Protocols in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a protocol in Swift?

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. Classes, structs, or enums can adopt protocols to provide actual implementations.

beginner

How do you declare a protocol in Swift?

Use the protocol keyword followed by the protocol name and curly braces. Inside, list the required methods or properties without implementations.

intermediate

Can a Swift protocol require properties? If yes, what kind?

Yes, protocols can require properties. They can specify whether the property must be gettable or gettable and settable, but they do not provide storage or implementation.

beginner

What does it mean for a type to conform to a protocol?

It means the type agrees to implement all the requirements defined in the protocol, such as methods and properties, ensuring it meets the protocol's contract.

intermediate

Can protocols be used as types in Swift? Give an example.

Yes, protocols can be used as types to define variables, parameters, or return types. For example, a variable of type MyProtocol can hold any instance of a type that conforms to MyProtocol.

Which keyword is used to define a protocol in Swift?

A class

B protocol

C interface

D struct

Swift uses the keyword protocol to define protocols.

What must a type do to conform to a protocol?

A Implement all required methods and properties

B Inherit from the protocol

C Override the protocol

D Declare the protocol as a variable

Conforming means implementing all the protocol's required methods and properties.

Can a protocol specify property requirements without providing storage?

A No, it must provide storage

B Only for classes

C Yes, it only specifies get/set requirements

D Only for structs

Protocols specify whether properties must be gettable or settable but do not provide storage.

Which of these can adopt a protocol in Swift?

A Class, struct, and enum

B Only classes

C Only structs

D Only enums

Classes, structs, and enums can all adopt protocols.

Can you use a protocol as a type for a variable?

A Only inside functions

B No, protocols are not types

C Only if the protocol is a class

D Yes, to hold any conforming instance

Protocols can be used as types to hold any instance that conforms to them.

Explain what a protocol is in Swift and why it is useful.

Think about how protocols help different types agree on a set of rules.

Describe how a type conforms to a protocol and what happens if it does not implement all requirements.

Consider what the compiler expects when you say a type adopts a protocol.

Practice

1. What is the main purpose of a protocol in Swift?

easy

A. To create a new class with default behavior

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

C. To store data persistently on the device

D. To handle user interface layout automatically

Solution

1. Step 1: Understand what a protocol defines

   A protocol specifies methods and properties that a type must implement, acting as a blueprint.

2. Step 2: Differentiate from other concepts

   Protocols do not create classes or store data; they only define requirements.

3. Final Answer:

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

4. Quick Check:

   Protocol purpose = blueprint [OK]

Hint: Protocols define required methods and properties [OK]

Common Mistakes:

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

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

easy

A. struct Drivable { func drive() }

B. class Drivable { func drive() }

C. func protocol Drivable() { drive() }

D. protocol Drivable { func drive() }

Solution

1. Step 1: Identify correct protocol declaration syntax

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

2. Step 2: Check method declaration inside protocol

   Methods inside protocols are declared without implementation, just the signature.

3. Final Answer:

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

4. Quick Check:

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

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

Common Mistakes:

• Using class or struct instead of protocol
• Adding method implementation inside protocol
• Incorrect keyword order

3. Consider this Swift code:

protocol Greetable {
  func greet() -> String
}

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

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

What will be printed when this code runs?

medium

A. Hello!

B. greet()

C. Error: Missing protocol implementation

D. nil

Solution

1. Step 1: Check protocol and struct conformance

   The struct Person adopts Greetable and implements the required greet() method returning "Hello!".

2. Step 2: Understand the print statement output

   Calling p.greet() returns "Hello!", which is printed.

3. Final Answer:

   Hello! -> Option A

4. Quick Check:

   Implemented method returns "Hello!" [OK]

Hint: Implemented protocol methods run normally [OK]

Common Mistakes:

• Expecting protocol name printed
• Thinking missing implementation error occurs
• Assuming nil is printed

4. What is wrong with this Swift code?

protocol Runnable {
  func run()
}

struct Athlete: Runnable {
  // Missing run() implementation
}

medium

A. No error, code is valid

B. Error: run() must have a return type

C. Error: Athlete does not implement required method run()

D. Error: Protocol Runnable cannot be adopted by struct

Solution

1. Step 1: Check protocol requirements

   The protocol Runnable requires a method run() with no return value.

2. Step 2: Verify struct conformance

   The struct Athlete adopts Runnable but does not implement run(), causing a compile error.

3. Final Answer:

   Error: Athlete does not implement required method run() -> Option C

4. Quick Check:

   Missing required method causes error [OK]

Hint: All protocol methods must be implemented [OK]

Common Mistakes:

• Assuming structs can't adopt protocols
• Thinking missing method is allowed
• Expecting run() to require return type

5. You want to create a protocol Vehicle with a property speed that can be read and set. Which of these protocol declarations is correct?

hard

A. protocol Vehicle { var speed: Int { get set } }

B. protocol Vehicle { var speed: Int }

C. protocol Vehicle { func speed() -> Int }

D. protocol Vehicle { let speed: Int }

Solution

1. Step 1: Understand property requirements in protocols

   To require a property to be readable and writable, use { get set } in the protocol.

2. Step 2: Analyze each option

   protocol Vehicle { var speed: Int { get set } } correctly declares speed as a variable property with get set. protocol Vehicle { var speed: Int } misses get set, so it's read-only. protocol Vehicle { func speed() -> Int } declares a method, not a property. protocol Vehicle { let speed: Int } uses let, which is not allowed in protocols.

3. Final Answer:

   protocol Vehicle { var speed: Int { get set } } -> Option A

4. Quick Check:

   Writable property = var with get set [OK]

Hint: Use 'var' with { get set } for read-write properties [OK]

Common Mistakes:

• Using 'let' in protocol properties
• Omitting { get set } for writable properties
• Confusing methods with properties