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.