Messanger differences in WP8 and Win8

Question

I have a simple questions why on these two platforms my code is working in different way. In first step I am sending a message, using command binded to the button in UI from MainViewModel.

private RelayCommand<Set> _setCommand;
    public RelayCommand<Set> SetCommand
    {
        get
        {
            return _setCommand
                ?? (_setCommand = new RelayCommand<Set>(
                    set =>
                    {
                        _navigationService.Navigate("QuestionView", set);
                        Messenger.Default.Send<Set, QuestionViewModel>(set);
                    }));
        }
    }

In the second step I register message in constructor in QuestionViewModel like this:

Messenger.Default.Register<Set>(this, Load);

In this solution I use PCL to sharing the code between platforms.

In the third step I would like to fire Load method, simple:

private void Load(Set set)
    {
        Load(set, 1);
    }

And in the Windows Store project everything works as I expected, but in the Windows Phone project the Load method is firied not at the first time, only at second. So I have to go to QuestionView - nothing, go back and enter again and now everything is working. Load method is fired. What is the problem?

Best, Tomasz

Answer 1

Here is the solution I found usefull: ViewModelLocator

#if NETFX_CORE //WINDOWS 8
        SimpleIoc.Default.Register<QuestionViewModel>();
#else //WINDOWS_PHONE
        SimpleIoc.Default.Register<QuestionViewModel>(true);
#endif

and registering services in App.xaml.cs like this:

 public App()
    {
        SimpleIoc.Default.Register<IQuestionService, QuestionService>();
    }

This solution is pretty nice no code-behind in Views and all functionalities of ViewModels are stored in PCL project.

Answer 2

The ViewModel behind QuestionView is probably not instantiated yet and can therefore not receive messages. Only after going to QuestionView the ViewModel gets instantiated (by the ViewModelLocator). If you want to send messages to the QuestionViewModel you should instantiate it immediately in the ViewModelLocator like this:

SimpleIoc.Default.Register<QuestionViewModel>(true);

I Hope this helps.