The goal of MVVM is to abstract away your Views from your Business Logic. This ensures great code reuse, testability, and is pretty awesome. Many MVVM Frameworks offer tools to enhance this such as data binding and dependency services to make our lives easier. These are built into Xamarin.Forms, which is awesome, but one feature less talked about is the Messaging Center. It’s entire goal is to enable ViewModels or other components to communicate with each other without having to know anything about each other besides a simple Message contract.
So for instance, let’s say you are in a master/detail setup where your MasterViewModel has a list of items and your DetailViewModel allows you to create a new item, update an item, or delete an item. When your user is on the detail page and triggers an event you need to somehow message back to your MasterViewModel that has a list of Items so the UI can react on the Master page when we navigate back.
This is where the MessagingCenter comes in. And enables anyone to “Subscribe” to message that has arguments and then enables anyone to send a message with arguments and have the Messaging Center handle the communication. This is what it looks like in image form:
So let’s say our MasterViewModel subscribes to “Update” and “AddNew” message events. It will then update it’s observable collection based on when it receives messages. Our DetailViewModel would then send a message in our SaveCommand to notify anyone that is subscribed to these specific messages:
There you have it, messaging made easy! Don’t forget to unsubscribe if you no longer wish to receive notifications.