Capture lists in closures in Swift - Step-by-Step Execution

Concept Flow - Capture lists in closures

Define variable x = 10

↓

Create closure with capture list [x

↓

Closure captures current x value

↓

Change x to 20

↓

Call closure

↓

Closure uses captured x (10), not current x (20)

↓

End

This flow shows how a closure captures a variable's value at creation using a capture list, preserving it even if the variable changes later.

Execution Sample

Swift

var x = 10
let closure = { [x] in
  print(x)
}
x = 20
closure()

This code creates a closure that captures x's value at 10, then changes x to 20, but the closure prints the captured 10.

Execution Table

Step	Action	Variable x	Closure Capture	Output
1	Initialize x	10	N/A	N/A
2	Create closure with capture list [x]	10	Captures x=10	N/A
3	Change x to 20	20	Captured x=10 (unchanged)	N/A
4	Call closure	20	Captured x=10	10
5	End	20	Captured x=10	N/A

💡 Closure uses captured x=10, ignoring current x=20

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
x	10	10	20	20
closure capture x	N/A	10	10	10

Key Moments - 2 Insights

Why does the closure print 10 instead of 20 after x changes?

Because the closure captured x's value at creation (step 2), it keeps that value (10) even though x later changes to 20 (step 3). See execution_table rows 2 and 4.

What if we omit the capture list [x] in the closure?

Without a capture list, the closure captures x by reference, so it would print the current value of x (20) when called. This differs from the capture list behavior shown in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what value does the closure capture at step 2?

A 10

B 20

C N/A

D Closure does not capture x

💡 Hint

Check the 'Closure Capture' column at step 2 in the execution_table.

At which step does variable x change to 20?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Variable x' column in execution_table to find when x changes.

If we remove the capture list [x], what will the closure print when called?

A Error

B 10

C 20

D Nothing

💡 Hint

Recall that without capture list, closure captures variable by reference, so it uses current x value at call time.

Concept Snapshot

Capture lists in closures:
- Syntax: { [var] in ... }
- Captures variable's value at closure creation
- Later changes to variable do not affect captured value
- Useful to fix variable state inside closure
- Without capture list, closure captures by reference

Full Transcript

This example shows how Swift closures can capture variables using capture lists. We start with variable x = 10. When we create a closure with [x], it captures the current value 10. Then we change x to 20. When we call the closure, it prints 10, the captured value, not 20. This behavior helps keep a fixed value inside the closure even if the original variable changes later. Without the capture list, the closure would print the current value of x when called.

Practice

1. What is the main purpose of using a capture list in a Swift closure?

easy

A. To control how variables are stored and referenced inside the closure

B. To define the return type of the closure

C. To specify the closure's parameter types

D. To make the closure run asynchronously

Solution

1. Step 1: Understand what capture lists do

   Capture lists let you specify how variables are captured inside closures, either by strong, weak, or unowned references.

2. Step 2: Identify the main purpose

   The main purpose is to control how variables are stored and referenced to avoid memory leaks or unexpected behavior.

3. Final Answer:

   To control how variables are stored and referenced inside the closure -> Option A

4. Quick Check:

   Capture list purpose = control variable capture [OK]

Hint: Capture lists manage variable references inside closures [OK]

Common Mistakes:

• Confusing capture lists with closure parameters
• Thinking capture lists define return types
• Assuming capture lists affect closure execution timing

2. Which of the following is the correct syntax to declare a capture list with a weak reference to self in a Swift closure?

easy

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

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

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

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

Solution

1. Step 1: Recall capture list syntax

   Capture lists are declared inside square brackets before the closure parameters, e.g., [weak self].

2. Step 2: Match correct syntax

   let closure = { [weak self] in print(self) } uses the correct syntax: [weak self] inside square brackets before the closure body.

3. Final Answer:

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

4. Quick Check:

   Capture list syntax = [weak self] [OK]

Hint: Capture lists go inside square brackets before closure body [OK]

Common Mistakes:

• Omitting square brackets around capture list
• Placing 'weak' outside the brackets
• Using parentheses instead of brackets

3. What will be the output of the following Swift code?

class Example {
    var value = 10
    lazy var closure: () -> Void = { [unowned self] in
        print(self.value)
    }
}
let example = Example()
example.value = 20
example.closure()

medium

A. 20

B. 10

C. Runtime error

D. nil

Solution

1. Step 1: Understand unowned self capture

   The closure captures self as unowned, meaning it references the original object without increasing retain count.

2. Step 2: Check value at closure call

   example.value is updated to 20 before calling closure, so print(self.value) outputs 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Unowned self captures current value = 20 [OK]

Hint: Unowned self captures current object state, not copy [OK]

Common Mistakes:

• Thinking closure captures old value copy
• Confusing unowned with weak causing nil
• Expecting runtime error without object deallocation

4. Identify the error in this Swift closure using a capture list:

class Test {
    var count = 0
    lazy var closure: () -> Void = { [weak self] in
        self.count += 1
        print(self.count)
    }
}

medium

A. No error, code is correct

B. Capture list syntax is incorrect

C. Closure cannot be lazy with capture list

D. Cannot use weak self without unwrapping optional

Solution

1. Step 1: Analyze weak self usage

   When capturing self as weak, self becomes an optional inside the closure.

2. Step 2: Check code for optional unwrapping

   The code uses self.count directly without unwrapping, causing a compile error.

3. Final Answer:

   Cannot use weak self without unwrapping optional -> Option D

4. Quick Check:

   Weak self requires optional unwrapping [OK]

Hint: Weak self is optional; unwrap before use [OK]

Common Mistakes:

• Using weak self as non-optional
• Ignoring optional chaining or guard let
• Assuming lazy closures can't have capture lists

5. You want to create a closure that captures a variable by making a copy of it, so changes outside the closure don't affect the closure's value. Which capture list syntax should you use in Swift?

hard

A. [weak variable]

B. [value = variable]

C. [unowned variable]

D. [copy variable]

Solution

1. Step 1: Understand capture by copy

   To capture a variable by making a copy, you assign it inside the capture list using syntax like [value = variable].

2. Step 2: Evaluate options

   [value = variable] uses the correct syntax to copy the variable's value into the closure's capture list.

3. Final Answer:

   [value = variable] -> Option B

4. Quick Check:

   Copy capture syntax = [value = variable] [OK]

Hint: Use [newName = oldVar] to capture a copy [OK]

Common Mistakes:

• Using weak or unowned which capture references
• Assuming [copy variable] is valid syntax
• Not assigning variable inside capture list