Why protocol-oriented design matters in Swift - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol extension method call

What will be printed when this Swift code runs?

Swift

protocol Greetable {
    func greet()
}

extension Greetable {
    func greet() {
        print("Hello from protocol extension")
    }
}

struct Person: Greetable {
    func greet() {
        print("Hello from Person struct")
    }
}

let p: Greetable = Person()
p.greet()

A Hello from protocol extension

B Runtime error: method not found

C Compilation error: greet() not implemented

D Hello from Person struct

💡 Hint

Think about how protocol extensions provide default implementations and how method dispatch works with protocol-typed variables.

When a variable is typed as the protocol itself, the method called is the one defined in the protocol extension, not the struct's own implementation. This is because protocol extension methods are statically dispatched.

🧠 Conceptual

intermediate

Why use protocols instead of base classes?

Which of the following is the main advantage of using protocols over base classes in Swift's protocol-oriented design?

A Protocols can be instantiated directly, classes cannot.

B Protocols can store properties with default values, classes cannot.

C Protocols allow multiple inheritance of behavior, while classes do not.

D Protocols enforce implementation of stored properties, classes do not.

💡 Hint

Think about how Swift handles inheritance and how protocols differ from classes.

Protocols allow a type to conform to multiple protocols, effectively enabling multiple inheritance of behavior, which classes cannot do because Swift classes support only single inheritance.

🔧 Debug

advanced

Fix the unexpected output in protocol extension

This Swift code prints "Hello from protocol extension" but the developer expects "Hello from Dog struct". What is the cause?

Swift

protocol Animal {
    func speak()
}

extension Animal {
    func speak() {
        print("Hello from protocol extension")
    }
}

struct Dog: Animal {
    func speak() {
        print("Hello from Dog struct")
    }
}

let pet: Animal = Dog()
pet.speak()

A Dog struct does not implement speak(), so protocol extension is used.

B The variable pet is typed as Animal, so protocol extension method is called due to static dispatch.

C The speak() method in Dog is private, so protocol extension is called.

D The protocol Animal requires a default implementation, so Dog's method is ignored.

💡 Hint

Consider how Swift decides which method to call when using protocol-typed variables.

When calling a method on a variable typed as a protocol, Swift uses static dispatch and calls the protocol extension method, ignoring the struct's own implementation.

📝 Syntax

advanced

Identify the syntax error in protocol conformance

Which option contains the correct syntax to make a struct conform to a protocol with a method requirement?

Swift

protocol Runner {
    func run()
}

struct Athlete {
    // Conformance here
}

A

struct Athlete: Runner {
    func run() {
        print("Running")
    }
}

B

struct Athlete implements Runner {
    func run() {
        print("Running")
    }
}

C

struct Athlete extends Runner {
    func run() {
        print("Running")
    }
}

D

struct Athlete conforms Runner {
    func run() {
        print("Running")
    }
}

💡 Hint

Remember the Swift keyword to declare protocol conformance.

In Swift, a type conforms to a protocol using a colon ':' followed by the protocol name. Keywords like implements, extends, or conforms are invalid.

🚀 Application

expert

Determine the number of methods available via protocol composition

Given the protocols and struct below, how many unique methods can be called on the variable 'obj' typed as 'A & B'?

Swift

protocol A {
    func methodA()
}

protocol B {
    func methodB()
    func methodC()
}

struct C: A, B {
    func methodA() { print("A") }
    func methodB() { print("B") }
    func methodC() { print("C") }
    func methodD() { print("D") }
}

let obj: A & B = C()

A 3

B 4

C 2

D 1

💡 Hint

Count only the methods declared in the protocols A and B accessible via the protocol composition type.

The variable obj is typed as A & B, so only methods declared in protocols A and B are accessible. methodD() is not accessible because it's only in struct C.