Protocol declaration syntax in Swift - Mini Project: Build & Apply

Protocol declaration syntax

📖 Scenario: Imagine you are creating a simple app that manages different types of vehicles. You want to make sure all vehicles can start and stop, but each vehicle might do it differently.

🎯 Goal: You will create a Vehicle protocol that requires two functions: start() and stop(). Then, you will create a struct that follows this protocol.

📋 What You'll Learn

Create a protocol named Vehicle

Add two function declarations inside the protocol: start() and stop()

Create a struct named Car that conforms to the Vehicle protocol

Implement the start() and stop() functions inside Car

Print messages when start() and stop() are called

💡 Why This Matters

🌍 Real World

Protocols are used in apps to define common behaviors for different types, like vehicles, animals, or user interface elements.

💼 Career

Understanding protocols is essential for Swift developers to write flexible and reusable code, especially in app development for iOS and macOS.

1

Create the Vehicle protocol

Write a protocol named Vehicle with two function declarations: start() and stop(). Do not add any code inside the functions.

Swift

# Write the protocol Vehicle with start() and stop() function declarations
// Your code here

Hint

Use the protocol keyword followed by the protocol name. Inside curly braces, declare the functions without bodies.

2

Create the Car struct conforming to Vehicle

Create a struct named Car that conforms to the Vehicle protocol.

Swift

protocol Vehicle {
    func start()
    func stop()
}

// Create struct Car conforming to Vehicle
// Your code here

Hint

Use struct Car: Vehicle to declare the struct and protocol conformance.

3

Implement start() and stop() in Car

Inside the Car struct, implement the start() function to print "Car started" and the stop() function to print "Car stopped".

Swift

protocol Vehicle {
    func start()
    func stop()
}

struct Car: Vehicle {
    // Implement start() to print "Car started"
    // Implement stop() to print "Car stopped"
    // Your code here
}

Hint

Use print("Car started") inside start() and print("Car stopped") inside stop().

4

Create a Car instance and call start() and stop()

Create a variable named myCar as an instance of Car. Then call myCar.start() and myCar.stop() to print the messages.

Swift

protocol Vehicle {
    func start()
    func stop()
}

struct Car: Vehicle {
    func start() {
        print("Car started")
    }
    func stop() {
        print("Car stopped")
    }
}

// Create myCar instance and call start() and stop()
// Your code here

Hint

Create myCar with let myCar = Car(). Then call myCar.start() and myCar.stop().

Practice

1. What keyword is used in Swift to declare a protocol?

easy

A. interface

B. class

C. protocol

D. struct

Solution

1. Step 1: Recall Swift syntax for protocols

   In Swift, protocols are declared using the keyword protocol.

2. Step 2: Compare options with known syntax

   Options like interface are from other languages, and class or struct declare types, not protocols.

3. Final Answer:

   protocol -> Option C

4. Quick Check:

   Keyword for protocol declaration = protocol [OK]

Hint: Remember: Swift uses 'protocol' keyword for protocols [OK]

Common Mistakes:

• Confusing 'protocol' with 'interface' from other languages
• Using 'class' or 'struct' instead of 'protocol'
• Misspelling the keyword

2. Which of the following is the correct syntax to declare a protocol named Vehicle in Swift?

easy

A. protocol Vehicle {}

B. protocol Vehicle() {}

C. protocol Vehicle[] {}

D. protocol Vehicle<> {}

Solution

1. Step 1: Understand protocol declaration syntax

   Protocols are declared with the keyword protocol, followed by the name and curly braces without parentheses or brackets.

2. Step 2: Check each option

   protocol Vehicle {} uses correct syntax: protocol Vehicle {}. Options B, C, and D incorrectly add parentheses or brackets.

3. Final Answer:

   protocol Vehicle {} -> Option A

4. Quick Check:

   Protocol declaration uses curly braces only [OK]

Hint: Protocols use curly braces, no parentheses or brackets [OK]

Common Mistakes:

• Adding parentheses after protocol name
• Using square or angle brackets incorrectly
• Omitting curly braces

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

protocol Greetable {
    func greet() -> String
}

struct Person: Greetable {
    func greet() -> String {
        return "Hello!"
    }
}

let p = Person()
print(p.greet())

medium

A. Hello!

B. greet()

C. Error: Missing protocol method implementation

D. nil

Solution

1. Step 1: Understand protocol and struct implementation

   The protocol Greetable requires a method greet() returning a String. The struct Person implements this method returning "Hello!".

2. Step 2: Analyze the print statement

   Creating an instance p of Person and calling p.greet() returns "Hello!", which is printed.

3. Final Answer:

   Hello! -> Option A

4. Quick Check:

   Implemented protocol method returns "Hello!" [OK]

Hint: Protocol methods must be implemented; output is returned string [OK]

Common Mistakes:

• Assuming protocol methods run automatically without implementation
• Expecting error due to protocol conformance
• Confusing method name with output

4. Identify the error in this protocol declaration:

protocol Drawable {
    func draw()
    var color: String
}

medium

A. func draw() must have a return type

B. Protocol properties must specify { get } or { get set }

C. Protocol name must start with uppercase letter

D. Missing parentheses after func draw

Solution

1. Step 1: Check protocol property declaration rules

   In Swift, protocol properties must declare if they are readable or writable using { get } or { get set }.

2. Step 2: Analyze the given property

   The property color: String lacks { get } or { get set }, causing a syntax error.

3. Final Answer:

   Protocol properties must specify { get } or { get set } -> Option B

4. Quick Check:

   Protocol property syntax requires access specifiers [OK]

Hint: Always add { get } or { get set } to protocol properties [OK]

Common Mistakes:

• Omitting { get } or { get set } in protocol properties
• Adding parentheses to property declarations
• Thinking protocol names must be uppercase

5. You want to declare a protocol Identifiable that requires a read-only id property of type String and a method identify() that returns a String. Which is the correct declaration?

hard

A. protocol Identifiable {
let id: String
func identify() -> String
}

B. protocol Identifiable {
var id: String
func identify()
}

C. protocol Identifiable {
var id: String { get set }
func identify() -> String
}

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

Solution

1. Step 1: Define read-only property in protocol

   A read-only property in a protocol must be declared with { get } to indicate it can be read but not written.

2. Step 2: Check method signature

   The method identify() must specify return type String as required.

3. Step 3: Evaluate options

   protocol Identifiable { var id: String { get } func identify() -> String } correctly declares var id: String { get } and func identify() -> String. protocol Identifiable { var id: String func identify() } misses { get } and return type. protocol Identifiable { let id: String func identify() -> String } uses let which is invalid in protocols. protocol Identifiable { var id: String { get set } func identify() -> String } allows writing with { get set }, not read-only.

4. Final Answer:

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

5. Quick Check:

   Read-only property uses { get } and method return type matches [OK]

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

Common Mistakes:

• Using 'let' instead of 'var' in protocol
• Omitting { get } for read-only properties
• Missing return type in method declaration