Windows Phone App MVVM, updating UI from an async method in the viewmodel

Question

I have the following ViewModel

public class MainViewModel : ViewModelBase
{

    public bool Processing
    {
        get;
        set;
    }

    private IEnumerable<ExternalLoginModel> _authenticationProviders;
    /// <summary>
    /// 
    /// </summary>
    public IEnumerable<ExternalLoginModel> AuthenticationProviders
    {
        get
        {
            return _authenticationProviders;
        }
        set
        {
            _authenticationProviders = value;               
            RaisePropertyChanged(() => AuthenticationProviders);
        }
    }

    /// <summary>
    /// 
    /// </summary>
    public MainViewModel(IAuthenticationService authService)
    {
        Processing = true;
        authService.GetSupportedAuthenticationProvidersAsync().ContinueWith(authProviders => { PopulateAuthProviders(authProviders.Result); });
    }

    /// <summary>
    /// 
    /// </summary>
    /// <param name="authProviders"></param>
    private void PopulateAuthProviders(IEnumerable<ExternalLoginModel> authProviders)
    {
        AuthenticationProviders = authProviders;
        Processing = false;
    }
}

The property AuthenticationProviders is bound to a ListView. The problem is that once the AuthenticationProviders are populated through the Async method (via) the constructor the ListView does not get updated. I tried RaisePropertyChanged but then I get the error The application called an interface that was marshalled for a different thread. This makes sense as well since its actually calling from another thread. How do I fix this?

Why aren't you scheduling the continuation delegate to run on the calling thread, i.e. add TaskScheduler.FromCurrentSynchronizationContext() to ContinueWith and then call RaisePropertyChanged ? — Marko Devcic
– Marko Devcic, Commented Aug 13, 2014 at 12:19

user3547774 · Accepted Answer · 2014-08-14 04:25:03Z

2

This literally contains the answer

http://msdn.microsoft.com/en-us/magazine/dn630646.aspx

Basically you have to use

    DispatcherHelper.Initialize

And then whenever you need to update you need to use DispatcherHelper.CheckBeginInvokeOnUI

Given is the sample

    DispatcherHelper.CheckBeginInvokeOnUI(() =>
            {
                RaisePropertyChanged(() => AuthenticationProviders);
            });

answered Aug 14, 2014 at 4:25

user3547774

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