I am trying to get my head around MVVM and the Navigation-based project template in SL4. At the moment I am trying to move the ContentFrame_Navigated event handler into the ViewModel. Basically this event handler checks each hyperlink button in the menu bar against the current page and adjusts the style accordingly. To do this it seems I need to pass the EventArgs as well as another object. I see MVVM Light has the PassEventArgsToCommand bit, but what about passing another object/control? In this case it’s the StackPanel hosting the list of menu item hyperlinks. I’m just getting my head around the MVVM concept, what’s the best practice in this case?
Cheers,
Dany.
With MVVM you have to think more abstract. You are not daling with a list of HyperLink elements, but with a list of Navigation Targets. I.e. you should separate the presentation (HyperLink elemen) from the data (the URL, the Title, whether the item is selected or not, etc.). You now hold the data in a list on your ViewModel (normally you would see this data as the ViewModel of your Hyperlinks, and name it accordingly). The items are held in a ObservableCollection so that you can track changes if a item is added etc.
To display this list you can use a class inheriting from ItemsPresnter (e.g. a ListBox) and use binding to set the properties. Now you navigate to a page you can set the IsSelected property of the relevant item in the list, resetting all other IsSelected properties.
As the navigation targets can be seen as a global collection, you can also hold it in a property on the ViewLocator, so that all Views and ViewModel can access this list, and setting the IsSelectedProperty is applicable to all Views. This global collection represent you navigation state.
And, BTW, the EventToCommand, the RelayCommand, and the Command attribute ony support one parameter. Furthermore, from experience I can advise you that it is not good practice (although you obviously can do it) to mix View objects, such as EventArgs, or elements and your ViewModel.