Closures causing retain cycles in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a retain cycle in Swift closures?

A retain cycle happens when two objects hold strong references to each other, preventing them from being deallocated. In closures, this often occurs when a closure captures self strongly, and self holds the closure, causing a loop.

beginner

How can you prevent retain cycles caused by closures in Swift?

Use a capture list with [weak self] or [unowned self] inside the closure to capture self weakly or unowned, breaking the strong reference cycle.

intermediate

Explain the difference between [weak self] and [unowned self] in closure capture lists.

[weak self] creates an optional reference that can become nil if self is deallocated, so you must unwrap it safely. [unowned self] assumes self will never be nil during closure execution and is non-optional, but can cause a crash if self is deallocated.

beginner

What happens if you don’t use [weak self] or [unowned self] in a closure that captures self?

The closure holds a strong reference to self, and if self also holds the closure, they keep each other alive. This causes a memory leak because neither can be freed.

intermediate

Show a simple Swift example of a retain cycle caused by a closure capturing self strongly.

class MyClass {
  var closure: (() -> Void)?
  func setup() {
    closure = {
      print(self)
    }
  }
}

Here, self holds closure, and closure captures self strongly, causing a retain cycle.

What keyword can you use in a closure capture list to avoid a retain cycle by capturing self weakly?

A [strong self]

B [weak self]

C [unowned self]

D [static self]

[weak self] captures self weakly, preventing strong reference cycles.

What is the risk of using [unowned self] in a closure?

A It can cause a crash if self is nil when the closure runs.

B It makes self optional inside the closure.

C It creates a strong reference cycle.

D It automatically unwraps self safely.

[unowned self] assumes self is alive; if not, accessing it causes a crash.

Why do retain cycles cause memory leaks in Swift?

A Because closures cannot capture variables.

B Because weak references increase memory usage.

C Because objects keep each other alive with strong references.

D Because ARC disables deallocation.

Strong references between objects prevent ARC from freeing memory.

Which of these is NOT a way to break a retain cycle in closures?

A Making self a global variable.

B Using [weak self] in the capture list.

C Using [unowned self] in the capture list.

D Setting the closure to nil when done.

Making self global does not prevent retain cycles; it can worsen memory issues.

In the example: closure = { print(self) }, what causes the retain cycle?

A Closure is a global function.

B Closure captures self weakly.

C Closure does not capture self.

D Closure captures self strongly and self holds closure.

Self and closure hold strong references to each other, causing a cycle.

Explain what a retain cycle is and how closures can cause them in Swift.

Think about how two things holding strong references to each other can never be freed.

Describe how to use capture lists to prevent retain cycles in Swift closures and the difference between weak and unowned captures.

Focus on how to tell the closure to hold a weaker reference to self.