Extension syntax in Swift - Mini Project: Build & Apply

Using Extension Syntax in Swift

📖 Scenario: Imagine you have a simple app that manages books. You want to add a new feature to calculate the age of a book since its publication without changing the original Book class.

🎯 Goal: Learn how to use extension syntax in Swift to add a new computed property to an existing class.

📋 What You'll Learn

Create a Book class with title and yearPublished properties

Add an extension to Book that adds a computed property age

Calculate the book's age based on the current year

Print the book's title and its age

💡 Why This Matters

🌍 Real World

Extensions let you add new features to existing code without changing it. This is useful when working with libraries or system classes.

💼 Career

Understanding extensions is important for Swift developers to write clean, modular, and maintainable code.

1

Create the Book class

Create a class called Book with two properties: title of type String and yearPublished of type Int. Initialize these properties with an initializer.

Swift

# Your code here

Hint

Remember to define properties and an initializer inside the class.

2

Add an extension with a computed property

Add an extension to the Book class that adds a computed property called age of type Int. This property should calculate the difference between the current year (2024) and yearPublished.

Swift

class Book {
    let title: String
    let yearPublished: Int
    
    init(title: String, yearPublished: Int) {
        self.title = title
        self.yearPublished = yearPublished
    }
}

// Your code here

Hint

Use extension Book { var age: Int { ... } } to add the computed property.

3

Create a Book instance and access age

Create a constant called myBook that is an instance of Book with the title "Swift Programming" and yearPublished 2015. Then, create a constant called bookAge that gets the age property from myBook.

Swift

class Book {
    let title: String
    let yearPublished: Int
    
    init(title: String, yearPublished: Int) {
        self.title = title
        self.yearPublished = yearPublished
    }
}

extension Book {
    var age: Int {
        return 2024 - yearPublished
    }
}

// Your code here

Hint

Create the instance with Book(title: "Swift Programming", yearPublished: 2015) and access age.

4

Print the book's title and age

Write a print statement that outputs: "Swift Programming is 9 years old." using the title and bookAge variables with string interpolation.

Swift

class Book {
    let title: String
    let yearPublished: Int
    
    init(title: String, yearPublished: Int) {
        self.title = title
        self.yearPublished = yearPublished
    }
}

extension Book {
    var age: Int {
        return 2024 - yearPublished
    }
}

let myBook = Book(title: "Swift Programming", yearPublished: 2015)
let bookAge = myBook.age

// Your code here

Hint

Use print("\(myBook.title) is \(bookAge) years old.") to show the message.

Practice

1. What is the main purpose of using extension in Swift?

easy

A. To add new functionality to an existing type without modifying its original code

B. To create a new subclass from an existing class

C. To replace the original implementation of a type

D. To delete methods from a type

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new features to existing types without changing their original source code.

2. Step 2: Compare options with this definition

   Only adding new functionality to an existing type without modifying its original code correctly describes this purpose. Replacing original implementation refers to overriding, creating subclasses is inheritance, and deleting methods is unrelated to extensions.

3. Final Answer:

   To add new functionality to an existing type without modifying its original code -> Option A

4. Quick Check:

   Extension purpose = Add features without changing original code [OK]

Hint: Extensions add features without changing original code [OK]

Common Mistakes:

• Confusing extensions with subclassing
• Thinking extensions replace original code
• Believing extensions can remove methods

2. Which of the following is the correct syntax to start an extension for the String type in Swift?

easy

A. extend String { }

B. extension String { }

C. extension String() { }

D. extension String[] { }

Solution

1. Step 1: Recall Swift extension syntax

   The correct syntax starts with the keyword extension followed by the type name without parentheses or brackets.

2. Step 2: Check each option

   extension String { } uses correct syntax. extend String { } uses invalid keyword. extension String() { } adds invalid parentheses. extension String[] { } uses invalid array notation.

3. Final Answer:

   extension String { } -> Option B

4. Quick Check:

   Extension syntax = extension TypeName { } [OK]

Hint: Use 'extension TypeName { }' without parentheses or brackets [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Adding parentheses after type name
• Using brackets after type name

3. What will be the output of this Swift code?

extension Int {
    func squared() -> Int {
        return self * self
    }
}

let number = 4
print(number.squared())

medium

A. 16

B. 8

C. 4

D. Compile error

Solution

1. Step 1: Understand the extension method

   The extension adds a method squared() to Int that returns the number multiplied by itself.

2. Step 2: Calculate the output for number = 4

   Calling 4.squared() returns 4 * 4 = 16.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply the number by itself in the extension method [OK]

Common Mistakes:

• Confusing squared with doubling
• Expecting syntax error due to extension
• Forgetting to return the value

4. Identify the error in this Swift extension code:

extension Double {
    func half() -> Double {
        self / 2
    }
}

medium

A. Missing return keyword in the method

B. Extension cannot add methods to Double

C. Method name 'half' is invalid

D. No error, code is correct

Solution

1. Step 1: Check method syntax in extension

   In Swift, single-expression functions can omit the return keyword.

2. Step 2: Verify if extension on Double is allowed

   Extensions can add methods to Double. The method name 'half' is valid.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Single-expression methods can omit return [OK]

Hint: Single-expression methods can omit 'return' keyword [OK]

Common Mistakes:

• Assuming 'return' is always required
• Thinking extensions can't add methods to built-in types
• Believing method names have restrictions

5. You want to add a computed property isEven to Int using an extension that returns true if the number is even, otherwise false. Which code correctly implements this?

hard

A. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } }

B. extension Int { func isEven() -> Bool { return self % 2 == 0 } }

C. extension Int { var isEven: Bool { self % 2 == 0 } }

D. extension Int { var isEven: Bool { self / 2 == 0 } }

Solution

1. Step 1: Understand computed property syntax

   Computed properties use var with a getter. extension Int { var isEven: Bool { self % 2 == 0 } } uses correct concise syntax; the func version declares a method, the if version lacks an else branch causing compile error, the division version has wrong logic.

2. Step 2: Check logic correctness

   extension Int { var isEven: Bool { self % 2 == 0 } } uses concise expression self % 2 == 0 which returns true if even. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } } omits else branch, causing compiler error. extension Int { var isEven: Bool { self / 2 == 0 } } incorrectly uses division instead of modulo.

3. Step 3: Verify distractors

   The method requires parentheses to call like num.isEven(). Incomplete if fails to compile. Division fails for even numbers like 2 (2/2 == 1 != 0).

4. Final Answer:

   extension Int { var isEven: Bool { self % 2 == 0 } } -> Option C

5. Quick Check:

   Computed property with modulo check = extension Int { var isEven: Bool { self % 2 == 0 } } [OK]

Hint: Use computed property with 'self % 2 == 0' for even check [OK]

Common Mistakes:

• Using division instead of modulo
• Confusing method with computed property
• Forgetting to return a Bool value