Question

Well after fiddling with MVVM light to get my button to enable and disable when I want it to… I sort of mashed things together until it worked.

However, I just know I’m doing something wrong here. I have RaiseCanExecuteChanged and CanExecute in the same area being called. Surely this is not how it’s done?

Here’s my xaml

<Button Margin="10, 25, 10, 25" VerticalAlignment="Center" HorizontalAlignment="Center" Width="50" Height="50" Grid.Column="1" Grid.Row="3" Content="Host">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Click">
            <mvvmLight:EventToCommand Command="{Binding HostChat}" MustToggleIsEnabled="True" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Button>

And here’s my code

public override void InitializeViewAndViewModel()
{
    view = UnityContainer.Resolve<LoginPromptView>();
    viewModel = UnityContainer.Resolve<LoginPromptViewModel>();
    view.DataContext = viewModel;
    InjectViewIntoRegion(RegionNames.PopUpRegion, view, true);

    viewModel.HostChat = new DelegateCommand(ExecuteHostChat, CanHostChat);
    viewModel.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(ViewModelPropertyChanged);
}

void ViewModelPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Name" || e.PropertyName == "Port" || e.PropertyName == "Address")
    {
        (viewModel.HostChat as DelegateCommand).RaiseCanExecuteChanged();
        (viewModel.HostChat as DelegateCommand).CanExecute();
    }
}

public void ExecuteHostChat()
{
}

public bool CanHostChat()
{
    if (String.IsNullOrEmpty(viewModel.Address) ||
        String.IsNullOrEmpty(viewModel.Port) ||
        String.IsNullOrEmpty(viewModel.Name))
    {
        return false;
    }
    else
        return true;
}

See how these two are together? Surely that can’t be right. I mean… it WORKS for me… but something seems wrong about it. Shouldn’t RaiseCanExecuteChanged call CanExecute? It doesn’t… and so if I don’t have that CanExecute in there, my control never toggles its IsEnabled like I need it to.

(viewModel.HostChat as DelegateCommand).RaiseCanExecuteChanged();
(viewModel.HostChat as DelegateCommand).CanExecute();

EDIT:

If I end up using the Command property of button to bind my command to… everything works fine. I can remove the CanExecute and just leave the RaiseCanExecuteChanged and everything works brilliantly.

Like this… this works just fine.

<Button Command="{Binding HostChat}" Margin="10, 25, 10, 25" VerticalAlignment="Center" HorizontalAlignment="Center" Width="50" Height="50" Grid.Column="1" Grid.Row="3" Content="Host">
</Button>

void ViewModelPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Name" || e.PropertyName == "Port" || e.PropertyName == "Address")
    {
        (viewModel.HostChat as DelegateCommand).RaiseCanExecuteChanged();
        //(viewModel.HostChat as DelegateCommand).CanExecute();
        //CommandManager.InvalidateRequerySuggested();
    }
}

Answer 1

CanExecute method only returns whether the command can be executed or not. It doesn’t do anything else. You only need RaiseCanExecuteChanged. RaiseCanExecuteChanged will call CanHostChat in your case.