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MessagingCenter_in_Xamarin_Forms.md

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Messaging Center

Description:

MessagingCenter allows us to publish/send a message from one part of the code and receive it in another from a subscriber/listener. They work independently of each other. With message publishers not needing to know of any pre-existing listeners, and similarly, subscribers not needing to be aware of any extra potential senders. The only thing that each component cares about is the actual message string that is being sent or looked for to receive.

One of the neat things about the MessagingCenter is that it allows us to pass data to previous pages as well. Microsoft's .Net MAUI will implement a way to do this but until upgrading to .Net MAUI from Xamarin.Forms the best way to pass data to previous pages in a form of "backwards navigation" is to utilize the MessagingCenter.

How we utilize it:

We use MessagingCenter in the RightToAsk application as a way to indicate where we want to navigate to on future pages, based on which pages the user has previously come from. In the app there are many ways in which the user can start creating an account. Depending on where they came from previously determines where we would like to place them back into the flow of the application after finishing the account creation process so that it appears as seamless as possible.

This requires the use of a ContineWith() method call because we need to complete the navigation to the next page so that the page is created. Most of our Subscribers are made in the constructors of a page's ViewModel. If we try to send the message before a subcriber is created, the message will not be received. Thus, the ContinueWith() method allows us to send the message after we create and push the new page onto the navigation stack.

Example Publisher:

MessagingCenter.Send(this, "MessageToSend");

In these examples "this" is the instance of the ViewModel that the message is being sent from. That line could also be written as the following:

MessagingCenter.Send<NameOfViewModel>(this, "MessageToSend");

Though shorthand generally forgoes the part.

Example Subscriber:

MessagingCenter.Subscribe<NameOfViewModel>(this, "MessageToSend", (sender) =>
{
  // Do something here
  CameFromMainPage = true;
  // unsubscribe
  MessagingCenter.Unsubscribe<NameOfViewModel>(this, "MessageToSend");
});

Here the name of the ViewModel in <> is supposed to be the ViewModel where you expect the message to be coming from. The "MessageToSend" is the incoming message you are looking for from that particular ViewModel location

Why bother Unsubscribing?

Depending on where the Subscriber is created, it could end up getting created multiple times. For example, in the RTA app we create the message listeners in a ViewModel's constructor. Every time we navigate to a new page we create a new instance of that page, which also creates a new instance of the ViewModel that the page's data is bound to (except in the case of our singleton view models such as the QuestionViewModel.Instance). This means that there will be X number of listeners created for the same X number of times we have navigated to that page, which also means that whatever we do within that listener also occurs that many times... For some cases this isn't a problem, but it can slow down the application if not cleaned up.

Here's what a sampe unsubscribe line looks like:

MessagingCenter.Unsubscribe<NameOfViewModel>(this, "MessageToSend");

I've found that one of the easiest ways to manage this, is to unsubscribe from the message inside of the actual subscriber, after we've finished doing what we want to with the data from the message.

MessagingCenter.Subscribe<NameOfViewModel>(this, "MessageToSend", (sender) =>
{
  // Do something here
  CameFromReadingPage = true;
  // unsubscribe
  MessagingCenter.Unsubscribe<NameOfViewModel>(this, "MessageToSend");
});

Example with Args:

If you wish to pass an object with data along with the message, you may send it as an argument/parameter in the following manner:

MessagingCenter.Send<NameOfViewModel, ObjectTypeToSend>(this, "MessageToSend", objectValueToSend);

MessagingCenter.Send<NameOfViewModel, bool>(this, "MessageToSend", true);

MessagingCenter.Subscribe<NameOfViewModel, bool>(this, "MessageToSend", (sender, arg) =>
{
  // Do something here
  var data = arg.ToString();
});

Link To Microsoft Docs:

https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center