Xamarin - 不能使用 PopModalAsync

Question

I am trying to use PopModalAsync to remove the modal page. However, the Navigation.ModalStack.Count is 0. If I use PopModalAsync , it will throw an exception:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index

I am using Xamarin.Forms . Here is some sample code:

App.cs (Potable)

public class App : Application
{
    public App()
    {
        // The root page of your application
        MainPage = new View.LoginPage();
    }
}

LoginPage.xaml.cs (Potable)

public partial class LoginPage : ContentPage
{
    public INavigation _Navigate;
    public LoginPage()
    {
        InitializeComponent();
        _Navigate = Navigation;
    }

    async void LoginBtnClicked(object sender, EventArgs args)
    {
        await _Navigate.PushModalAsync(new AuthenicationBrowser());
        //await _Navigate.PopModalAsync(); it is work at here
        Debug.WriteLine("Navigation.NavigationStack  LoginBtnClicked ===> {0}", Navigation.NavigationStack.Count); //getting 0
         Debug.WriteLine("Navigation.ModalStack  LoginBtnClicked ===> {0}", Navigation.ModalStack.Count);  // getting 1    
    }

    public async void PopModal()
    {
        Debug.WriteLine(Navigation.NavigationStack.Count);
        await Navigation.PopModalAsync();
    }



}

AuthenicationBrowser.cs (Potable) * Edited: Put PopModalAsync *

public partial class AuthenicationBrowser : ContentPage
{
    public AuthenicationBrowser()
    {
      InitializeComponent();
    }
    public async void PopModal()
    {
       Debug.WriteLine("Navigation.ModalStack  AuthenicationBrowser .PopModal===> {0}", Navigation.ModalStack.Count);  // getting 0    
       await Navigation.PopModalAsync();
    }
}

BrowserView.cs (Potable)

public class BrowserView : WebView
{
    public BrowserView()
    {

    }
}

AuthenicationBrowserRenderer.cs (Droid) * Edited: Calling PopModal *

  [assembly: ExportRenderer(typeof(BrowserView), typeof(AuthenicationBrowserRenderer))] 
 namespace App.Droid
 {
     class AuthenicationBrowserRenderer : WebViewRenderer
     {
       ... // Doing some Auth in OnElementChanged and using JavaScriptCallBack class after received json in Webview
     }
     public class JavaScriptCallBack: Java.Lang.Object, IValueCallback
     {
        public JavaScriptCallBack()
        {

        }
        public async void OnReceiveValue(Java.Lang.Object result)
        {
            Java.Lang.String json = (Java.Lang.String)result;
            string raw_json = json.ToString();
            Debug.WriteLine("raw_json  ====>>> {0}", raw_json);
            var login_page = new LoginPage();
            var auth_page = new AuthenicationBrowser();

            Debug.WriteLine(login_page.Navigation.ModalStack.Count); // getting 0
            Debug.WriteLine(auth_page.Navigation.ModalStack.Count); // getting 0
            auth_page.PopModal(); // Trying to do PopModalAsync 


         }
     }
 }

Answer 1

Finally, I may get the answer that App.Current.MainPage.Navigation.PopModalAsync(); can do the trick. The reason is that the new LoginPage() is called as a new Content Page not existing page.

If I call it from the App.Current.MainPage (The existing LoginPage), it can get the existing modal from Modal Stack.

So the solution can be :

    public partial class LoginPage : ContentPage
    {

        public LoginPage()
        {
            InitializeComponent();

        }


        async void LoginBtnClicked(object sender, EventArgs args)
        {
            await Navigation.PushModalAsync(new AuthenicationBrowser());
        }

        public async void PopModal()
        {

            Debug.WriteLine("Navigation.ModalStack  PopModal ===> {0}", App.Current.MainPage.Navigation.ModalStack.Count);
            await App.Current.MainPage.Navigation.PopModalAsync();

        }



    }

Answer 2

As it seems you had the wrong Navigation object. In my case I also had this wrong.

I displayed a modal page, which had it's own navigation bar (or Navigation object). So when I wanted to dismiss this modal page I got the mentioned exception, because there were no other pages on the navigation stack. It was the wrong object ...

Answer 3

对我来说，当我应该调用 PushAsync 时，我正在调用 PushModalAsync 没有要推送 PushModelAsync 的项目