在应用启动时在OnNavigatedTo中使用IPageDialogService

Question

我使用Prism Template Pack创建了一个Prism Unity App，并为其添加了另一个View和ViewModel（OtherPage和OtherPageViewModel）。这是我的代码：

App.xaml.cs

public partial class App : PrismApplication
{
    public App(IPlatformInitializer initializer = null) : base(initializer) { }

    protected override void OnInitialized()
    {
        InitializeComponent();

        NavigationService.NavigateAsync("MainPage?title=Hello%20from%20Xamarin.Forms");
    }

    protected override void RegisterTypes()
    {
        Container.RegisterTypeForNavigation<MainPage>();
        Container.RegisterTypeForNavigation<OtherPage>();
    }
}

MainPageViewModel.cs

public class MainPageViewModel : BindableBase, INavigationAware
{
    private INavigationService _navigationService;
    private IPageDialogService _pageDialogService;
    public DelegateCommand NavigateToOtherPageCommand { get; set; }
    private string _title;
    public string Title
    {
        get { return _title; }
        set { SetProperty(ref _title, value); }
    }

    public MainPageViewModel(INavigationService navigationService,
                             IPageDialogService pageDialogService)
    {
        _navigationService = navigationService;
        _pageDialogService = pageDialogService;
        NavigateToOtherPageCommand = new DelegateCommand(async () => await NavigateToOtherPage());
    }

    public void OnNavigatedFrom(NavigationParameters parameters)
    {

    }

    public async void OnNavigatedTo(NavigationParameters parameters)
    {
        if (parameters.ContainsKey("title"))
            Title = (string)parameters["title"] + " and Prism";

        await _pageDialogService.DisplayAlertAsync("Message", "Message for the user", "Ok");
    }

    private async Task NavigateToOtherPage()
    {
        await _navigationService.NavigateAsync("OtherPage");
    }
}

OtherPageViewModel.cs

public class OtherPageViewModel : BindableBase, INavigationAware
{
    private IPageDialogService _pageDialogService;
    public OtherPageViewModel(IPageDialogService pageDialogService)
    {
        _pageDialogService = pageDialogService;
    }

    public void OnNavigatedFrom(NavigationParameters parameters)
    {
    }

    public async void OnNavigatedTo(NavigationParameters parameters)
    {
        await _pageDialogService.DisplayAlertAsync("Message", "Message for the user", "Ok");
    }
}

我正在尝试使用IPageDialogService在MainPageViewModels的OnNavigatedTo方法中向用户显示消息，但不显示任何消息，也不会引发错误。

但是，如果我使用NavigateToOtherPageCommand从MainPage导航到OtherPage，则会正确显示该消息。为什么IPageDialogService不能在启动页面的OnNavigatedTo方法中工作（即MainPageViewModel）？它似乎在任何其他页面的OnNavigatedTo方法中工作正常（即OtherPageViewModel）。

Answer 1

尝试使用await Task.Yield()。有关此问题的详细信息，请参阅此链接：pageDialogService MainPage reference is always null inside OnNavigatedTo after app is launched