How to implement dialog architecture in MVVM

Question

I am developing a WPF 4.0 - MVVM application based on PRISM framework (Unity Container).

I was wondering what is the best way to implement dialogs in the mvvm pattern. I am planning to use quite a few in my application so I want something reusable.

Answer 1

Since you are using Prism/Unity implement the mediator pattern for your View Models.

1. Add a DialogService (IDialogService) module to your project.
2. Modules containing dialogs register them with the IDialogService. Don't forget to declare DialogServiceModule as a ModuleDependency.

3. ViewModels now use the IDialogService to show the required dialog.

   public interface IDialogService
   {
       void    RegisterDialog  (string dialogID, Type type);
       bool?   ShowDialog      (string dialogID);
   }
   
   public class DialogService : IDialogService
   {
       private IUnityContainer       m_unityContainer;
       private DialogServiceRegistry m_dialogServiceRegistry;
   
       public DialogService(IUnityContainer unityContainer)
       {
           m_unityContainer = unityContainer;
           m_dialogServiceRegistry = new DialogServiceRegistry();
       }
   
       public void RegisterDialog(string dialogID, Type type)
       {
           m_dialogServiceRegistry.RegisterDialog(dialogID, type);
       }
   
       public bool? ShowDialog(string dialogID)
       {
           Type type = m_dialogServiceRegistry[dialogID];
           Window window  = m_unityContainer.Resolve(type) as Window;
           bool? dialogResult = window.ShowDialog();
   
           return dialogResult;
       }
   }
   

If you use ViewModel events & handlers in the View, use the WeakEventHandler pattern to eliminate a potential resource leak. Also, it is possible for multiple Views to be attached to the same ViewModel. I've worked on projects with one ViewModel -> one View. But also one ViewModel -> multiple Views. Just something to consider when making your design decisions.