Closures as function parameters in Swift

Introduction

Closures let you pass small pieces of code into functions to run later. This helps make your code flexible and reusable.

When you want to run some code after a task finishes, like loading data.

When you want to customize how a function works without changing its code.

When you want to handle events like button taps in your app.

When you want to sort or filter a list with custom rules.

When you want to run code multiple times with different actions.

Syntax

Swift

func functionName(parameterName: (ParameterTypes) -> ReturnType) {
    // function body
    parameterName(arguments) // call the closure
}

The closure is written as a parameter type using arrow syntax: (input) -> output.

You call the closure inside the function like a normal function.

Examples

This function takes a closure with no inputs and no return. It prints a greeting, then runs the closure.

Swift

func greetUser(action: () -> Void) {
    print("Hello!")
    action()
}

greetUser {
    print("Welcome to the app!")
}

This function takes a closure that receives an integer. It runs the closure multiple times, passing the current count.

Swift

func repeatTask(times: Int, task: (Int) -> Void) {
    for i in 1...times {
        task(i)
    }
}

repeatTask(times: 3) { count in
    print("Task number \(count)")
}

Sample Program

This program defines a function that takes two numbers and a closure to perform an operation on them. It then prints the result. We call it twice: once to add, once to multiply.

Swift

func performOperation(a: Int, b: Int, operation: (Int, Int) -> Int) {
    let result = operation(a, b)
    print("Result: \(result)")
}

performOperation(a: 5, b: 3) { x, y in
    x + y
}

performOperation(a: 5, b: 3) { x, y in
    x * y
}

Important Notes

Closures can capture values from their surrounding context.

Trailing closure syntax lets you write the closure outside the parentheses if it is the last parameter.

Closures make your functions more flexible and reusable.

Summary

Closures are blocks of code you can pass into functions as parameters.

They let you customize what a function does without changing its code.

Use closures to run code later, handle events, or customize behavior.

Practice

1.

What is the main purpose of passing a closure as a function parameter in Swift?

easy

A. To allow the function to execute custom code provided by the caller

B. To make the function run faster

C. To prevent the function from returning a value

D. To declare a global variable inside the function

Solution

1. Step 1: Understand what a closure is

   A closure is a block of code that can be passed around and executed later.

2. Step 2: Understand why closures are passed to functions

   Passing a closure lets the function run custom code without changing its own code.

3. Final Answer:

   To allow the function to execute custom code provided by the caller -> Option A

4. Quick Check:

   Closures customize function behavior = A [OK]

Hint: Closures let functions run your code inside them [OK]

Common Mistakes:

• Thinking closures speed up the function
• Confusing closures with variables
• Believing closures stop function returns

2.

Which of the following is the correct syntax to declare a function that takes a closure parameter with no input and no return value?

func performAction(____) { }

easy

A. action: () => Void

B. action: Void -> ()

C. action: () -> () -> Void

D. action: () -> Void

Solution

1. Step 1: Recall closure syntax in Swift

   A closure with no input and no return is written as () -> Void.

2. Step 2: Match syntax to function parameter

   The parameter should be named and typed as action: () -> Void.

3. Final Answer:

   action: () -> Void -> Option D

4. Quick Check:

   Closure type syntax = D [OK]

Hint: Closure type: inputs in (), arrow, then return type [OK]

Common Mistakes:

• Using 'Void -> ()' which is invalid
• Adding extra arrows like '() -> () -> Void'
• Using '=>' which is not Swift syntax

3.

What will be printed when the following Swift code runs?

func greet(action: () -> String) {
    print("Greeting: " + action())
}

greet { "Hello, Swift!" }

medium

A. Hello, Swift!

B. Greeting: action

C. Greeting: Hello, Swift!

D. Error: Missing return type

Solution

1. Step 1: Understand the function call

   The function greet takes a closure that returns a String and prints "Greeting: " plus the closure's result.

2. Step 2: Evaluate the closure passed

   The closure returns "Hello, Swift!", so the print statement outputs "Greeting: Hello, Swift!".

3. Final Answer:

   Greeting: Hello, Swift! -> Option C

4. Quick Check:

   Closure returns string printed with prefix = A [OK]

Hint: Closure returns string, function prints with prefix [OK]

Common Mistakes:

• Expecting just the closure string without prefix
• Confusing closure name with output
• Thinking code causes error

4.

Identify the error in this Swift function that takes a closure parameter:

func repeatTask(times: Int, task: () -> Void) {
    for _ in 1...times {
        task
    }
}

medium

A. The closure 'task' is not called with parentheses

B. The loop range should start from 0

C. The closure parameter must return a value

D. The function must return a value

Solution

1. Step 1: Check how the closure is used inside the loop

   The code uses 'task' without parentheses, so the closure is not executed.

2. Step 2: Correct closure call syntax

   To run the closure, it must be called as 'task()' with parentheses.

3. Final Answer:

   The closure 'task' is not called with parentheses -> Option A

4. Quick Check:

   Call closure with () to execute = C [OK]

Hint: Call closures with () to run them [OK]

Common Mistakes:

• Forgetting parentheses to call closure
• Changing loop range unnecessarily
• Expecting closure to return value when void

5.

Given this function that filters numbers using a closure, how would you call it to get only even numbers?

func filterNumbers(_ numbers: [Int], using condition: (Int) -> Bool) -> [Int] {
    var result = [Int]()
    for number in numbers {
        if condition(number) {
            result.append(number)
        }
    }
    return result
}

Choose the correct call:

hard

A. filterNumbers([1,2,3,4]) { number in number * 2 }

B. filterNumbers([1,2,3,4]) { $0 % 2 == 0 }

C. filterNumbers([1,2,3,4]) { _ in true }

D. filterNumbers([1,2,3,4]) { $0 > 5 }

Solution

1. Step 1: Understand the closure parameter

   The closure takes an Int and returns Bool to decide if number is included.

2. Step 2: Identify condition for even numbers

   Even numbers satisfy number % 2 == 0, so closure should return true for those.

3. Step 3: Match correct call

   filterNumbers([1,2,3,4]) { $0 % 2 == 0 } uses shorthand $0 % 2 == 0, which returns true for even numbers.

4. Final Answer:

   filterNumbers([1,2,3,4]) { $0 % 2 == 0 } -> Option B

5. Quick Check:

   Closure returns true for even numbers = B [OK]

Hint: Use $0 % 2 == 0 to filter even numbers [OK]

Common Mistakes:

• Returning number * 2 instead of Bool
• Returning always true to get all numbers
• Filtering numbers greater than 5 instead of even