RelayCommand not firing CanExecute

Question

I'm trying to create a command that binds to a button, and depends on a hierarchy of viewmodels to represent a pivot and pivotitems in a wp7 app. It goes something like this (i'll omit as much boilerplate code as possible):

Parent viewmodel :

    public RelayCommand RefreshCommand
    {
        get
        {
            return _refreshCommand;
        }

        set
        {
            if (_refreshCommand == value)
            {
                return;
            }

            var oldValue = _refreshCommand;
            _refreshCommand = value;

            // Update bindings, no broadcast
            RaisePropertyChanged(RefreshCommandPropertyName);
        }
    }


    public RelayCommand<RoutedEventArgs> LoadedCommand
    {
        get
        {
            Action<RoutedEventArgs> getCurrentViewmodel = (args) =>
            {
                CurrentViewModel = SearchByLocationViewModel; // maybe something better...
                RefreshCommand = CurrentViewModel.RefreshResultsCommand;
                RefreshCommand.RaiseCanExecuteChanged();
            };
            return new RelayCommand<RoutedEventArgs>(getCurrentViewmodel);
        }
    }

    public RelayCommand<SelectionChangedEventArgs> PivotItemChangedCommand
    {
        get
        {
            Action<SelectionChangedEventArgs> pivotChanged = (args) =>
            {
                if (args != null)
                {
                    // update current viewmodel
                    CurrentViewModel = ((Microsoft.Phone.Controls.PivotItem) args.AddedItems[0]).DataContext as ISearchViewModelBase;
                    RefreshCommand = CurrentViewModel.RefreshResultsCommand;
                }
            };
            return new RelayCommand<SelectionChangedEventArgs>(pivotChanged);
        }
    }

So, i'm setting the parent view model's RefreshCommand property to the child command each time it loads, and I do the same when the selected pivotitem changes.

Xaml

<Button Command="{Binding RefreshCommand}" />

Child viewmodel

public Func<bool> CanRefresh
{
    get { return () => !IsLoading; }
}

private RelayCommand refreshResultsCommand;

public RelayCommand RefreshResultsCommand
{
    get
    {
        Action doRefresh = () =>
        {
            SearchResults.Clear();
            this._pageNumber = 0;
            AddItems();
        };
        return refreshResultsCommand = refreshResultsCommand ?? new RelayCommand(doRefresh, CanRefresh);
    }
}

What happens is, the CanRefresh is NEVER called. The commands work fine, depending on the pivotitem it calls one AddItems (that's a virtual method implemented differently in each child viewmodel) or the other, but it's like CanExecute is never raised. I'm using MVVM Light toolkit. Thanks!

When you say 'the CanRefresh is NEVER called', do you mean the CanRefresh property, or the lambda within it? How can you tell that it is never called? — Luke Woodward

Luke Woodward Luke Woodward · Accepted Answer · 2012-02-18T17:41:43

I suspect that the CanExecute method is never being called because the CanExecuteChanged event of the RelayCommand is never being fired on a view-model object that the view-layer is currently bound to. Since you're creating commands on most property accesses, there are a lot of commands about, and it's quite possible that you're firing the event on a command that isn't yet bound to by the view-layer or one that is no longer bound to.

I've never seen anyone create new commands every time the property getter is called. Could you try creating the commands once in a constructor instead of on every property access instead, and see if that fixes things?

RelayCommand not firing CanExecute

2 Answers