Protocol as types in Swift - Mini Project: Build & Apply

Using Protocol as Types in Swift

📖 Scenario: You are building a simple app that manages different types of vehicles. Each vehicle can start its engine, but the way it starts might differ. You want to use a protocol to represent any vehicle type and treat them uniformly.

🎯 Goal: Create a protocol called Vehicle with a startEngine() method. Then create two structs, Car and Motorcycle, that conform to Vehicle. Finally, use the protocol as a type to store different vehicles in an array and start their engines.

📋 What You'll Learn

Create a protocol named Vehicle with a method startEngine() that returns a String.

Create a struct Car that conforms to Vehicle and implements startEngine() returning "Car engine started".

Create a struct Motorcycle that conforms to Vehicle and implements startEngine() returning "Motorcycle engine started".

Create an array called vehicles of type [Vehicle] containing one Car and one Motorcycle.

Use a for loop to call startEngine() on each vehicle in vehicles and print the result.

💡 Why This Matters

🌍 Real World

Protocols let you write flexible code that can work with many types sharing the same behavior, like different vehicles in a transport app.

💼 Career

Understanding protocols and using them as types is key for Swift developers to write clean, reusable, and scalable code.

1

Create the Vehicle protocol

Create a protocol called Vehicle with a method startEngine() that returns a String.

Swift

// Create the Vehicle protocol with startEngine() method
// Your code here

Hint

Use the protocol keyword and define startEngine() as a function returning String.

2

Create Car and Motorcycle structs conforming to Vehicle

Create a struct called Car that conforms to Vehicle and implements startEngine() returning "Car engine started". Also create a struct called Motorcycle that conforms to Vehicle and implements startEngine() returning "Motorcycle engine started".

Swift

protocol Vehicle {
    func startEngine() -> String
}

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

Hint

Define structs with struct Name: Vehicle and implement startEngine() returning the exact strings.

3

Create an array of Vehicle type

Create an array called vehicles of type [Vehicle] containing one Car() and one Motorcycle().

Swift

protocol Vehicle {
    func startEngine() -> String
}

struct Car: Vehicle {
    func startEngine() -> String {
        return "Car engine started"
    }
}

struct Motorcycle: Vehicle {
    func startEngine() -> String {
        return "Motorcycle engine started"
    }
}

// Create an array vehicles of type [Vehicle] with Car() and Motorcycle()
// Your code here

Hint

Use let vehicles: [Vehicle] = [Car(), Motorcycle()] to create the array.

4

Loop through vehicles and print engine start messages

Use a for loop with variable vehicle to iterate over vehicles. Inside the loop, call startEngine() on vehicle and print the returned string.

Swift

protocol Vehicle {
    func startEngine() -> String
}

struct Car: Vehicle {
    func startEngine() -> String {
        return "Car engine started"
    }
}

struct Motorcycle: Vehicle {
    func startEngine() -> String {
        return "Motorcycle engine started"
    }
}

let vehicles: [Vehicle] = [Car(), Motorcycle()]

// Use a for loop to print startEngine() result for each vehicle
// Your code here

Hint

Use for vehicle in vehicles and print(vehicle.startEngine()).

Practice

1. What does it mean to use a protocol as a type in Swift?

easy

A. Any object that follows the protocol's rules can be used as that type.

B. Only classes can conform to the protocol.

C. Protocols can only be used to create new classes.

D. Protocols define stored properties for types.

Solution

1. Step 1: Understand what a protocol defines

   A protocol defines a set of rules or features that a type must implement.

2. Step 2: Using protocol as a type

   When a protocol is used as a type, any object that implements those rules can be assigned to that type.

3. Final Answer:

   Any object that follows the protocol's rules can be used as that type. -> Option A

4. Quick Check:

   Protocol as type means flexible object use = A [OK]

Hint: Protocol as type means any conforming object fits [OK]

Common Mistakes:

• Thinking protocols only apply to classes
• Believing protocols define stored properties
• Confusing protocol usage with inheritance

2. Which of the following is the correct way to declare a variable of a protocol type named Drawable?

easy

A. var shape: Drawable

B. var shape: Drawable()

C. var shape: Drawable = Drawable()

D. var shape: Drawable[]

Solution

1. Step 1: Understand protocol type declaration

   To declare a variable of a protocol type, use the protocol name without parentheses or brackets.

2. Step 2: Analyze options

   var shape: Drawable correctly declares a variable of type Drawable. Options A and B incorrectly try to instantiate a protocol, which is not allowed. var shape: Drawable[] uses invalid syntax.

3. Final Answer:

   var shape: Drawable -> Option A

4. Quick Check:

   Protocol type variable = var name: ProtocolName [OK]

Hint: Declare protocol type without parentheses or initialization [OK]

Common Mistakes:

• Trying to instantiate a protocol directly
• Using parentheses after protocol name
• Using array brackets incorrectly

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

protocol Greetable {
    func greet() -> String
}

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

let greeter: Greetable = Person()
print(greeter.greet())

medium

A. greet()

B. Compile error

C. Person

D. Hello

Solution

1. Step 1: Understand protocol conformance

   Person struct conforms to Greetable and implements greet() returning "Hello".

2. Step 2: Using protocol as type and calling method

   Variable greeter is of type Greetable but holds a Person instance, so calling greet() returns "Hello".

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Protocol method call returns implemented string = Hello [OK]

Hint: Protocol variable calls conforming type's method [OK]

Common Mistakes:

• Expecting protocol name or method name as output
• Thinking protocol cannot be used as variable type
• Confusing compile error with runtime behavior

4. Identify the error in this Swift code using protocol as a type:

protocol Vehicle {
    func start()
}

class Car: Vehicle {
    func start() {
        print("Car started")
    }
}

var myVehicle: Vehicle = Vehicle()
myVehicle.start()

medium

A. Variable myVehicle should be a class, not protocol

B. Missing method implementation in Car

C. Cannot instantiate protocol directly with Vehicle()

D. start() method should return a value

Solution

1. Step 1: Check protocol instantiation

   Protocols cannot be instantiated directly, so Vehicle() causes an error.

2. Step 2: Analyze variable assignment

   Variable myVehicle must hold an instance of a type conforming to Vehicle, like Car(), not Vehicle() itself.

3. Final Answer:

   Cannot instantiate protocol directly with Vehicle() -> Option C

4. Quick Check:

   Protocols can't be created with () = Error [OK]

Hint: Protocols can't be instantiated directly [OK]

Common Mistakes:

• Trying to create an instance of a protocol
• Confusing protocol with class instantiation
• Ignoring method return types

5. Given the protocol and classes below, which code correctly stores different types conforming to Animal in an array and calls sound() on each?

protocol Animal {
    func sound() -> String
}

class Dog: Animal {
    func sound() -> String { "Bark" }
}

class Cat: Animal {
    func sound() -> String { "Meow" }
}

hard

A. let animals: [Animal] = [Dog(), Cat()]
for animal in animals {
print(animal.sound)
}

B. let animals: [Animal] = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

C. let animals: [Dog] = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

D. let animals = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

Solution

1. Step 1: Understand protocol array usage

   To store different types conforming to Animal, declare array as [Animal].

2. Step 2: Call method correctly

   Use animal.sound() with parentheses to call the method. let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } does this correctly.

3. Final Answer:

   let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } -> Option B

4. Quick Check:

   Protocol array with method calls = let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } [OK]

Hint: Use protocol array type and call methods with () [OK]

Common Mistakes:

• Using concrete type array instead of protocol type
• Calling method without parentheses
• Mixing incompatible types in array