Closures as function parameters in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a closure in Swift?

A closure is a block of code that can be passed around and used in your code. It can capture and store references to variables and constants from the surrounding context.

beginner

How do you pass a closure as a parameter to a function in Swift?

You define the function parameter with a closure type, for example: (Int) -> Int, then you can pass a closure when calling the function.

intermediate

What is the trailing closure syntax in Swift?

If the last parameter of a function is a closure, you can write the closure outside the parentheses when calling the function. This makes the code cleaner and easier to read.

beginner

Why are closures useful as function parameters?

Closures let you customize behavior by passing code to functions. This helps write flexible and reusable code, like sorting or filtering with custom rules.

intermediate

What does it mean that closures can capture values?

Closures can remember and use variables from the place where they were created, even if that place no longer exists when the closure runs.

How do you declare a function in Swift that takes a closure parameter which accepts an Int and returns a String?

A func example(closure: (Int) -> String) {}

B func example(closure: String -> Int) {}

C func example(closure: Int) -> String {}

D func example(closure: () -> ()) {}

The closure parameter type is written as (Int) -> String, meaning it takes an Int and returns a String.

What is the trailing closure syntax used for in Swift?

A To declare a closure inside a function

B To write the closure outside the parentheses when it is the last parameter

C To capture variables inside a closure

D To convert a closure to a function

Trailing closure syntax lets you write the closure after the function call parentheses if it is the last parameter.

Which of these is a correct way to call a function with a closure parameter using trailing closure syntax?

A doSomething() { print("Hello") }

B doSomething({ print("Hello") })

C doSomething(print("Hello"))

D doSomething(print) { "Hello" }

Trailing closure syntax places the closure outside the parentheses, like doSomething() { ... }.

What does it mean when a closure 'captures' a variable?

A It copies the variable to a new function

B It deletes the variable after use

C It keeps a reference to the variable from its surrounding context

D It converts the variable to a constant

Capturing means the closure keeps a reference to variables from where it was created, so it can use them later.

Why use closures as function parameters?

A To store data permanently

B To make the function run faster

C To avoid writing functions

D To customize what the function does with code you provide

Closures let you pass custom code to functions, making them flexible and reusable.

Explain how to define and use a closure as a function parameter in Swift.

Think about how you write the function signature and how you call it with a closure.

Describe what it means that closures capture values and why this is useful.

Imagine a closure remembering something from where it was created.