Protocol requirements (methods and properties) in Swift

Introduction

Protocols define a set of rules that classes, structs, or enums must follow. They make sure certain methods and properties are present.

When you want different types to share common behavior.

When you want to write flexible code that works with many types.

When you want to define a blueprint for methods and properties without implementation.

When you want to ensure certain methods or properties exist in multiple types.

When you want to use polymorphism to treat different types the same way.

Syntax

Swift

protocol SomeProtocol {
    var mustHaveProperty: Int { get set }
    func requiredMethod() -> String
}

Properties in protocols can be read-only ({ get }) or read-write ({ get set }).

Methods in protocols only declare the signature, not the body.

Examples

This protocol requires a read-only property and a method.

Swift

protocol Vehicle {
    var numberOfWheels: Int { get }
    func drive()
}

This protocol requires a read-write property and a method that returns a string.

Swift

protocol Person {
    var name: String { get set }
    func greet() -> String
}

This protocol only requires a method with a parameter.

Swift

protocol Logger {
    func log(message: String)
}

Sample Program

This program defines a protocol Animal with a property and a method. The struct Dog follows the protocol by implementing the property and method. It then prints the dog's name and sound.

Swift

protocol Animal {
    var name: String { get }
    func sound() -> String
}

struct Dog: Animal {
    var name: String
    func sound() -> String {
        return "Woof!"
    }
}

let myDog = Dog(name: "Buddy")
print("\(myDog.name) says \(myDog.sound())")

Important Notes

When a type adopts a protocol, it must implement all required methods and properties.

Properties can be declared as { get } or { get set } to control if they are read-only or read-write.

Protocols help write code that is easier to test and maintain.

Summary

Protocols define required methods and properties without implementation.

Types that adopt protocols must implement all requirements.

Use protocols to create flexible and reusable code.

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