Closures causing retain cycles in Swift - Step-by-Step Execution

Concept Flow - Closures causing retain cycles

Create Object A

↓

Object A holds Closure

↓

Closure captures Object A strongly

↓

Strong reference cycle formed

↓

Neither Object A nor Closure deallocated

↓

Memory leak occurs

This flow shows how an object holding a closure that captures the object strongly creates a cycle, preventing deallocation.

Execution Sample

Swift

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

This code creates an object that holds a closure capturing itself strongly, causing a retain cycle.

Execution Table

Step	Action	Object References	Closure Capture	Result
1	Create MyClass instance 'obj'	obj: 1 strong reference	No closure yet	Object created
2	Call obj.setup()	obj: 1 strong reference	Closure assigned capturing 'self' strongly	Closure holds strong reference to obj
3	Closure stored in obj.closure	obj: 1 strong ref, closure: 1 strong ref	Closure captures obj strongly	Strong reference cycle formed
4	End of scope	obj and closure still hold each other	Closure still captures obj	Neither deallocated, memory leak
Exit	Program ends or obj set to nil	Cycle prevents deallocation	Strong retain cycle remains	Memory leak due to retain cycle

💡 Strong reference cycle prevents obj and closure from being deallocated

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
obj	nil	MyClass instance (1 strong ref)	Same (1 strong ref)	Same (1 strong ref + captured by closure)	Not deallocated
closure	nil	nil	Closure assigned capturing obj	Stored in obj.closure, captures obj strongly	Not deallocated

Key Moments - 3 Insights

Why does the closure capturing 'self' cause a retain cycle?

Because the closure is stored in the object and also captures the object strongly, they keep each other alive, as shown in execution_table step 3.

Why doesn't the object get deallocated after going out of scope?

Because the closure still holds a strong reference to the object, preventing deallocation, as seen in execution_table step 4.

How can we avoid this retain cycle?

By capturing 'self' weakly or unowned in the closure, breaking the strong reference cycle (not shown in this trace).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what references does the closure hold?

A A weak reference to obj

B No reference to obj

C A strong reference to obj

D A copy of obj

💡 Hint

Check the 'Closure Capture' column at step 3 in execution_table

At which step does the strong reference cycle form?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for when closure captures obj strongly in execution_table

If we change the closure to capture 'self' weakly, what would happen to the retain cycle?

A The retain cycle would be broken

B The retain cycle would still occur

C The closure would not capture anything

D The object would be deallocated immediately

💡 Hint

Refer to key_moments about breaking retain cycles by weak capture

Concept Snapshot

Closures causing retain cycles:
- Objects holding closures that capture 'self' strongly create cycles.
- This prevents deallocation, causing memory leaks.
- Use [weak self] or [unowned self] in closures to avoid cycles.
- Always check closure capture lists to manage memory.

Full Transcript

This visual execution trace shows how closures in Swift can cause retain cycles. When an object holds a closure that captures the object strongly, they keep each other alive. The execution table traces creating an object, assigning a closure that captures 'self' strongly, and how this forms a strong reference cycle. The variable tracker shows the references held by the object and closure over time. Key moments clarify why the cycle happens and how to avoid it by weak capturing. The quiz tests understanding of when and why the cycle forms and how to fix it. The snapshot summarizes the key points about closures and retain cycles in Swift.

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