Advanced Swift: Retain Cycles

What causes a retain cycle?

Swift uses Automatic Reference Counting (ARC) for memory management. Under ARC, memory is allocated to an instance of a class when it’s created and deallocated when there are no more references to it. But sometimes, two or more instances refer to each other, creating a loop that can't be unlinked - a scenario termed as a "retain cycle".

class Person {

    var dog: Dog?

}

class Dog {

    var owner: Person?

}

In this example, a Person might have a Dog, and a Dog might have an Owner. If the Person owns a Dog and that Dog refers to the Person as its owner, then we have a simple retain cycle. Neither the Person nor the Dog instance will ever have their reference counts go to zero, and thus, will never be deallocated.

var personInstance: Person? = Person()
var dogInstance: Dog? = Dog()

personInstance?.dog = dogInstance
dogInstance?.owner = personInstance

These are the foundational blocks of a retain cycle. But the solution isn't merely avoiding mutual references. It's about proper management of them. This is where weak and unowned references come into play.

Weak references can make strong code.

Weak and unowned references are tools Swift gives us to break these cycles. A weak reference allows one instance to refer to another without incrementing its reference count. An unowned reference is similar, but with some key differences we'll soon address.

class Person {

    var dog: Dog?

}

class Dog {

    weak var owner: Person?

}

By declaring the owner variable in the Dog class as a weak reference, we’ve effectively broken the retain cycle. Now, when an instance of Person is de-initialized, the dog instance's owner reference will automatically be set to nil, allowing the dog instance to be de-initialized as well.

That said, using weak references can lead to their own issues. The main one being that since they can be set to nil, they have to be optional. This can lead to unexpected behavior if you're not anticipating a nil value. Unowned references are a way to work around this issue. An unowned reference is similar to a weak reference in that it doesn't increase the reference count, but it's guaranteed to always have a value.

class Person {

    var dog: Dog?

}

class Dog {

    unowned var owner: Person

}

However, unowned references can be dangerous. Since they’re guaranteed to always have a value, trying to access an unowned reference after the instance it refers to has been deallocated will result in a runtime error. Therefore, unowned references should only be used when you’re absolutely sure that the reference will always be valid during its lifetime.

Now that we've talked about the problem and its solutions, let's take a step back and think about why retain cycles are important to understand. A retain cycle might not seem like a big deal in a simple two-object scenario, but consider a complex application where objects create strong references to each other in a web-like structure. Over time, the memory taken up by these objects could grow and lead to sluggish performance or even crashes due to memory pressure. A deep understanding of retain cycles is crucial in preventing these types of issues.

Moreover, understanding retain cycles and their implications helps you write more efficient and cleaner code. It encourages you to think critically about the relationships between objects: Is an owner-ownership relationship appropriate here? Is it okay if this reference is nil at some point? These considerations lead to code that's easier to understand, maintain, and debug.

[ Zach Coriarty ]