I know I'm doing something wrong with RaiseCanExecuteChanged and CanExecute

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();
    }
}

On further thought, it's not even working properly. It enables the button... but then it doesn't disable it. — Cowman, Sep 04 '12 at 03:47
If I use MVVM light's RelayCommand instead of DelegateCommand... things seem to work fine. Could this really be the root of the problem? If so, that seems rather silly. — Cowman, Sep 04 '12 at 04:28

score 0 · Accepted Answer · answered Sep 04 '12 at 05:53

0

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.

answered Sep 04 '12 at 05:53

Vale

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It doesn't seem to call it though, if I'm using a Microsoft Prism DelegateCommand with a MVVM Light EventToCommand. It only works if I change out DelegateCommand with MVVM Light's RelayCommand. I couldn't find anything in the code that would cause this, but sure enough that's how it works. Any idea why? – Cowman Sep 04 '12 at 06:04
I have never used EventToCommand. Maybe you have to use RelayCommand. But looks like a bug to me... It should work. – Vale Sep 04 '12 at 08:09

I know I'm doing something wrong with RaiseCanExecuteChanged and CanExecute

1 Answers1