如何保留导航事件的数据上下文

Question

我设置了一个属性，当调用它的setter时，它会触发自定义导航事件（NavigateTo）。到目前为止，导航在第一次尝试时工作正常，即从列表视图中选择一个项目，其中所选项目绑定到属性，然后属性设置器启动，进而触发导航事件。

但是，当我导航回主页并从列表视图中选择一个项目时，导航事件不会触发秒。

我已经调试了"NavigateTo"的调用，第二次导航回主页并从列表视图中选择时没有调用它。这向我指出，页面的构造函数没有被调用，VM的数据上下文也没有设置，这反过来意味着不会调用setter导航事件。

问题：

如何在页面之间的导航生命周期中保留数据上下文？

MainViewModel-在SelectedCouncilName setter中调用导航事件，该事件传递一个用于导航的int参数：

    private CouncilName _selectedCouncilName;
    public CouncilName SelectedCouncilName
    {
        get
        {
            return _selectedCouncilName;
        }
        set
        {
            if (_selectedCouncilName != value)
            {
                _selectedCouncilName = value;
                RaisePropertyChanged("SelectedCouncilName");
                _navCallBack.NavigateTo(_selectedCouncilName.ID);            
            }
        }
    }

MainPage.xaml列表视图-所选项目绑定到SelectedCouncilName属性：

<ListView x:Name="ZonesListView"
                      Margin="0,0,-12,0"
                      ItemsSource="{Binding CouncilNameItems}"
                      SelectedItem="{Binding SelectedCouncilName,
                                             Mode=TwoWay}">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Margin="0,0,0,17">
                            <TextBlock Style="{ThemeResource ListViewItemTextBlockStyle}"
                                       Text="{Binding CouncilAcronym}"
                                       TextWrapping="Wrap" />
                            <TextBlock Margin="12,-6,12,0"
                                       Style="{ThemeResource ListViewItemSubheaderTextBlockStyle}"
                                       Text="{Binding CouncilFullName}"
                                       TextWrapping="Wrap" />
                        </StackPanel>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>

MainPage.xaml.cs-在SelectedCouncilName属性上触发setter之后，在其中触发导航事件的代码。（我在后面包含了整个页面的代码，以显示如何设置数据上下文和导航方法的详细信息）：

public sealed partial class MainPage : Page, INavigationCallback
{
    MainViewModel vm;
    private NavigationHelper navigationHelper;
    public MainPage()
    {
        this.navigationHelper = new NavigationHelper(this);
        this.navigationHelper.LoadState += navigationHelper_LoadState;
        this.navigationHelper.SaveState += navigationHelper_SaveState;
        //init data context
        this.NavigationCacheMode = NavigationCacheMode.Required;
        this.InitializeComponent();
        vm = new MainViewModel(this);
        this.DataContext = vm;
        vm.LoadCouncilNamesData();
    }
    private void navigationHelper_SaveState(object sender, SaveStateEventArgs e)
    {
        throw new NotImplementedException();
    }
    void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
    {
        throw new NotImplementedException();
    }
    /// <summary>
    /// Invoked when this page is about to be displayed in a Frame.
    /// </summary>
    /// <param name="e">Event data that describes how this page was reached.
    /// This parameter is typically used to configure the page.</param>
    protected override void OnNavigatedTo(NavigationEventArgs e)
    {

        // TODO: Prepare page for display here.
        // TODO: If your application contains multiple pages, ensure that you are
        // handling the hardware Back button by registering for the
        // Windows.Phone.UI.Input.HardwareButtons.BackPressed event.
        // If you are using the NavigationHelper provided by some templates,
        // this event is handled for you.
    }
    void INavigationCallback.NavigateTo(string ItemID)
    {
        Frame.Navigate(typeof(RequestTagPage), ItemID);
    }
}

Answer 1

删除this.NavigationCacheMode = NavigationCacheMode.Required用于在导航回时调用构造函数

Answer 2

如果问题是没有调用构造函数，那么我建议将视图模型设置为StaticResource或将其放置在viewmodel定位器中。当您在XAML中绑定它时，数据上下文将保持不变。

然后可以在视图模型的构造函数中安装填充，因为它将在需要时生成。