wpf mvvm passing parameters between viewmodels using commands

Question

This is my first attempt at MVVM. My application's core is loosely based Josh Smith's msdn article. And I am also using the mvvm light framework.

I have a main window containing a command list area and a workspace area which shows usercontrols/views as tabitems, each usercontrol has a corresponding viewmodel. The mainWindow also has a viewmodel containing my command list, and the workspace viewmodels have a base workspace viewmodel.

My default view has a master datagrid, of MappingSets, that can have one selected item. The commands launch new tabitems with views that handle MappingSet detail based on that selected item. I have a View/ViewModel that, depending on the command used should return either a tabitem for creating a new MappingSet with no existing data, or a tabitem containing the detail of the selected item for editing, or a tabitem containing detail the selected item as the base for a new MappingSet.

Having Set the scene, what I have not managed to work out is command dependent way to pass parameters, such as the identifier of the selected MappingSet object, to instantiate my viewmodel in one of the three states mentioned above? For instance would the mvvmlight messenger be appropriate for this task?

Answer 1

This is a perfect scenario for the messenger/eventaggregator. However, your message chain might be a bit convoluted. From what I'm understanding, your Main window holds a list of commands (like a menu or a ribbon). Here is how I see the chain of events.

1. You select a MappingSet from the datagrid, this causes a MappingSetSelected message to be fired (with a payload of the selected MappingSet)
2. The main window listens for that message and stores the currently selected MappingSet
3. When the user clicks the button a "EditMappingSet" or "CreateNewMappingSet" message is fired (or if the Window is responsible for creating the new views, it creates them itself).

Answer 2

If there are only three options, you could have them binding to three different commands and within the commands do the passing of your self-defined variable.

private RelayCommand _openMappingSetCommand;

//Command that one of your options is bound to
public ICommand ViewMappingSetOption1
    {
        get
        {
            if (_openMappingSetCommand == null)
            {
                _openMappingSetCommand = new RelayCommand(param => this.DoTabRequest("your parameter");
            }
            return _openMappingSetCommand ;
        }
    }


// Method that creates your viewmodel
private void DoTabRequest(parameterType parameter)
    {
        WorkspaceViewModel viewModel = null;

        if (viewModel == null)
        {
            viewModel = (WorkspaceViewModel)Activator.CreateInstance(typeof (viewModelType), parameter);
            this.Workspaces.Add(viewModel);
        }

        this.ActiveWorkspace = viewModel;
    }

Then allow for that parameter on the constructor of your viewmodel and do whatever else you need based on that.