Weak self and unowned self patterns in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using weak self in Swift closures?

Using weak self prevents strong reference cycles by creating a weak reference to self inside a closure. This means the closure does not keep self alive, avoiding memory leaks.

intermediate

How does unowned self differ from weak self in Swift?

unowned self is a non-optional reference that assumes self will always exist when the closure runs. Unlike weak self, it does not become nil. Use it when self is guaranteed to outlive the closure.

beginner

What happens if you access self inside a closure without weak or unowned?

Accessing self strongly inside a closure creates a strong reference cycle if self also holds the closure. This causes a memory leak because neither can be released.

intermediate

When should you use weak self instead of unowned self?

Use weak self when self might become nil before the closure finishes. This makes self optional inside the closure, so you must safely unwrap it.

beginner

Explain a real-life analogy for weak and unowned references.

Imagine self is a person and the closure is a friend holding a photo. weak means the friend holds a photo that can fade (become nil) if the person leaves. unowned means the friend holds a photo that never fades because the person is always around.

What does weak self create inside a closure?

A A weak, optional reference to self

B A strong reference to self

C A non-optional unowned reference to self

D A copy of self

weak self creates a weak, optional reference to self to avoid strong reference cycles.

When should you use unowned self in a closure?

A When you want to copy self

B When self might be nil during closure execution

C When self is guaranteed to exist during closure execution

D When you want to create a strong reference cycle

unowned self is used when self will definitely exist while the closure runs.

What risk do you take when using unowned self incorrectly?

A Crash due to accessing nil reference

B Memory leak

C Slower performance

D No risk at all

If self is nil but accessed as unowned, the app will crash.

Which keyword makes self optional inside a closure?

A strong

B unowned

C let

D weak

weak makes self optional because it can become nil.

Why do we avoid strong references to self inside closures?

A To improve code readability

B To prevent strong reference cycles and memory leaks

C To make code run faster

D To copy self

Strong references inside closures can cause memory leaks by creating cycles.

Explain the difference between weak self and unowned self in Swift closures.

Think about whether self can be nil or not during closure execution.

Describe a situation where using weak self is necessary to avoid a memory leak.

Consider when self and closure keep each other alive.

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