Use MVVM Light's Messenger to Pass Values Between View Model

Question

Could someone be so kind as to explain MVVM Light's Messenger for me? I was reading a post on StackOverflow here: MVVM pass values between view models trying to get this. The documentation on MVVM Light's not that great at this point so I'm completely unsure where to go.

Say I have two ViewModels and a ViewModelLocator. I want to be able to pass parameters between all three without issue. How would I go about doing this with the messenger? Is it capable of that?

Edit: Here's my new implementation. As of now, it looks as if MessengerInstance doesn't call for a token. I'm terribly confused.

In the first ViewModel:

MessengerInstance.Send<XDocument>(SelectedDocument);

And in the second:

MessengerInstance.Register<XDocument>(this, xdoc => CopySettings(xdoc));

Could be completely wrong. Haven't gotten a chance to test it, but visual studio gets less angry with me when I do it this way. Also the MessengerInstance does register before the Message is sent.

Answer 1

Say I have two ViewModels and a ViewModelLocator. I want to be able to pass parameters between all three without issue. How would I go about doing this with the messenger? Is it capable of that?

That's exactly what it's for, yes.

To send a message:

MessengerInstance.Send(payload, token);

To receive a message:

MessengerInstance.Register<PayloadType>(
    this, token, payload => SomeAction(payload));

There are many overloads, so without knowing exactly what you're trying to accomplish via the messenger, I won't go into all of them, but the above should cover the simple case of wanting to send and receive a message with a payload.

Note that "token" can be really anything that identifies the message. While a string is often used for this, I prefer to use an enum because it's a little safer and enables intellisense, "find usages", etc.

For example:

public enum MessengerToken
{
    BrushChanged,
    WidthChanged,
    HeightChanged
}

Then your send/receive would be something like:

// sending view model
MessengerInstance.Send(Brushes.Red, MessengerToken.BrushChanged);

// receiving view model

// put this line in the constructor
MessengerInstance.Register<Brush>(this, token, brush => ChangeColor(brush));

public void ChangeColor(Brush brush)
{
    Brush = brush;
}

[EDIT] URL to devuxer's comment below changed to: http://blog.galasoft.ch/posts/2009/09/mvvm-light-toolkit-messenger-v2/