Protocol requirements (methods and properties) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a struct conforming to a protocol with property requirements

What is the output of this Swift code when run?

Swift

protocol Vehicle {
    var numberOfWheels: Int { get }
    func description() -> String
}

struct Bike: Vehicle {
    var numberOfWheels: Int = 2
    func description() -> String {
        return "Bike with \(numberOfWheels) wheels"
    }
}

let myBike = Bike()
print(myBike.description())

A Runtime error

B Bike with wheels

C Compilation error: missing property implementation

D Bike with 2 wheels

💡 Hint

Check how the struct implements the protocol's property and method.

The struct Bike correctly implements the required property numberOfWheels and method description(). The print statement outputs the string with the number 2.

❓ Predict Output

intermediate

Protocol method requirement with mutating keyword

What will be printed when this Swift code runs?

Swift

protocol Toggleable {
    mutating func toggle()
}

struct LightSwitch: Toggleable {
    var isOn = false
    mutating func toggle() {
        isOn = !isOn
    }
}

var light = LightSwitch()
light.toggle()
print(light.isOn)

A true

B false

C Compilation error: missing mutating keyword

D Runtime error

💡 Hint

Look at how the toggle method changes the isOn property.

The toggle method flips the isOn boolean from false to true. The print statement outputs true.

🔧 Debug

advanced

Identify the error in protocol property implementation

Why does this Swift code fail to compile?

Swift

protocol Identifiable {
    var id: String { get set }
}

struct User: Identifiable {
    let id: String
}

A Missing method implementation

B Cannot assign let property 'id' to protocol requiring get set

C Protocol requires static property 'id'

D No error, code compiles fine

💡 Hint

Check the difference between 'let' and 'var' in property requirements.

The protocol requires a property 'id' that can be read and written (get set). The struct declares 'id' as a constant (let), which cannot be set after initialization, causing a compile error.

📝 Syntax

advanced

Correct syntax for protocol property with optional get and set

Which option correctly declares a protocol property that can be read and optionally written?

A var name: String { get set }

B var name: String { get }

C var name: String { set }

D var name: String { get optional set }

💡 Hint

Protocols specify property requirements with get and set keywords.

Option A declares a property that must be readable and writable. Option A is read-only. Option A is write-only (rare). Option A is invalid syntax.

🚀 Application

expert

Implement a protocol with a class and demonstrate property observer

Given this protocol, which class implementation correctly conforms and prints a message when the property changes?

Swift

protocol TemperatureMonitor {
    var temperature: Double { get set }
}

class Sensor: TemperatureMonitor {
    var temperature: Double = 0.0 {
        didSet {
            print("Temperature changed to \(temperature)")
        }
    }
}

let sensor = Sensor()
sensor.temperature = 25.5

A Compilation error: property observer not allowed

B No output printed

C Prints 'Temperature changed to 25.5'

D Runtime error

💡 Hint

Property observers can be used in classes to react to changes.

The class Sensor conforms to the protocol and uses didSet to print a message whenever temperature changes. Setting temperature to 25.5 triggers the print.

Practice

1. What does a protocol in Swift require from a type that adopts it?

easy

A. To declare new methods unrelated to the protocol

B. To inherit from a superclass

C. To override all existing methods

D. To implement all the methods and properties declared in the protocol

Solution

1. Step 1: Understand protocol purpose

   Protocols define a set of methods and properties that a type must implement.

2. Step 2: Check adoption requirements

   Any type adopting a protocol must provide implementations for all its requirements.

3. Final Answer:

   To implement all the methods and properties declared in the protocol -> Option D

4. Quick Check:

   Protocol adoption = implement all requirements [OK]

Hint: Protocols require full implementation of declared methods and properties [OK]

Common Mistakes:

• Thinking protocols require inheritance from a class
• Assuming methods can be skipped
• Confusing protocol adoption with subclassing

2. Which of the following is the correct way to declare a read-only property requirement in a Swift protocol?

easy

A. var name: String { get }

B. var name: String { get set }

C. let name: String

D. func name() -> String

Solution

1. Step 1: Recall property syntax in protocols

   Read-only properties use { get } to indicate they must be readable but not necessarily writable.

