DelegateCommand 的 CanExecute 逻辑

Question

Update: The focus became MVVM instead of the actual question so I'm updating it.

I'm having a problem with CanExecute for DelegateCommand . It doesn't update before I call RaiseCanExecuteChanged , is this the desired behavior?

I uploaded a simple sample project reproducing this problem here : http://dl.dropbox.com/u/39657172/DelegateCommandProblem.zip

The problem is this, I have two Buttons like this. One is Binding Command to a RelayCommand implementation and the other is binding to the Prism implementation of DelegateCommand

<Button Command="{Binding DelegateSaveCommand}"/>
<Button Command="{Binding RelaySaveCommand}"/>

The ViewModel ICommands

DelegateSaveCommand = new DelegateCommand(Save, CanSaveDelegate);
RelaySaveCommand = new RelayCommand(param => Save(), param => CanSaveRelay);

and the CanExecute method/predicate

public bool CanSaveDelegate()
{
    return HasChanges;
}
public bool CanSaveRelay
{
    get { return HasChanges; }
}

Both are using the property HasChanges . When HasChanges is updated, only the CanSaveRelay updates. Is this the way it's meant to be?

Answer 1

As it already was mentioned, this is intended behavior of DelagateCommand , not a bug. DelegateCommand doesn't raise CanExecuteChanged event automatically, you have to raise that event manually by calling RaiseCanExecuteChanged when appropriate. Whereas RelayCommand relays on CommandManager.RequerySuggested event for that. This event is raised every time the user clicks somewhere or presses a button.

For situations when it is not very convenient or there is no appropriate place for calling RaiseCanExecuteChanged (like in your scenario you have to subscribe to PropertyChanged event on the model, etc) I have created the following simple wrapper that ensures that the CanExecute method of the wrapped command is executed automatically on CommandManager.RequerySuggested event:

public class AutoCanExecuteCommandWrapper : ICommand
{
    public ICommand WrappedCommand { get; private set; }

    public AutoCanExecuteCommandWrapper(ICommand wrappedCommand)
    {
        if (wrappedCommand == null) 
        {
            throw new ArgumentNullException("wrappedCommand");
        }

        WrappedCommand = wrappedCommand;
    }

    public void Execute(object parameter)
    {
        WrappedCommand.Execute(parameter);
    }

    public bool CanExecute(object parameter)
    {
        return WrappedCommand.CanExecute(parameter);
    }

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

You can use it like this:

DelegateSaveCommand = new AutoCanExecuteCommandWrapper(new DelegateCommand(Save, CanSaveDelegate));

Answer 2

If you want to stick to DelegateCommand you can use ObservesCanExecute :

DelegateSaveCommand = new DelegateCommand(Save, CanSaveDelegate).ObservesCanExecute(CanSaveDelegate);

Note that there is also ObservesProperty available if you are using a property for your CanExecute check. But then your property has to call NotifyPropertyChanged.

Answer 3

There is a bug in the DelegateCommand provided by Prism which doesn't raise the CanExecute event. I beat my head against the wall for a day until I dove into the DelegateCommand class provided by the Prism framework. I don't have the code with me, but I can post my resolution in a bit.

The alternative is to use one of the other RelayCommand frameworks out there.

Edit
Rather than reposting the code, there are other SO questions that provide resolutions:

• WPF-Prism CanExecute method not being called

And Kent B. has a good article: MVVM Infrastructure: DelegateCommand