如何在onappearing（）方法中导航？

Question

我想避免将我的上登机在导航堆栈中查看，以解决按下返回按钮的问题。在我的mainpage中，我检查inboarding是否应出现。如果为true，我将pushmodalasync方法inppearting方法中的方法。但是，应用程序崩溃具有以下例外：

Objective-C exception thrown.  Name: System.InvalidOperationException Reason: Window should have a UIWindow set. (System.InvalidOperationException)
   at Microsoft.Maui.Controls.Window.get_NativeWindow()
   at Microsoft.Maui.Controls.Platform.ModalNavigationManager.EndEditing()
   at Microsoft.Maui.Controls.Platform.ModalNavigationManager.PushModalAsync(Page modal, Boolean animated)
   at Microsoft.Maui.Controls.Window.NavigationImpl.OnPushModal(Page modal, Boolean animated)
   at BytePesaApp.MainPage.OnAppearing() in MainPage.xaml.cs:line 18
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
   at Foundation.NSAsyncSynchronizationContextDispatcher.Apply()

以下是代码实现：

public partial class MainPage : ContentPage
{

    public MainPage()
    {
        InitializeComponent();
    }

    protected override async void OnAppearing()
    {
        base.OnAppearing();

        if (ShouldShowOnBoardingPage() == true)
        {
            await Navigation.PushModalAsync(new OnBoardingPage(), false);
        }
    }


    private bool ShouldShowOnBoardingPage()
    {
        return VersionTracking.IsFirstLaunchEver;
    }
}

Answer 1

听起来毛伊岛直到表现出来之前都没有良好的状态。推迟这样的导航：

Dispatcher.Dispatch(async() =>
    await Navigation.PushModalAsync(new OnBoardingPage(), false);
);

此代码将是排队，并将在启动返回后运行。

---

替代方案是在入职期间替换主页。
在app.xaml.cs中：

if (ShouldShowOnBoardingPage() == true)
    MainPage = new OnBoardingPage();
else
    MainPage = new MainPage();

然后，当入门完成时：

    Application.Current.MainPage = new MainPage();

如果可能的话，我更喜欢后一种方法。

Answer 2

onappeation（）以启动加载的事件;不幸的是，没有onload（）可以覆盖，因此您必须使用事件：

public partial class MainPage : ContentPage
{

    public MainPage()
    {
        InitializeComponent();

        this.Loaded += MainPage_Loaded;
    }

    protected async void MainPage_Loaded(object sender, EventArgs e)
    {
        if (ShouldShowOnBoardingPage() == true)
        {
            await Navigation.PushModalAsync(new OnBoardingPage(), false);
        }
    }

    private bool ShouldShowOnBoardingPage()
    {
        return VersionTracking.IsFirstLaunchEver;
    }
}

Answer 3

决定之一是使用任务：

protected override async void OnAppearing()
{
    base.OnAppearing();

    if (ShouldShowOnBoardingPage() == true)
    {
        Task.Run(async () => 
        {
             await Navigation.PushModalAsync(new OnBoardingPage(), false);
        });
        
    }
}