Retain cycle

A retain cycle, also known as a reference cycle, occurs when two or more objects hold strong references to each other, preventing them from being deallocated. This can lead to memory leaks in your application.

In the context of Swift, retain cycles often occur with closures. By default, closures capture references strongly, which can create reference cycles. For example, if a closure captures a reference to an object that also holds a reference to the closure, neither can be deallocated.

To break these cycles, you can use weak or unowned references. Weak references do not increase the reference count and are set to nil when the referenced object is deallocated. Unowned references also do not increase the reference count but are not set to nil, so they should only be used when you are certain the referenced object will outlive the reference.

Here are some relevant segments from the WWDC sessions that discuss retain cycles and how to resolve them:

1. Analyze heap memory:

   • Resolving leaks of Swift closure contexts
   • Comparing performance of weak and unowned

2. A Swift Tour: Explore Swift’s features and design:

   • Classes

3. Explore Swift performance:

   • Memory allocation

For a detailed explanation on how to identify and resolve retain cycles using the Xcode memory graph debugger, you can refer to the session Analyze heap memory.