Protocol-oriented architecture in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Protocol Extensions

What will be printed when the following Swift code runs?

iOS Swift

protocol Greetable {
  func greet() -> String
}

extension Greetable {
  func greet() -> String {
    return "Hello from protocol extension"
  }
}

struct Person: Greetable {}

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

A Hello from protocol extension

B Error: Protocol method not implemented

C Hello from Person struct

D Runtime crash

💡 Hint

Protocol extensions provide default implementations.

Since Person does not implement greet(), the default implementation from the protocol extension is used.

❓ ui_behavior

intermediate

Protocol and UIViewController Behavior

Given a protocol with a default method to setup UI, which option correctly applies the protocol to a UIViewController subclass and calls the setup method?

iOS Swift

protocol UISetup {
  func setupUI()
}

extension UISetup {
  func setupUI() {
    print("Default UI setup")
  }
}

class MyViewController: UIViewController, UISetup {
  override func viewDidLoad() {
    super.viewDidLoad()
    // Call setupUI here
  }
}

A override func viewDidLoad() { super.viewDidLoad(); self.setupUI }

B override func viewDidLoad() { super.viewDidLoad(); self.UISetup.setupUI() }

C override func viewDidLoad() { super.viewDidLoad(); setupUI() }

D override func viewDidLoad() { super.viewDidLoad(); UISetup.setupUI() }

💡 Hint

Call the protocol method directly on self.

The protocol method setupUI() is available on self because MyViewController conforms to UISetup. Calling setupUI() inside viewDidLoad works.

❓ lifecycle

advanced

Protocol-oriented Initialization

What error occurs when trying to compile this Swift code?

iOS Swift

protocol Identifiable {
  var id: String { get }
  init(id: String)
}

struct User: Identifiable {
  let id: String
  init(id: String) {
    self.id = id
  }
}

let user = User(id: "123")

A Error: Type 'User' does not conform to protocol 'Identifiable' because init(id:) is missing

B No error, compiles and runs fine

C Error: Cannot use init in protocol

D Runtime error: Missing initializer

💡 Hint

Structs do not inherit protocol initializers automatically.

User must implement the required init(id:) initializer explicitly to conform to Identifiable.

❓ navigation

advanced

Protocol and Navigation Controller

Which option correctly uses a protocol to define a navigation action and implements it in a UIViewController subclass?

iOS Swift

protocol Navigatable {
  func navigateToDetail()
}

class ListViewController: UIViewController, Navigatable {
  func navigateToDetail() {
    let detailVC = UIViewController()
    // Navigate to detailVC
  }
}

A navigationController?.pushViewController(detailVC, animated: true)

B self.show(detailVC)

C present(detailVC, animated: true)

D detailVC.navigationController?.pushViewController(self, animated: true)

💡 Hint

Use navigationController to push new view controller.

To navigate in a navigation stack, pushViewController is used on navigationController.

📝 Syntax

expert

Protocol Composition and Method Resolution

What is the output of this Swift code?

iOS Swift

protocol A {
  func foo() -> String
}

protocol B {
  func foo() -> String
}

extension A {
  func foo() -> String { "From A" }
}

extension B {
  func foo() -> String { "From B" }
}

struct C: A, B {}

let c = C()
print(c.foo())

A From A

B From B

C Runtime crash

D Error: Ambiguous use of 'foo()'

💡 Hint

Multiple protocol extensions provide the same method.

The compiler cannot decide which foo() to use because both A and B provide default implementations, causing ambiguity.

Practice

1. What is the main purpose of a protocol in Swift's protocol-oriented architecture?

easy

A. To store data persistently in an app

B. To define a blueprint of methods and properties that conforming types must implement

C. To create a concrete class with implemented methods

D. To handle user interface layout automatically

Solution

1. Step 1: Understand what a protocol is

   A protocol defines a set of rules or requirements for methods and properties that any conforming type must implement.

2. Step 2: Differentiate from other concepts

   Protocols do not implement functionality themselves but specify what must be implemented, unlike classes or structs.

