Can the Memory Graph tool help identify retain cycles in MvvmCross applications

Understanding Retain Cycles

Retain cycles occur when two or more objects hold strong references to each other, preventing them from being deallocated from memory. This is a common issue in iOS development, especially when using delegates or closures that capture objects strongly.

Using the Memory Graph Tool

1. Running the App and Taking Snapshots: To use the Memory Graph tool, run your MvvmCross application in Xcode and navigate through its core flows. Take memory snapshots at different points, especially after navigating away from views or view models that should be deallocated.

2. Analyzing Snapshots: In the Memory Graph Debugger, the left panel displays all objects currently in memory, along with their instance counts. Look for objects that should have been deallocated but are still present. An increase in the number of instances of a class over time can indicate a retain cycle.

3. Identifying Retain Cycles: Select an object in the left panel to view its object graph. The graph shows the references that keep the object in memory. Bold lines indicate strong references, while light gray lines indicate unknown references (which could be weak or strong). If you see a loop where objects reference each other strongly, it's likely a retain cycle.

4. Fixing Retain Cycles: Once you've identified a retain cycle, review your code to find where the strong references are being created. Often, this involves changing strong references to weak references, especially in delegates or closures.

Applying to MvvmCross

In MvvmCross applications, view models are tightly coupled to the lifecycle of views. This can sometimes lead to retain cycles if not managed properly. For example, if a view model holds a strong reference to a view, and the view also holds a strong reference to the view model (perhaps through a binding), a retain cycle can occur.

To avoid this in MvvmCross, ensure that any references from views to view models are weak. Use lifecycle methods provided by MvvmCross (like `Disappeared`) to clean up any strong references when views are dismissed. Additionally, use the Memory Graph tool to verify that view models are being deallocated correctly when their associated views are removed from the navigation stack.

Example of Identifying Retain Cycles in MvvmCross

Suppose you have a `MyViewModel` that is bound to a `MyView`. If `MyView` holds a strong reference to `MyViewModel`, and `MyViewModel` also holds a strong reference to `MyView` (perhaps through a delegate or a closure), you would see both objects remaining in memory even after navigating away from `MyView`. Using the Memory Graph tool, you would identify this cycle and then modify your code to use weak references where necessary.

By following these steps and using the Memory Graph tool, you can effectively identify and resolve retain cycles in MvvmCross applications, ensuring that your app's memory usage remains efficient and stable.