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How do you implement a messaging/event aggregator to decouple view models?

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Use CommunityToolkit.Mvvm's WeakReferenceMessenger to publish and subscribe to messages without direct references, avoiding memory leaks from strong event subscriptions. One view model sends a message; others register handlers. Unregister when appropriate, though weak references reduce leak risk.
WeakReferenceMessenger.Default.Send(new ItemAddedMessage(item));

WeakReferenceMessenger.Default.Register<ItemAddedMessage>(this,
    (r, m) => Items.Add(m.Value));
Real-world example Adding an item on an edit page notifies the list page to refresh via a message, with neither page holding a reference to the other.

Common follow-ups: Why weak references? | When should you unregister?

MVVM Toolkit Dependency Injection Performance & Optimization
All 7 Dependency Injection & Services questions