Understanding Retain Cycles and Memory Leaks in iOS Swift and SwiftUI

A Comprehensive Guide to Recognizing, Resolving, and Preventing Memory Management Pitfalls

Retain cycles (also known as strong reference cycles) are a common source of memory leaks in iOS development. Whether you’re working with traditional Swift using UIKit or embracing the modern SwiftUI paradigm, understanding how retain cycles form and how to avoid them is essential for building efficient, leak-free applications.

What Is a Retain Cycle?

In Swift, memory management is largely handled through Automatic Reference Counting (ARC). ARC keeps track of how many strong references exist to an instance. When the count drops to zero, the instance is deallocated. A retain cycle occurs when two (or more) objects hold strong references to each other, preventing ARC from ever releasing them — even when they are no longer needed.

For example, if Object A holds a strong reference to Object B, and Object B, in turn, holds a strong reference to Object A, neither can be deallocated. This circular dependency creates a memory leak.

How Do Retain Cycles Occur?

1. Mutual Strong References Between Classes

Consider two classes that reference each other:

class Parent {
    var child: Child?
}

class Child {
    var parent: Parent?
}