Mvvmcross: Multiple instances of viewmodels being created when navigating when using MvvmCross with Forms and Master-Detail

[nickrandolph]

To help us fix your issue, please provide the information in the below template.

Navigation parameters aren't being correctly passed in an Forms application when using Master-Detail. Navigation works when navigating from master but attempting to navigate between view models in the details pane doesn't work.

Steps to reproduce

1. Start with the TestProjects - Forms - MasterDetailExample

2. Add navigation parameters class
   public class CustomParms
   {
   public string Var1 { get; set; }
   }

3. Add command to Option1ViewModel - this will trigger navigation to Option2ViewModel, passing a parameter
   private MvxAsyncCommand goCommand;
   public MvxAsyncCommand GoCommand { get {
   return goCommand ?? (goCommand = new MvxAsyncCommand(() =>
   Navigation.Navigate(new CustomParms { Var1 ="Hello World!" })));
   }
   }

4. Add button to Option1Page to invoke command
   

5. Alter Option2ViewModel to implement IMvxViewModel
   public class Option2ViewModel : MvxViewModel, IMvxViewModel

6. Add Initialize method to Option2ViewModel
   public async Task Initialize(CustomParms parameter)
   {
   Message = parameter.Var1;
   await Task.FromResult(0);
   }

7. Run application, select Option1 from menu and then click on button to attempt to navigate to Option2

Expected behavior

Application should navigate the details pane to Option2ViewModel/Option2Page and the label on the page should update with the text "Hello World!"

Actual behavior

Application navigates the details pane to Option2ViewModel/Option2Page but the label does not *update

Configuration

Version: 5.0.3

Platform:

• [x] iOS
• [x] Android
• [x] UWP

[nickrandolph]

The LoadViewModel in MvxPresenterHelpers (MvvmCross.Forms.Presenters) seems to be generating a duplicate view model. Wondering if that's where the issue is??

[willsam100]

I have found the same problem, but just on navigation using the new IMvxNavigtionService. I have attached a reproduction.

As @nickrandolph said, a duplicate view model is created, the first one with the details, while the second one does not, resulting in the UI showing the duplicate viewModel without the details.

MvvmCross5repro.zip

[Cheesebaron]

Please no zip blobs. Make a new repository and chuck in the code there

[jimbobbennett]

@Cheesebaron - I've uploaded the reproduction from @willsam100 to https://github.com/jimbobbennett/MvvmCross5BugRepro

[dpalat]

I created another repo in case you need it https://github.com/dpalat/App.Template

[dgzornoza]

I have the same problem.
Is there any resolution?

[dgzornoza]

i think the problema is here:

public static class MvxPresenterHelpers
{
    public static IMvxViewModel LoadViewModel(MvxViewModelRequest request)
    {
        var viewModelLoader = Mvx.Resolve<IMvxViewModelLoader>();
        var viewModel = viewModelLoader.LoadViewModel(request, null);
        return viewModel;
    }

...

Alwais recreate VM instance.

[laeckerv]

I also was hunting this bug in my application and found a bug report on the official Xamarin tracker, so this issue might not be MvvmCross related.

Viewmodel instantiates twice using a ViewModelLocator

[dgzornoza]

Versión 5.1.0 is ok

[BradVause]

Hi guys, I upgraded to 5.1.0 today - and the view model is still instantiated twice.

[martijn00]

@BradVause Are you using the NavigationService? This should be fixed in this line: https://github.com/MvvmCross/MvvmCross/blob/develop/MvvmCross-Forms/MvvmCross.Forms/Presenters/MvxPresenterHelpers.cs#L22

[laeckerv]

@BradVause : I can confirm that the bug is fixed for me in 5.1.0. (using NavigationService)

[alexpetin]

5.1.1 - The duplication occurs in your example - https://github.com/martijn00/MvxForms
Under Android - start app, "back" to close and start again - the page duplicated.