Capture lists in closures in Swift - Mini Project: Build & Apply

Capture Lists in Closures

📖 Scenario: Imagine you are creating a simple Swift program that uses closures to remember values. Closures can capture variables from their surroundings. Sometimes, you want to control how these variables are captured to avoid unexpected changes.

🎯 Goal: You will build a Swift program that demonstrates how to use a capture list in a closure to keep a fixed value, even if the original variable changes later.

📋 What You'll Learn

Create a variable called number with the value 10

Create a closure called closure that captures number using a capture list

Change the value of number to 20 after the closure is created

Call the closure and print its result to show the captured value

💡 Why This Matters

🌍 Real World

Capture lists in closures are useful when you want to keep a snapshot of data at a certain moment, such as in UI event handlers or asynchronous code.

💼 Career

Understanding capture lists helps you write safer Swift code that avoids unexpected side effects, a valuable skill for iOS and macOS developers.

1

Create the initial variable

Create a variable called number and set it to 10.

Swift

# Your code here

Hint

Use var number = 10 to create the variable.

2

Create a closure with a capture list

Create a closure called closure that captures number using a capture list to keep its current value.

Swift

var number = 10
// Your code here

Hint

Use { [number] in return number } to capture number by value.

3

Change the variable after closure creation

Change the value of number to 20 after the closure is created.

Swift

var number = 10
let closure = { [number] in
    return number
}
// Your code here

Hint

Simply assign number = 20 after the closure.

4

Call the closure and print the result

Call the closure closure() and print its result using print().

Swift

var number = 10
let closure = { [number] in
    return number
}
number = 20
// Your code here

Hint

Use print(closure()) to show the captured value.

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