Capture lists in closures in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of closure capturing a variable

What is the output of this Swift code snippet?

Swift

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

A 10

B 20

C Compilation error

D Runtime error

💡 Hint

Think about when the value of x is captured by the closure.

The closure captures the value of x at the time of its creation because of the capture list [x]. Later changes to x do not affect the captured value.

❓ Predict Output

intermediate

Effect of capture list on reference type

What will this Swift code print?

Swift

class Counter {
    var count = 0
}
let counter = Counter()
let closure = { [counter] in
    print(counter.count)
}
counter.count = 5
closure()

A 5

B Runtime error

C Compilation error

D 0

💡 Hint

Consider how reference types behave when captured in closures.

The capture list captures the reference to the Counter instance, not a copy. So changes to counter.count are visible inside the closure.

🔧 Debug

advanced

Why does this closure cause a retain cycle?

Examine the code below. Why does it cause a retain cycle?

Swift

class Person {
    var name: String
    lazy var greet: () -> Void = {
        print("Hello, \(self.name)!")
    }
    init(name: String) {
        self.name = name
    }
}
let p = Person(name: "Alice")
p.greet()

A The closure does not capture self, so no retain cycle occurs.

B The lazy property causes a syntax error.

C The closure captures name by value, causing a retain cycle.

D The closure captures self strongly, causing a retain cycle.

💡 Hint

Think about how closures capture self by default in classes.

The closure captures self strongly by default, and since self owns the closure, this creates a retain cycle preventing deallocation.

📝 Syntax

advanced

Correct capture list syntax

Which option shows the correct syntax for capturing a variable weakly in a closure?

Swift

var value = 10
let closure = { /* capture list here */ in
    print(value)
}

A { [weak var value] in print(value) }

B { [value weak] in print(value) }

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

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

💡 Hint

The capture list syntax uses square brackets with capture specifiers before the variable name.

The correct syntax is [weak value]. Option C is invalid syntax, C is invalid because 'var' is not used in capture lists, and D captures self, not value.

🚀 Application

expert

Predict the output with multiple captures and mutation

What is the output of this Swift code?

Swift

var a = 1
var b = 2
let closure = { [a, b] in
    print("a = \(a), b = \(b)")
}
a = 3
b = 4
closure()

A a = 3, b = 2

B a = 1, b = 2

C a = 1, b = 4

D a = 3, b = 4

💡 Hint

Captured values are copied at closure creation time.

The closure captures copies of a and b at the time of its creation, so later changes do not affect the captured values.

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