MVVM Light - Passing Params to Target ViewModel Before Navigating

Overview
I use MVVM Light for my WP7 apps, it's great! I implement the following navigation service for controlling navigation from the view model layer:


http://blog.galasoft.ch/archive/2011/01/06/navigation-in-a-wp7-application-with-mvvm-light.aspx


Part 2 Here


The main problem I've faced is that I often want to pass parameters to the target view's view model to prepare it for what it needs to do. It is possible to access properties in the target view model through the ViewModelLocator as each view model has a static property used for the view binding via the non-static properties, however this means that the view models are tightly coupled through the VML and it seems like a bit of a hack.


The example code is from an article page which requires navigation to a links page.


Messaging
It's possible to fire a message out before navigating to pass parameters to the target view model, however, if the target view has never been hit, the associated vm will not not have instantiated so will not receive the message! This can be worked around by calling the VML CreateXXX method however this is still not the elegant solution I was hoping for.


2-Way Messaging
To solve this problem, I figured that when a VM instantiates, ask if any other VM has any parameters for it then register a message pipe for parameters when it is instantiated.

The Message
I created a message which would take a parameter to pass from the source to the target or an action to request a parameter from the source, by the target:

public class LinksInitMessage : MessageBase

{

    public Article Payload { get; set; }

    public Action<Article> Callback { get; set; }

    public LinksInitMessage(Article payload)

    {

        this.Payload = payload;

    }

    public LinksInitMessage(Action<Article> callback)

    {

        this.Callback = callback;

    }

}

Source VM
First off messages need registering (at constructor stage) so that the target VM may message back one it constructs:

private void RegisterMessenger()

{

    //Register any messages

    Messenger.Default.Register<LinksInitMessage>(this, msg =>

    {

        if (this._linksInitItem != null)

        {

            msg.Callback(this._linksInitItem);

            this._linksInitItem = null;

        }

    });

}

Then when the source VM is ready to navigate , it can send out a message and keep a parameter in case the VM calls back, then navigate:

private void Link()

{

    // Send message to Links (it will get it if it's there otherwise, the VM will request when open)

    Messenger.Default.Send<LinksInitMessage>(new LinksInitMessage(this._selectedItem));

    // Store article incase Links requests it

    this._linksInitItem = this._selectedItem;

    // Nav to links

    _navService.NavigateTo(ViewModelLocator.LinksUri);

} 

Target VM

First off when the VM constructs, it sends out a message to ask if any other VM has any a message for it, if it gets a response it can initialise itself. Second it registers a message type so once it is already instantiated, it can receive params again and re-initialise itself:

private void RegisterMessenger()

{

    //Send out a message to see if another VM has params we need

    Messenger.Default.Send<LinksInitMessage>(new LinksInitMessage(article =>

    {

        if (article != null)

        {

            this.SelectedItem = article;

        }

    }));

    //Register any message pipes

    Messenger.Default.Register<LinksInitMessage>(this, msg =>

    {

        this.ClearViewModel();

        this.SelectedItem = msg.Payload;

    });

}

Conclusion

That's it, seems like a nice loosely coupled way of initialising a view model on navigation.