MVVM Toolkit light Messenger chained Messages - mvvm

Question

this might be complicated to explain but I give it a try...
I would like to use the Messenger to navigate to a new Page and also create a new Object (or pass one). How is this possible or am I on the wrong path?
Basically:
Click on "add new person" button which should bring up the PersonView and also should hold a new instance of a person object.
Click on "add person" button which should bring up the same PersonView page and should receives the object which is selected.
Message 1 = open Uri / Message 2 send exisiting or new object.
So far I have MainPageViewModel which sends
Messenger.Default.Send<Uri>(...)...
And MainPage.cs which Registers Messenger.Default.Register<Uri>(...)and executes
Frame.Navigate(...targetUri)....
I tryed to Send a message to the PersonViewModel right after Frame.Navigate... but this runs out of sync... so the page was not loaded to receive the PersonMessage,...
So any tips, tricks, licks, approaches would be greate...
Thanks...

Answer 1

Hope this helps, basicaly it is a simple Singleton class that gets the navigation frame the page that contains, after that you are able to use it in your viewmodel and navigate, and get notified when the page changes, so with this you control in a better way the navigation
and send messages, and get aware about your page status.
public class NavigationFrameController {
private static NavigationFrameController _instance;
private MainPage _root;
public Frame NavFrame { get; set;}
private static object keyLock = new Object();
NavigationFrameController() {
_root = (MainPage)Application.Current.RootVisual;
NavFrame = _root.ContentFrame;
NavFrame.Navigated += new NavigatedEventHandler(ContentFrame_Navigated);
NavFrame.NavigationFailed += new NavigationFailedEventHandler(ContentFrame_NavigationFailed);
}
public static NavigationFrameController Instance {
get {
if (_instance == null)
lock (keyLock) {
_instance = new NavigationFrameController();
}
return _instance;
}
}
public void NavigateTo(Uri uri) {
NavFrame.Source = uri;
}
private void ContentFrame_Navigated(object sender, NavigationEventArgs e) {
//send your message
// get attached to this event and get notified
}
private void ContentFrame_NavigationFailed(object sender, NavigationFailedEventArgs e) {
}

Answer 2

I had the same problem - essentially where you receive the message to open a new window also create the viewmodel and add it to the view as datacontext. When you instantiate your viewmodel pass in the existing object or null etc... then in your viewmodel you can handle whether its a new or existing object.
If you are using dependency injection then call a resolve from the codebehind where you process the 'add person' messsage etc..

Answer 3

I think what you should do is keep the first message for navigation, and add to it the information about the object (person) you want to send. you can add a parameter to the query string, say "add=true" and then you can create the object normally in the view model, or the ID of the object to edit, and in this case, the viewmodel can retrieve the object itself and edit it.
To achieve this, in the code behind of the PersonView (associated with the PersonViewModel) has to send a message upon navigation (OnNavigatedTo) to its ViewModel containing the received info from the query string.