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Discover how to effectively manage `Xamarin Forms ViewModels Inheritance` and resolve common problems with MVVM pattern implementations.
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Managing Xamarin Forms ViewModels Inheritance Effectively

In the world of Xamarin Forms development, especially for applications utilizing the Model-View-ViewModel (MVVM) pattern, efficient management of ViewModels is crucial. One common challenge developers face is how to implement inheritance among ViewModels, ensuring that shared properties and behaviors are accessible in derived classes. This guide will dive deep into a scenario where a developer encountered issues with property inheritance in their ViewModels and how they resolved the problem.

The Problem: Inheriting ViewModels in Xamarin Forms

When working on a Xamarin Forms application, particularly in a pet project focused on video games, the developer realized that their code had repetitive properties across multiple ViewModels. To combat this redundancy, they created a base ViewModel—ViewModelBase—that implements the INotifyPropertyChanged interface, making property notification more manageable. The developer then set up a specialized base ViewModel named GamesViewModel, from which specific ViewModels such as NewGamesViewModel and SearchViewModel inherit.

However, a critical problem arose when the developer attempted to use a property called SearchGame defined in the GamesViewModel. Despite successfully assigning a value in GamesViewModel, the derived ViewModels (NewGamesViewModel, SearchViewModel) reflected a null value for SearchGame. This issue was puzzling, especially since the derived ViewModels are expected to inherit all properties from their base class.

Why Did This Happen?

The crux of the issue lies in how the instances of the ViewModels were created and referenced in the application. When a new SearchGamePage is instantiated, it creates a new instance of SearchViewModel which does not maintain the state of properties from the GamesViewModel properly. Each instantiation results in a fresh SearchViewModel starting with default/null values, leading to the inherited property being null.

The Solution: Using MessagingCenter and Dependency Service

The developer found an elegant solution to this problem by leveraging Xamarin's MessagingCenter for communication between ViewModels and the DependencyService to manage ViewModel instances. Here’s how the solution unfolds:

Step 1: Sending Messages with MessagingCenter

Within the GamesViewModel, whenever a value for SearchGame is set, the developer sends a message that other ViewModels can subscribe to. Here's how it looks in code:

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This approach sends a message indicating that a "search_game" event has occurred, allowing other components that listen to this message to react accordingly.

Step 2: Subscribing to Messages in the GamesViewModel

In the constructor of GamesViewModel, the developer subscribes to messages using the MessagingCenter. This subscription updates the SearchGame property when messages are received:

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Step 3: Registering ViewModels Using DependencyService

To maintain the instance of the SearchViewModel within SearchGamePage, it is necessary to avoid creating new instances each time the page is instantiated. The solution implemented was to register the ViewModels in the application's App.xaml.cs:

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Step 4: Assigning BindingContext with DependencyService

Finally, in the SearchGamePage, the developer binds the SearchViewModel instance using the DependencyService, ensuring that the same instance is used:

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By following these steps, whenever SearchGame is updated in the GamesViewModel, the changes are reflected across all derived ViewModels, including Search