Caliburn command evaluation only fired once

Question

I'm trying to enable a button on my view using Caliburns 'Can' convention for view model property evaluation.

View (excerpt)

<PasswordBox PasswordChanged="PasswordBox_OnPasswordChanged" Grid.Row="1" Grid.Column="1" />
...
<StackPanel Orientation="Horizontal" HorizontalAlignment="Right">
    <Button Content="Cancel" cal:Message.Attach="[Event Click] = [Action Cancel]" />
    <Button Content="Login" cal:Message.Attach="[Event Click] = [Action Login]" />
</StackPanel>

Code-behind

private void PasswordBox_OnPasswordChanged(object sender, RoutedEventArgs e)
{
    if (DataContext != null)
        ((dynamic) DataContext).Password = ((PasswordBox) sender).Password;
}

ViewModel

public class LoginSplashViewModel : Screen
{

    private string _username;
    private string _password;

    public string Username
    {
        get { return _username; }
        set
        {
            _username = value;
            NotifyOfPropertyChange();
        }
    }

    public string Password
    {
        get { return _password; }
        set
        {
            _password = value;
            NotifyOfPropertyChange();
        }
    }

    public LoginSplashViewModel()
    {
        DisplayName = "Login";
    }

    public bool CanLogin()
    {
        return  !string.IsNullOrEmpty(_username) || 
                !string.IsNullOrEmpty(_password);
    }

    public void Login()
    {
        TryClose(true);
    }

    public void Cancel()
    {
        TryClose(false);
    }

}

However the 'CanLogin()' method is only fired exactly once (when binding the view model to a view), and never again, thus the button staying disabled.

Am I missing something here?

Answer 1

 public string Password{
  get{ return _password;}
  set{
     _password = value;
     NotifyOfPropertyChange();
     NotifyOfPropertyChange(() => CanLogin);   // <--- Addition
   }
 } 

Likewise for UserName...

for what it's worth you don't have to the long form of the event either... you could do
<Button x:Name="Login" />

Answer 2

It is how the command is implemented in Caliburn. Basically there is a thing that can be called to force reevaluation of CanExecute methods in ICommand implementors .

Whenever there are changes that can be identified in code behind or viewModel. Also, you have an option to implement the command yourself and avoid this necessity.