Why Closures causing retain cycles in Swift? - Purpose & Use Cases

The Big Idea

Discover why your app might be stuck holding on too tightly and how to gently let go!

The Scenario

Imagine you have two friends who keep calling each other endlessly without stopping. In programming, this happens when two parts of your code keep holding on to each other, never letting go.

The Problem

When closures and objects hold strong references to each other, they create a loop. This means the memory they use never gets freed, causing your app to slow down or crash. Manually tracking and fixing these loops is tricky and error-prone.

The Solution

Understanding how closures capture variables and using tools like weak or unowned references helps break these loops. This way, your app uses memory efficiently and runs smoothly without unexpected crashes.

Before vs After

✗ Before

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

✓ After

class Person {
  var closure: (() -> Void)?
  func setup() {
    closure = { [weak self] in print(self) }
  }
}

What It Enables

It enables your app to manage memory smartly, preventing leaks and keeping performance high.

Real Life Example

Think of a photo app where a photo object holds a closure to update UI, and the closure holds the photo object back. Without fixing this, the app never frees memory, causing slowdowns.

Key Takeaways

Closures can hold strong references causing memory loops.

These loops prevent memory from being freed, hurting app performance.

Using weak or unowned references in closures breaks these loops.

Practice

1. What is the main reason closures cause retain cycles in Swift?

easy

A. Closures capture self strongly, preventing deallocation

B. Closures do not capture variables at all

C. Closures always run on a background thread

D. Closures automatically break retain cycles

Solution

1. Step 1: Understand closure capturing behavior

   Closures in Swift capture variables, including self, strongly by default.

2. Step 2: Recognize impact on memory

   If a closure captures self strongly and self holds the closure, neither can be released, causing a retain cycle.

3. Final Answer:

   Closures capture self strongly, preventing deallocation -> Option A

4. Quick Check:

   Strong capture causes retain cycle = B [OK]

Hint: Closures keep strong references by default, causing retain cycles [OK]

Common Mistakes:

• Thinking closures do not capture variables
• Assuming closures run on background threads
• Believing closures automatically break retain cycles

2. Which of the following is the correct syntax to declare a closure capturing self weakly in Swift?

easy

A. let closure = { weak self in print(self) }

B. let closure = { (weak self) -> Void in print(self) }

C. let closure = { [weak self] in print(self) }

D. let closure = { [self weak] in print(self) }

Solution

1. Step 1: Recall capture list syntax

   In Swift, capture lists are declared inside square brackets before the closure body.

2. Step 2: Identify correct weak capture syntax

   The correct syntax is [weak self] inside the capture list, followed by the closure code.

3. Final Answer:

   let closure = { [weak self] in print(self) } -> Option C

4. Quick Check:

   Capture list uses [weak self] = C [OK]

Hint: Use square brackets for capture lists like [weak self] [OK]

Common Mistakes:

• Using parentheses instead of brackets for capture list
• Writing weak self without brackets
• Swapping order to [self weak]

3. Consider this Swift code snippet:

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

What is the main problem with this code?

medium

A. It runs without any memory issues

B. It causes a syntax error due to missing capture list

C. It prints nil because self is not captured

D. It causes a retain cycle because closure captures self strongly

Solution

1. Step 1: Analyze closure capturing self

   The closure assigned to the property captures self strongly by default.

2. Step 2: Understand retain cycle effect

   Since self holds closure and closure holds self, neither is released, causing a retain cycle and memory leak.

3. Final Answer:

   It causes a retain cycle because closure captures self strongly -> Option D

4. Quick Check:

   Strong capture causes retain cycle = A [OK]

Hint: Closures capturing self strongly cause retain cycles unless weak/unowned used [OK]

Common Mistakes:

• Assuming no retain cycle without capture list
• Thinking it causes syntax error
• Believing closure prints nil

4. Given this code, what is the best fix to avoid the retain cycle?

class ViewController {
    var completion: (() -> Void)?
    func configure() {
        completion = {
            print(self)
        }
    }
}

medium

A. Call configure() twice

B. Use [weak self] in the closure capture list

C. Declare completion as a strong reference

D. Remove the closure property entirely

Solution

1. Step 1: Identify cause of retain cycle

   The closure captures self strongly, causing retain cycle with the property.

2. Step 2: Apply weak capture to break cycle

   Adding [weak self] in the capture list makes self a weak reference, breaking the cycle.

3. Final Answer:

   Use [weak self] in the closure capture list -> Option B

4. Quick Check:

   Weak capture breaks retain cycle = D [OK]

Hint: Add [weak self] to closure capture list to fix retain cycles [OK]

Common Mistakes:

• Removing closure property instead of fixing capture
• Making completion strong reference (already strong)
• Calling configure multiple times doesn't fix cycle

5. You have a class with a closure property that captures self unowned:

class Worker {
    var task: (() -> Void)?
    func start() {
        task = { [unowned self] in
            print("Working with \(self)")
        }
    }
}
var w: Worker? = Worker()
w?.start()
w = nil

What happens when w is set to nil and why?

hard

A. Worker instance is deallocated safely; unowned does not retain self

B. Runtime crash occurs because closure accesses deallocated self

C. Memory leak occurs due to strong capture

D. Nothing happens; closure is never called

Solution

1. Step 1: Understand unowned capture behavior

   Unowned references do not increase retain count but expect self to exist while closure runs.

2. Step 2: Analyze deallocation when w = nil

   Setting w to nil releases Worker instance; no retain cycle exists because closure holds unowned self.

3. Final Answer:

   Worker instance is deallocated safely; unowned does not retain self -> Option A

4. Quick Check:

   Unowned capture allows safe deallocation = A [OK]

Hint: Unowned captures don't retain but expect self alive during use [OK]

Common Mistakes:

• Confusing unowned with weak causing runtime crash
• Assuming strong capture causes leak here
• Thinking closure runs automatically on deallocation