Capture lists in closures in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to capture the variable x in the closure.

Swift

let x = 10
let closure = { [[1]] in
    print(x)
}
closure()

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Alet

Bself

Cvar

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' instead of the variable name.

Forgetting to use square brackets.

Using 'var' or 'let' inside the capture list.

The capture list [x] captures the variable x inside the closure.

2fill in blank

medium

Complete the code to capture the variable count as a copy inside the closure.

Swift

var count = 5
let increment = { [[1] count] in
    print(count + 1)
}
count = 10
increment()

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Alet

Bvar

Cself

Dinout

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' which captures a mutable reference.

Using 'self' which is for capturing the instance.

Using 'inout' which is not valid in capture lists.

Using let count in the capture list captures a copy of count at the time the closure is created.

3fill in blank

hard

Fix the error by completing the capture list to avoid a strong reference cycle.

Swift

class Person {
    var name = "Alice"
    lazy var greet: () -> Void = { [[1] self] in
        print("Hello, \(self.name)!")
    }
}
let p = Person()
p.greet()

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Avar

Bweak

Cunowned

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' or 'let' which do not affect reference strength.

Using 'unowned' without understanding lifetime risks.

Not capturing 'self' at all, causing a strong reference cycle.

Using [weak self] in the capture list prevents a strong reference cycle by capturing self weakly.

4fill in blank

hard

Fill both blanks to capture variables a and b as copies inside the closure.

Swift

var a = 1
var b = 2
let closure = { [[1] a, [2] b] in
    print(a + b)
}
a = 10
b = 20
closure()

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Alet

Bvar

Cweak

Dunowned

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' which captures mutable references.

Using 'weak' or 'unowned' which are for class instances.

Using let for both a and b captures copies of their values at closure creation.

5fill in blank

hard

Fill all three blanks to capture self weakly and variables x and y as copies.

Swift

class Calculator {
    var x = 3
    var y = 4
    lazy var compute: () -> Int = { [[1] self, [2] x, [3] y] in
        return x * y + (self?.x ?? 0)
    }
}
let calc = Calculator()
print(calc.compute())

Drag options to blanks, or click blank then click option'

Aweak

Blet

Cvar

Dunowned

Attempts:

3 left

💡 Hint

Common Mistakes

Capturing self strongly causing retain cycles.

Using 'var' instead of 'let' for variables.

Using 'unowned' without ensuring self is always alive.

Capture self weakly to avoid retain cycles, and capture x and y as copies with let.