Capture lists in closures in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a capture list in Swift closures?

A capture list is a way to define how variables are captured inside a closure. It lets you specify whether to capture variables strongly, weakly, or unowned to control memory management and avoid retain cycles.

beginner

Why do we use capture lists in closures?

We use capture lists to avoid strong reference cycles (retain cycles) by controlling how variables are captured. For example, capturing a variable as weak or unowned prevents the closure from keeping the variable alive forever.

beginner

How do you write a capture list in a Swift closure?

You write a capture list inside square brackets before the closure parameters. For example: {[weak self] in ...} captures self weakly inside the closure.

intermediate

What is the difference between capturing a variable as weak and unowned in a capture list?

Capturing as weak means the variable can become nil and is optional inside the closure. Capturing as unowned means the variable is expected to always exist and is non-optional, but if it doesn't exist, the program will crash.

intermediate

Show an example of a capture list preventing a retain cycle with self in a closure.

Example:

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

Here, self is captured weakly to avoid a retain cycle between the class and the closure.

What does the capture list [weak self] do inside a Swift closure?

A Prevents the closure from running

B Captures self strongly to keep it alive

C Makes self optional outside the closure

D Captures self weakly to avoid retain cycles

The capture list [weak self] captures self weakly inside the closure to avoid strong reference cycles.

Where do you place the capture list in a Swift closure?

A At the end of the function

B Inside square brackets before the closure parameters

C Inside the closure body

D After the closure body

Capture lists are placed inside square brackets before the closure parameters.

What happens if you capture a variable as unowned but it becomes nil?

A The program crashes

B The variable becomes optional

C The closure ignores the variable

D The variable is captured weakly instead

Capturing as unowned assumes the variable always exists; if it becomes nil, the program crashes.

Why might you want to use a capture list in a closure?

A To control how variables are captured and avoid memory leaks

B To make the closure run faster

C To change the closure's return type

D To make the closure synchronous

Capture lists help control variable capture to avoid retain cycles and memory leaks.

Which of these is a correct capture list syntax?

A { weak self in print(self) }

B { (weak self) -> Void in print(self) }

C { [weak self] in print(self) }

D { [self weak] in print(self) }

The correct syntax uses square brackets: { [weak self] in ... }

Explain what a capture list is and why it is important in Swift closures.

Think about how closures keep references to variables and how that can cause problems.

Describe the difference between capturing a variable as weak versus unowned in a closure's capture list.

Consider safety and lifetime guarantees of the captured variable.

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