Understanding Retain Cycles in Swift iOS

A Guide to Understanding and Preventing Memory Leaks in Your iOS Apps

A retain cycle occurs when two (or more) objects hold strong references to each other, preventing ARC (Automatic Reference Counting) from deallocating them even when they should no longer exist. This results in a memory leak because the reference count for each object never reaches zero.

How a Retain Cycle Happens

Consider two classes, Person and Apartment, where a Person has an apartment property and an Apartment has a tenant property. If both properties are strong references (the default in Swift), they hold on to each other, creating a cycle.

Example Code

  class Person {
    let name: String
    var apartment: Apartment?  // Strong reference to Apartment

    init(name: String) {
        self.name = name
    }

    deinit {
        print("\(name) is being deinitialized")
    }
}

class Apartment {
    let unit: String
    var tenant: Person?  // Strong reference to Person

    init(unit: String) {
        self.unit = unit
    }

    deinit {
        print("Apartment \(unit) is being deinitialized")
    }
}

// Creating instances
var john: Person? = Person(name: "John")
var unit4A: Apartment? = Apartment(unit: "4A")

// Setting up the mutual references (retain cycle!)…