ViewModel with async Command

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Hi guys,

If I have a ViewModel with an async method, how can I define a Command to bind to a button in my View?
The problem I see is that when I click the Login button, I am not navigated from my login View to my main view, but if I click somewhere on the page, then the navigation happens indeed. If I remove async from the method, this works from the first click.
I am reading in the docs about Application.Update(this), but I don’t think it is reasonable to call Application.Update(this) from the ViewModel, what “this” should be in that case?

Is this even possible, or am I trying in vain?

Best regards,
Alex

For example, I have a LoginViewModel with the following method.

private async Task LoginAsync()
    {
        try
        {
            IsBusy = true;
            ErrorMessage = string.Empty;
            var success = await _authService.LoginAsync(Username, Password);
            if (!success)
                ErrorMessage = “Invalid username or password.”;
            // If success, AuthenticationService will raise event and ApplicationViewModel will handle shell swap
        }
        catch (Exception ex)
        {
            ErrorMessage = $”Login failed: {ex.Message}”;
        }
        finally
        {
            IsBusy = false;
        }
    }

And I am trying to define my command in the constructor:

public LoginViewModel(IAuthenticationService authService)
    {
        _authService = authService;
        LoginCommand = new Command(
            execute: async () => { await LoginAsync(); },
            canExecute: () => !IsBusy && !string.IsNullOrWhiteSpace(Username) && !string.IsNullOrWhiteSpace(Password));
    }

and in my LoginView:

btnLogin.Command = _vm.LoginCommand;

 

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You’re correct that the call to Application.Update does not have to happen in the ViewModel.

Here’s a sample showing how to do async commands in Wisej MVVM.

– The async method should be in the ViewModel,
– Navigation or shell swapping should happen in an application-level ViewModel reacting to authentication state
– Any Wisej-specific Application.Update(…) usage should be hidden in infrastructure that restores/pushes the Wisej context rather than called from the ViewModel itself.

– In the sample, look at:
– LoginViewModel.cs for the async login logic
– ApplicationViewModel.cs for shell swapping (MainViewModel and LoginViewModel are the shells)
– WisejAsyncCommand.cs and WisejUiDispatcher.cs for the Wisej-aware command/context handling
– WisejUIDispatcher for the call to Application.Update
– Window1.cs for the shell host.

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MVVMAsyncSample.zip
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Dear Julie,
thanks for your answer.

So you mean in the definition of the LoginCommand in the ViewModel’s constructor:

LoginCommand = new Command(
            execute: async () => { var context = Application.Current; await LoginAsync(); Application.Update(context); },…

However I find it a bit strange, in the sense I would expect my ViewModel to be unaware of Wisej’s Application.Current etc. What if I wanted to use the same ViewModel for a WinForms application and not a Wisej one?

Best regards,
Alex

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Application.Update(this) works when this is a Wisej control. But in your case, this is the ViewModel. So don’t use Application.Update(this). Instead, you can save the context before the await by doing:

var context = Application.Current.

Then when you need to update the UI you can do:

Application.Update(context);

More information on Application.Current: https://docs.wisej.com/api/wisej.web/general/application#current

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