MVVM Light Messaging Made Absurdly Easy

I wanted to send a message from a ViewModel to its View so that themegaphone1 View could pop up a dialog box.  To do this, I used MVVM Light’s messaging bus.

At first, this seemed difficult because I was over thinking it.  It turns out to be painfully easy.

In the ViewModel I created a NotificaitonMessage.  You can pass any kind of object through the Message Bus, but to keep things simple, I used a string as my token.  All I had to do was instantiate a NotificationMessage object and then call a static method on the supplied  Messenger class:

var myMessage = new NotificationMessage("change");
 Messenger.Default.Send(myMessage);

This sends off my message like a message in a bottle.  The sender (my ViewModel) has no idea if the message will be received by any other class (ViewModel or View).

In the View I registered to receive this notification.  To do so, I put one line in my constructor,

 Messenger.Default.Register<NotificationMessage> (this, NotifyMe);

NotifyMe is a delegate, pointing to a method named NotifyMe.  You could, of course, just use a lambda expression.

NotifyMe receives a parameter of type NotificationMessage.  That message has a property Notification which contains the object you sent (e.g., “change”).

       public void NotifyMe (NotificationMessage message)
       {
          string token = message.Notification;  // "change"
          DisplayAlert ("Test", "Hi!", "OK");
       }

In the code shown, I extract the string “change” but I don’t do anything with it.  I just extracted it to show how it is done.

That’s it.  I’ve successfully told the View to display an alert by firing off a message in the ViewModel.  Handy, quick, easy.

 

 

About Jesse Liberty

Jesse Liberty is a Senior Consultant with Wintellect, with three decades of experience writing and delivering software projects. He is the author of 2 dozen books and multiple Pluralsight & LinkedIn Learning courses, and has been a Senior Technical Evangelist for Microsoft, a Distinguished Software Engineer for AT&T, a VP for Information Services for Citibank and a Software Architect for PBS. He is a Xamarin Certified Mobile Developer and a Xamarin MVP and a Microsoft MVP.
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