Weak self and unowned self patterns in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to capture self weakly inside the closure.

Swift

someAsyncFunction { [[1] self] in
    print(self?.description ?? "No self")
}

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

Aweak

Bunowned

Cstrong

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'unowned' when self might be nil causes crashes.

Forgetting to capture self weakly leads to retain cycles.

Using weak captures self weakly, preventing retain cycles and allowing self to become nil.

2fill in blank

medium

Complete the code to safely unwrap weak self inside the closure.

Swift

someAsyncFunction { [weak self] in
    guard let [1] = self else { return }
    print([1].description)
}

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

AweakSelf

BselfStrong

Cself

DstrongSelf

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name 'self' for the constant causes confusion.

Not unwrapping weak self leads to optional usage errors.

Assigning self to a strong constant like strongSelf inside the closure safely unwraps weak self.

3fill in blank

hard

Fix the error in the closure capture list to avoid a retain cycle.

Swift

someAsyncFunction { [[1] self] in
    print(self.description)
}

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

Aunowned

Bweak

Cstrong

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'strong' causes retain cycles.

Using 'weak' requires optional unwrapping.

Using unowned captures self without increasing retain count and assumes self will not be nil.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters and maps with weak self.

Swift

let results = items.compactMap { item in
    guard let [1] = self else { return nil }
    return item.value [2] 10 ? item.key : nil
}

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

AstrongSelf

B>

C<

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' directly without unwrapping weak self.

Using the wrong comparison operator.

Unwrap weak self as strongSelf and use the greater than operator to filter values above 10.

5fill in blank

hard

Fill both blanks to create a closure that captures unowned self and uses it safely.

Swift

someAsyncFunction { [[1] self] in
    let message = self.[2] + " is ready"
    print(message)
}

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

Aunowned

Bstatus

C)

Dweak

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'weak' requires optional unwrapping.

Forgetting to close the print statement causes syntax errors.

Capture self as unowned, access the status property, and close the print statement with a parenthesis.

Practice

1. What is the main reason to use weak self inside a closure in Swift?

easy

A. To make self immutable inside the closure

B. To guarantee self will never be nil inside the closure

C. To prevent strong reference cycles and allow self to become nil

D. To create a copy of self for thread safety

Solution

1. Step 1: Understand strong reference cycles

   Strong reference cycles happen when closures capture self strongly, causing memory leaks.

2. Step 2: Role of weak self

   Using weak self makes the reference to self optional and allows it to become nil, breaking the cycle.

3. Final Answer:

   To prevent strong reference cycles and allow self to become nil -> Option C

4. Quick Check:

   Weak self avoids memory leaks = A [OK]

Hint: Weak self means self can be nil inside closures [OK]

Common Mistakes:

• Thinking weak self guarantees non-nil self
• Confusing weak with unowned self
• Assuming weak self copies self

2. Which of the following is the correct syntax to capture self as unowned in a Swift closure?

easy

A. [weak self] in

B. [unowned self] in

C. [self unowned] in

D. [unowned] self in

Solution

1. Step 1: Recall capture list syntax

   Swift uses square brackets with capture specifiers before in in closures.

2. Step 2: Correct unowned syntax

   The correct syntax is [unowned self] in to capture self as unowned.

3. Final Answer:

   [unowned self] in -> Option B

4. Quick Check:

   Unowned capture syntax = [unowned self] in [OK]

Hint: Capture lists use square brackets with 'unowned self' [OK]

Common Mistakes:

• Swapping order like [self unowned]
• Missing brackets or 'in' keyword
• Using weak instead of unowned when asked

3. Consider this Swift code snippet:

class Example {
    var closure: (() -> Void)?
    func setup() {
        closure = { [weak self] in
            print(self == nil ? "nil" : "not nil")
        }
    }
}
let example = Example()
example.setup()
example.closure?()

What will be printed when example.closure?() is called?

medium

A. not nil

B. Compilation error

C. nil

D. Runtime crash

Solution

1. Step 1: Analyze weak self capture

   The closure captures self weakly, so inside the closure self is optional.

2. Step 2: Check self lifetime

   The example instance is still alive when closure?() is called, so self is not nil.

3. Final Answer:

   not nil -> Option A

4. Quick Check:

   Weak self is alive here = not nil [OK]

Hint: If instance exists, weak self is not nil inside closure [OK]

Common Mistakes:

• Assuming weak self is always nil
• Confusing weak self with unowned self
• Expecting runtime crash due to optional

4. Identify the problem in this Swift code and choose the best fix:

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

medium

A. Make greet a regular function instead of closure

B. Use [weak self] in closure to avoid strong reference cycle

C. Use [unowned self] in closure to avoid strong reference cycle

D. No problem; code works fine

Solution

1. Step 1: Detect strong reference cycle risk

   The closure captures self strongly, and greet is lazy var, so cycle possible.

2. Step 2: Choose unowned self for non-optional self

   Since self will always exist when greet is called, unowned self is safe and avoids optional unwrapping.

3. Final Answer:

   Use [unowned self] in closure to avoid strong reference cycle -> Option A

4. Quick Check:

   Unowned self for guaranteed self = C [OK]

Hint: Use unowned self if self always exists in closure [OK]

Common Mistakes:

• Ignoring strong reference cycles
• Using weak self and forcing unwrap unnecessarily
• Thinking no problem exists

5. You have a Swift class that manages a network request with a closure callback. You want to avoid memory leaks but also avoid optional unwrapping inside the closure. Which pattern should you use and why? Choose the best explanation.

hard

A. Do not capture self at all to avoid any reference issues

B. Use weak self to avoid leaks and unwrap self safely inside closure

C. Use strong self to keep the instance alive during the closure execution

D. Use unowned self because self will outlive the closure, avoiding optionals

Solution

1. Step 1: Understand lifetime of self and closure

   If self will definitely exist while closure runs, unowned self is safe and avoids optionals.

2. Step 2: Compare weak vs unowned for this case

   weak self requires optional unwrapping, but unowned self assumes self exists, simplifying code and avoiding leaks.

3. Final Answer:

   Use unowned self because self will outlive the closure, avoiding optionals -> Option D

4. Quick Check:

   Unowned self for guaranteed lifetime = A [OK]

Hint: Use unowned if self guaranteed alive, else weak [OK]

Common Mistakes:

• Using weak self but not unwrapping safely
• Using strong self causing memory leaks
• Ignoring self capture causing leaks