3. Final Answer:

   To define a blueprint of methods and properties that conforming types must implement -> Option B

4. Quick Check:

   Protocol = Blueprint for behavior [OK]

Hint: Protocols specify required methods and properties only [OK]

Common Mistakes:

• Confusing protocols with classes or structs
• Thinking protocols store data
• Assuming protocols handle UI layout

2. Which of the following is the correct syntax to declare a protocol named Drivable with a method drive()?

easy

A. func Drivable() { drive() }

B. class Drivable { func drive() }

C. struct Drivable { func drive() }

D. protocol Drivable { func drive() }

Solution

1. Step 1: Recall protocol declaration syntax

   Protocols are declared with the keyword protocol followed by the protocol name and curly braces containing method signatures.

2. Step 2: Identify correct method declaration

   The method drive() is declared without a body inside the protocol.

3. Final Answer:

   protocol Drivable { func drive() } -> Option D

4. Quick Check:

   Protocol syntax = protocol + name + method signatures [OK]

Hint: Protocols use 'protocol' keyword, not class or struct [OK]

Common Mistakes:

• Using class or struct instead of protocol keyword
• Adding method bodies inside protocol
• Incorrect function declaration syntax

3. Given the code below, what will be printed when car.start() is called?

protocol Startable {
  func start()
}

struct Car: Startable {
  func start() {
    print("Car started")
  }
}

let car = Car()
car.start()

medium

A. Car started

B. start() method not implemented

C. Compilation error

D. No output

Solution

1. Step 1: Check protocol conformance

   The struct Car conforms to Startable and implements the required start() method.

2. Step 2: Analyze method call output

   Calling car.start() executes the method which prints "Car started" to the console.

3. Final Answer:

   Car started -> Option A

4. Quick Check:

   Implemented method prints output [OK]

Hint: Implemented protocol methods run normally [OK]

Common Mistakes:

• Assuming missing implementation error
• Thinking protocol methods print automatically
• Confusing compilation errors with runtime

4. Identify the error in the following code snippet:

protocol Runnable {
  func run()
}

struct Athlete: Runnable {
  func run(distance: Int) {
    print("Running \(distance) meters")
  }
}

medium

A. Structs cannot conform to protocols

B. Protocol Runnable cannot have methods

C. Method signature in Athlete does not match protocol requirement

D. Missing return type in run() method

Solution

1. Step 1: Compare protocol and struct method signatures

   The protocol requires func run() with no parameters, but Athlete implements func run(distance: Int) which has a parameter.

2. Step 2: Understand conformance rules

   Method signatures must match exactly for conformance; differing parameters cause a mismatch error.

3. Final Answer:

   Method signature in Athlete does not match protocol requirement -> Option C

4. Quick Check:

   Method signatures must match exactly [OK]

Hint: Method signatures must match protocol exactly [OK]

Common Mistakes:

• Adding parameters to protocol methods in implementation
• Thinking structs can't conform to protocols
• Ignoring method signature differences

5. You want to create a flexible system where different types can be "playable" with a play() method, and some can also be "recordable" with a record() method. Which protocol-oriented approach below best supports this design?

hard

A. Define separate protocols Playable and Recordable, then have types conform to one or both as needed

B. Create a single protocol with both play() and record() methods, and require all types to implement both

C. Use a base class with play() and record() methods and inherit from it

D. Implement play() and record() as global functions

Solution

1. Step 1: Understand protocol composition

   Defining separate protocols allows types to conform only to behaviors they support, making the design flexible and reusable.

2. Step 2: Evaluate other options

   Single protocol forces all types to implement all methods, base class limits flexibility, and global functions do not enforce behavior contracts.

3. Final Answer:

   Define separate protocols Playable and Recordable, then have types conform to one or both as needed -> Option A

4. Quick Check:

   Separate protocols = flexible conformance [OK]

Hint: Use multiple protocols for flexible behavior [OK]

Common Mistakes:

• Forcing all types to implement all methods
• Using inheritance instead of protocols
• Not leveraging protocol composition