In MvvmCross, as in other frameworks, the choice between using WeakReference and StrongReference is crucial for managing memory and preventing memory leaks. Here's a detailed explanation of the differences and how they apply in the context of MvvmCross:
Strong References
**Strong References are the default type of reference in most programming languages, including C#. When an object is referenced strongly, the garbage collector cannot collect it until all strong references to the object are removed. This means that as long as there is at least one strong reference to an object, it will remain in memory.
In MvvmCross, strong references can lead to memory leaks if not managed properly. For example, if a ViewModel holds a strong reference to a View, and the View also holds a strong reference to the ViewModel, a retain cycle is created. This prevents both objects from being garbage collected even when they are no longer needed.
Weak References
**Weak References, on the other hand, allow the garbage collector to collect the referenced object if no strong references exist. This is particularly useful in scenarios where you want to allow an object to be garbage collected if it is no longer needed, but still want to access it if it happens to be available.
In MvvmCross, using weak references is encouraged to avoid retain cycles between ViewModels and Views. For instance, instead of having a ViewModel strongly reference a View, you can use a weak reference or a messaging system to communicate between them. This ensures that the View can be garbage collected when it is no longer needed, even if the ViewModel still exists.
Practical Example in MvvmCross
Consider a scenario where a ViewModel needs to notify a View when some data is loaded. A common approach is to use an event in the ViewModel that the View subscribes to. However, if the View subscribes strongly to this event, it creates a strong reference from the ViewModel back to the View, which can prevent the View from being garbage collected.
To avoid this, MvvmCross provides methods like `WeakSubscribe`, which allows the View to subscribe to the ViewModel's events weakly. This way, the ViewModel does not hold a strong reference to the View, preventing memory leaks.
Guidelines for Using Weak References in MvvmCross
- Use Weak References for Views: When a ViewModel needs to interact with a View, use weak references or messaging systems to avoid retain cycles.
- Avoid Strong References to Views: Ensure that ViewModels do not hold strong references to Views to allow Views to be garbage collected when they are no longer needed.
- Use Messaging Systems: Consider using messaging systems like MvvmCross's `IMvxMessenger` to communicate between ViewModels and Views without creating strong references.
By following these guidelines, you can effectively manage memory in MvvmCross applications and prevent common issues like memory leaks.
Citations:[1] https://stackoverflow.com/a/19379912
[2] https://forums.swift.org/t/unowned-references-have-more-overhead-than-strong-references/72765
[3] https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/weak-references
[4] https://www.reddit.com/r/swift/comments/8ifwy9/ios_mvvm_breaking_retainer_cycle_between/
[5] https://stackoverflow.com/questions/40966271/weak-reference-and-strong-reference
[6] https://langdev.stackexchange.com/questions/1351/approaches-for-implementing-weak-references
[7] https://putridparrot.com/blog/weakreferences-in-net/
[8] http://java-latte.blogspot.com/2014/07/difference-between-StrongReference-WeakReference-SoftReference-PhantomReference-in-java.html