MessagingCenter

Xamarin.Forms has a built-in messaging mechanism to promote decoupled code. This way, view models and other components do not need to know each other. They can communicate by a simple messaging contract.

There a basically two main ingredients for using the MessagingCenter.

Subscribe; listen for messages with a certain signature (the contract) and execute code when a message is received. A message can have multiple subscribers.

Send; sending a message for subscribers to act upon.

Simple example

Here we will see a simple example of using the MessagingCenter in Xamarin.Forms.

First, let's have a look at subscribing to a message. In the FooMessaging model we subscribe to a message coming from the MainPage. The message should be "Hi" and when we receive it, we register a handler which sets the property Greeting. Lastly this means the current FooMessaging instance is registering for this message.

public class FooMessaging
{
    public string Greeting { get; set; }

    public FooMessaging()
    {
        MessagingCenter.Subscribe<MainPage> (this, "Hi", (sender) => {
            this.Greeting = "Hi there!";
        });
    }
}

To send a message triggering this functionality, we need to have a page called MainPage, and implement code like underneath.

public class MainPage : Page
{
    private void OnButtonClick(object sender, EventArgs args)
    {
        MessagingCenter.Send<MainPage> (this, "Hi");
    }
}

In our MainPage we have a button with a handler that sends a message. this should be an instance of MainPage.

Passing arguments

You can also pass arguments with a message to work with.

We will use the classed from our previous example and extend them. In the receiving part, right behind the Subscribe method call add the type of the argument you are expecting. Also make sure you also declare the arguments in the handler signature.

public class FooMessaging
{
    public string Greeting { get; set; }

    public FooMessaging()
    {
        MessagingCenter.Subscribe<MainPage, string> (this, "Hi", (sender, arg) => {
            this.Greeting = arg;
        });
    }
}

When sending a message, make sure to include the argument value. Also, here you add the type right behind the Send method and add the argument value.

public class MainPage : Page
{
    private void OnButtonClick(object sender, EventArgs args)
    {
        MessagingCenter.Send<MainPage, string> (this, "Hi", "Hi there!");
    }
}

In this example a simple string is used, but you can also use any other type of (complex) objects.

Unsubscribing

When you no longer need to receive messages, you can simply unsubscribe. You can do it like this:

MessagingCenter.Unsubscribe<MainPage> (this, "Hi");

When you are supplying arguments, you have to unsubscribe from the complete signature, like this:

MessagingCenter.Unsubscribe<MainPage, string> (this, "Hi");