MvvmCross ensures garbage collection for event subscriptions primarily through the use of weak references. Here's a detailed explanation of how it works:
Overview of MvvmCross Messenger
The MvvmCross Messenger plugin is designed to facilitate event aggregation between components. It allows components to communicate by publishing and subscribing to messages. The Messenger uses weak references for event subscriptions, which helps prevent memory leaks by allowing the garbage collector to remove objects that are no longer referenced.
Weak References in MvvmCross
When you subscribe to a message using the MvvmCross Messenger, you can specify whether to use a weak or strong reference. By default, MvvmCross uses weak references (`MvxReference.Weak`) for subscriptions. This means that the Messenger does not hold a strong reference to the callback method, allowing the garbage collector to remove the subscription if the object that subscribed goes out of scope.
Subscription and Unsubscription
Subscriptions can be managed using several methods:
- Subscribe: Messages are passed directly on the publishing thread.
- SubscribeOnMainThread: Messages are marshaled to the main UI thread.
- SubscribeOnThreadPoolThread: Messages are queued for thread pool processing.
Each subscription method returns a `MvxSubscriptionToken`, which can be used to unsubscribe from the message. However, if the token is not stored, the subscription may be garbage collected immediately.
Garbage Collection Mechanism
The garbage collection mechanism in MvvmCross relies on the `MvxSubscriptionToken` being garbage collected when it goes out of scope. If a subscription token is not stored, it will be eligible for garbage collection, which can lead to the unsubscription of the event handler. This approach ensures that objects are not kept in memory unnecessarily due to event subscriptions.
Observing Subscription Status
MvvmCross also provides methods to observe the current subscription status, such as `HasSubscriptionsFor` and `CountSubscriptionsFor`. Additionally, the Messenger publishes `MvxSubscriberChangeMessage` messages whenever subscriptions are added, removed, or expire, allowing components to adapt their behavior based on the current subscription state.
Best Practices for Memory Management
To ensure effective garbage collection and prevent memory leaks:
- Use weak references for event subscriptions whenever possible.
- Store subscription tokens if you need to manage subscriptions explicitly.
- Unsubscribe from events when they are no longer needed, especially if using strong references.
- Avoid keeping strong references to objects that should be garbage collected, such as view models or views.
By following these practices and leveraging MvvmCross's weak subscription mechanism, developers can help ensure that event subscriptions do not prevent objects from being garbage collected, thus maintaining efficient memory management in their applications.
Citations:[1] https://www.mvvmcross.com/documentation/plugins/messenger
[2] https://stackoverflow.com/questions/39315395/mvvmcross-best-way-to-raise-event-in-viewmodel
[3] https://stackoverflow.com/questions/12494804/helping-the-gc-in-mono-droid-using-mvvmcross
[4] https://gregshackles.com/weak-subscription-gotcha-in-mvvmcross/
[5] https://github.com/MvvmCross/MvvmCross/issues/2145
[6] https://livebook.manning.com/book/xamarin-in-action/b-using-mvvm-light-instead-of-mvvmcross/v-15/
[7] https://stackoverflow.com/questions/30798429/mvvmcross-android-viewmodel-never-garbage-collected
[8] https://www.mvvmcross.com/documentation/advanced/customizing-using-App-and-Setup
[9] https://www.spicelogic.com/Blog/net-event-handler-memory-leak-16