Xamarin forms MVVM Remove the previous VM in my current VM

Deos69 · July 2019

I have a main page : HomeVM. When the app starts, I use PushModalAsync for RegistrationVM In my RegistrationVM, I have a button for pushModalAsync -> LoginVM

When the user is connected, I want go back in the VM HomeVM, but impossible, with PopModalAsync, I come back to RegistrationVM.

I would like to close all popmodalasync.

I have tried this

Here in my app.xaml :

var mainPage = (Page)ViewFactory.CreatePage(typeof(HomeVM));
            MainPage = new NavigationPage(mainPage)
            {
                BarBackgroundColor = (Color)Resources["PrimaryColor"],
                BarTextColor = Color.White,
            };
            Navigation = MainPage.Navigation;


 protected override void OnStart()
        {

   var registrationpage = (Page)ViewFactory.CreatePage(typeof(RegistrationVM));
            MainPage.Navigation.PushModalAsync(registrationpage);
        }

RegistrationVM :

public RegistrationVM()
        {            

            Task.Run(async () => await ConnectionAPI());


        }

 async Task ConnectionAPI()
        {
            try
            {


                        applicationContext.Device = mydevice;                                           
                        await Navigation.PushModalAsync<LoginVM>(async (vm, p) => await vm.InitializeAsync(this));                     


            }
            catch (Exception e)
            {
                Log.Error(e, "Unhandled exception while loading touch points : {e}", e);
            }
            finally
            {

            }
        }

LoginVM : When the user click on "Login" PreviousVM = RegistrationVM.

async Task Login()
        {
            try
            {

            Log.Information("Logging in");              

                     applicationContext.User = employe;



                        await Navigation.PopModalAsync();
                        await Navigation.RemoveAsync(previousVM);                     





            }
            catch (Exception e)
            {
                Log.Error(e, "Unhandled exception during log in : {e}", e);
            }
            finally
            {

            }

        }

I come back to Registration view

I have tried by using "PushAsync" and "PopAsync" and try this in LoginVM.cs : await Navigation.PopToRootAsync();

Nothing works correctly. It always comeback to RegistrationVM.

Graverobber · July 2019

I have the theory that you Main.Navigation, the one where you push your Regpage to is not the same as the navigation of Regpage.
Therefore when you tried popToRoot in your login, it just popped back to Regpage.
Edit: To test this theory you could put a breakpoint into your LoginVM and check the Navigation params stack there.
Here are some thoughts to your code

Only assign the MainPage attribute of your App in the App class. Do the Push/Navigation to a new ContentPage only within a ContentPage.
Why always like this (Page)ViewFactory.CreatePage(typeof(HomeVM))? Does it have an advantage? You can just write MainPage = new NavigationPage(new HomeVM());

As you're working with modal pages, one thing you could try:

In your RegistrationVM register for the Apps ModalPopped event. When it fires pop the registration page as well
Application.Current.ModalPopped += (sender, e) =>
{
await Navigation.PopModalAsync();
};

If you try this, remove this line from your LoginVM await Navigation.RemoveAsync(previousVM);

Graverobber · July 2019

I have the theory that you Main.Navigation, the one where you push your Regpage to is not the same as the navigation of Regpage.
Therefore when you tried popToRoot in your login, it just popped back to Regpage.
Edit: To test this theory you could put a breakpoint into your LoginVM and check the Navigation params stack there.
Here are some thoughts to your code

Only assign the MainPage attribute of your App in the App class. Do the Push/Navigation to a new ContentPage only within a ContentPage.
Why always like this (Page)ViewFactory.CreatePage(typeof(HomeVM))? Does it have an advantage? You can just write MainPage = new NavigationPage(new HomeVM());

As you're working with modal pages, one thing you could try:

In your RegistrationVM register for the Apps ModalPopped event. When it fires pop the registration page as well
Application.Current.ModalPopped += (sender, e) =>
{
await Navigation.PopModalAsync();
};

If you try this, remove this line from your LoginVM await Navigation.RemoveAsync(previousVM);

Deos69 · July 2019

Thanks for your answer!

First, I can't write MainPage = new NavigationPage(new HomeVM());
HomeVM is is not recognized

I have tried :

Application.Current.ModalPopped += (sender, e) =>
{
await Navigation.PopModalAsync();
};

It works for me cause Registration is the first Page I think, so with this code, i'm waiting LoginVM closure and RegistrationVM close at his turn.

Graverobber · July 2019

Great glad it worked.

Don't forget to unsubscribe the event in Registration OnDisappearing.

protected override void OnAppearing()
{
    Application.Current.ModalPopped += OnPop;
    base.OnAppearing();
}

protected override void OnDisappearing()
{
    Application.Current.ModalPopped -= OnPop;
    base.OnDisappearing();
}

protected void OnPop(object sender, ModalPoppedEventArgs e)
{
    await Navigation.PopModalAsync();
}

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