private var alphabets = ["A","B","C"]
    // 用AppStorage替代State，指定存储键名
    @AppStorage("selectedAlphabet") private var alphabetChosen: String?
 
    var body: some View {
        NavigationSplitView(columnVisibility: .constant(NavigationSplitViewVisibility.all)) {
            List(alphabets, id:\.self, selection: $alphabetChosen) { alphabet in
                Text(alphabet)
            }
        } detail: {
            Text(alphabetChosen ?? "0")
        }
    }
}

方案2：使用@SceneStorage存储场景状态 #

@SceneStorage是专门为场景状态设计的存储方案，仅保留当前场景的状态，更轻量化，适合仅需后台恢复时保留状态的场景：

import SwiftUI
 
struct NavigationSplitViewExample: View {
    private var alphabets = ["A","B","C"]
    // 用SceneStorage替代State，指定存储键名
    @SceneStorage("selectedAlphabet") private var alphabetChosen: String?
 
    var body: some View {
        NavigationSplitView(columnVisibility: .constant(NavigationSplitViewVisibility.all)) {
            List(alphabets, id:\.self, selection: $alphabetChosen) { alphabet in
                Text(alphabet)
            }
        } detail: {
            Text(alphabetChosen ?? "0")
        }
    }
}

两种方案对比 #

• @AppStorage：数据持久化到系统偏好设置，跨场景、跨应用重启有效；
• @SceneStorage：数据绑定到当前场景，仅在当前场景后台恢复时有效，不会影响其他场景，更适合多窗口场景。

内容的提问来源于stack exchange，提问作者nirav dinmali