Question

I am using Prism 2, trying to add four navigation buttons (First Record, Last Record, Previous Record, Next Record) in shell to be used by modules. I also want these buttons to be disable if active View/ViewModel does not provide these functions.

I tried using events but didn’t know how to achieve my second goal regarding disabling buttons. It seems I need to check current active View/ViewModel to see if they subscribed the click event during View switch. But I think publisher should be unaware of subscriber…

Somehow I tried my own way. I create an IDocNavigation interface which has four method corresponding to my four buttons. At runtime I check modules’ ViewModel if they implemented that interface or not, and change the ICommand on fly. Below is my code. I include one LastRecordCommand only:

public ShellViewModel(Views.Shell shell)
{
this.Shell = shell;
shell.DataContext = this;

shell.MainDocking.ActivePaneChanged += (s, e) =>
{
    if (e.NewPane.Content is UserControl &&
        ((UserControl)e.NewPane.Content).DataContext is IDocumentNavigate)
    {

        IDocumentNavigate vm = ((UserControl)e.NewPane.Content).DataContext as IDocumentNavigate;
        LastRecordCommand = new RelayCommand(x => vm.GotoLastRecord(), x => true);
    }
    else
    {
        LastRecordCommand = new RelayCommand(x => { }, x => false);
    }

};
//...

I feel these are quite ugly. Creating an empty RelayCommand is also stupid. How can I improve ? or how can I achieve disabling command if event is more suitable in my case ?

Answer 1

You can make use of CompositeCommand in prism.

Define a globally available CompositeCommand

public static readonly CompositeCommand FirstRecord= new CompositeCommand(true);

Then in your your module view models

    class Module1
     {


        public DelegateCommand Module1Firstrecord{ get; set; }
        Module1()
        {
          Module1Firstrecord = new DelegateCommand(this.FirstRecord, CanExecute);
        }

        private void FirstRecord()
        {
        //do whatever you want

        }

        private bool CanExecute()
        {
                 return true;
        }

        private void Module1_IsActiveChanged(object sender, EventArgs e)
        {
         //Find if your window is acive
         // if it is active Module1Firstrecord.IsActive = true    
         //else false.
        }
}

With IActiveAware you can handle the active window scenario easily. According to whether your active module have a handler for the command on not the buttons will enable/disable.