WPF Custom ICommand implementation and the CanExecuteChanged event

Question

in my WPF UI, I use RoutedCommands that I refer to in my xaml via the following code:

Command=\"viewModel:MessageListViewModel.DeleteMessagesCommand\"


        

Answer 1

Just implement the CanExecuteChanged event as follows:

public event EventHandler CanExecuteChanged
{
    add { CommandManager.RequerySuggested += value; }
    remove { CommandManager.RequerySuggested -= value; }
}

When you assign the command to a control, it subscribes to the CanExecuteChanged event. If you "redirect" it to the CommandManager.RequerySuggested event, the control will be notified whenever CommandManager.RequerySuggested is triggered.

Answer 2

I very much prefer the DelegateCommand implementation of Prism for viewmodel binding (http://msdn.microsoft.com/en-us/library/ff654132.aspx). You can invoke CanExecute() on every command invoker by calling RaiseCanExecuteChanged on it.

Simple usage example:

public class ViewModel
{
   public ViewModel()
   {
      Command = new DelegateCommand<object>(x => CommandAction(), x => CanCommandAction());
   }

   bool state;

   public void ChangeState(bool value)
   {
      state = value;
      Command.RaiseCanExecuteChanged();
   }

   public DelegateCommand<object> Command {get; private set;}

   private void CommandAction()
   {
      //do smthn
   }

   private bool CanCommandAction() { return true == state; }
}

//and binding as usual
Command="{Binding Command}"