I have a boolean variable in my ViewModel and I want to observe any changes to it. ViewModel:

    internal class AuthenticationViewModel : ViewModelBase
    {
        APIClient apiClient;
        bool _loginStatus;

        public AuthenticationViewModel()
        {
        }

        public bool LoginStatus { get { return _loginStatus; } 
            set { 
                _loginStatus = value; 
                NotifyPropertyChanged("LoginStatus"); 
            } }
}
public class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected void NotifyPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

I am trying to use it in my View as following:

public MainWindow()
        {
            InitializeComponent();
            ViewModel = new AuthenticationViewModel();
            _ = ViewModel.GetReadyForUnlockWithBLEAsync();
            if(ViewModel.LoginStatus)
            {
                AuthenticationSuccess();
            }
        }

But I am not able to observe the variable from ViewModel. I can't get its updated value in View on any changes in ViewModel.

CodePudding user response：

The code in your MainWindow constructor checks the value of the LoginStatus property once after creating the view model. There is nothing that would register a PropertyChanged event, like for example a data binding.

You may manually register a PropertyChanged handler like shown below, and also await the awaitable method call in an async handler of the Window's Loaded event.

public MainWindow()
{
    InitializeComponent();

    ViewModel = new AuthenticationViewModel();

    ViewModel.PropertyChanged  = OnViewModelPropertyChanged;

    Loaded  = OnWindowLoaded;
}

private async void OnWindowLoaded(object sender, RoutedEventArgs e)
{
    await ViewModel.GetReadyForUnlockWithBLEAsync();
}

private void OnViewModelPropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == nameof(ViewModel.LoginStatus))
    {
        AuthenticationSuccess();
    }
}

Perhaps you don't need the PropertyChanged handling at all. Assuming that LoginStatus is updated by the GetReadyForUnlockWithBLEAsync method, this may also work:

public MainWindow()
{
    InitializeComponent();

    ViewModel = new AuthenticationViewModel();

    Loaded  = OnWindowLoaded;
}

private async void OnWindowLoaded(object sender, RoutedEventArgs e)
{
    await ViewModel.GetReadyForUnlockWithBLEAsync();

    AuthenticationSuccess();
}