2. Step 2: Analyze options

   var name: String { get } uses { get } correctly; var name: String { get set } requires read-write; let name: String is invalid in protocols; func name() -> String is a method, not a property.

3. Final Answer:

   var name: String { get } -> Option A

4. Quick Check:

   Read-only property = { get } [OK]

Hint: Use { get } for read-only property in protocols [OK]

Common Mistakes:

• Using let instead of var in protocol
• Confusing methods with properties
• Using { get set } for read-only properties

3. Consider the following Swift code:

protocol Vehicle {
    var speed: Double { get set }
    func description() -> String
}

struct Car: Vehicle {
    var speed: Double
    func description() -> String {
        return "Car speed is \(speed) km/h"
    }
}

let myCar = Car(speed: 80)
print(myCar.description())

What is the output when this code runs?

medium

A. Error: Protocol requirements not met

B. Car speed is 80.0 km/h

C. Car speed is 80 km/h

D. Car speed is km/h

Solution

1. Step 1: Check protocol conformance

   Car struct implements the required property speed and method description(), fulfilling Vehicle protocol.

2. Step 2: Analyze print output

   The description() returns "Car speed is \(speed) km/h". Since speed is 80.0 (Double), string interpolation formats it as "80.0".

3. Final Answer:

   Car speed is 80.0 km/h -> Option B

4. Quick Check:

   Protocol methods implemented = correct output [OK]

Hint: String interpolation for Double 80.0 prints '80.0' [OK]

Common Mistakes:

• Expecting 80 instead of 80.0
• Thinking protocol conformance causes error
• Missing string interpolation syntax

4. Given this protocol and struct:

protocol Shape {
    var area: Double { get }
    func perimeter() -> Double
}

struct Square: Shape {
    var side: Double
    var area: Double {
        side * side
    }
}

What is the error in the code?

medium

A. area property should be var with get set

B. side property must be a constant (let)

C. Missing implementation of perimeter() method

D. Square cannot have computed properties

Solution

1. Step 1: Check protocol requirements

   Shape protocol requires a read-only property area and a method perimeter().

2. Step 2: Verify Square struct implementation

   Square implements area as computed property but does not implement perimeter() method.

3. Final Answer:

   Missing implementation of perimeter() method -> Option C

4. Quick Check:

   All protocol methods must be implemented [OK]

Hint: All protocol methods must be implemented, or error occurs [OK]

Common Mistakes:

• Assuming computed properties are not allowed
• Thinking get set is required for read-only
• Ignoring missing method implementations

5. You want to create a protocol Identifiable that requires a read-only id property of type String and a method identify() that prints the id. Which of the following is the correct protocol and extension to provide a default implementation of identify()?

hard

A. protocol Identifiable {
var id: String { get }
func identify()
}
extension Identifiable {
func identify() {
print("ID: \(id)")
}
}

B. protocol Identifiable {
var id: String { get set }
func identify()
}
extension Identifiable {
func identify() {
print(id)
}
}

C. protocol Identifiable {
let id: String
func identify()
}
extension Identifiable {
func identify() {
print("ID is " + id)
}
}

D. protocol Identifiable {
var id: String { get }
func identify() -> String
}
extension Identifiable {
func identify() -> String {
return id
}
}

Solution

1. Step 1: Define protocol requirements

   id must be read-only, so use { get } and identify() method with no return.

2. Step 2: Provide default implementation in extension

   Extension implements identify() printing "ID: \(id)" matching protocol signature.

3. Final Answer:

   protocol Identifiable { var id: String { get }; func identify() } extension Identifiable { func identify() { print("ID: \(id)") } } -> Option A

4. Quick Check:

   Read-only property + default method in extension [OK]

Hint: Use protocol extension for default method implementations [OK]

Common Mistakes:

• Using let in protocol property
• Changing method signature in extension
• Making id writable when only read-only needed