Protocol declaration syntax in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a protocol in Swift?

A protocol in Swift is like a blueprint that defines methods, properties, or other requirements that a class, struct, or enum can adopt and implement.

beginner

How do you declare a protocol in Swift?

Use the keyword protocol followed by the protocol name and curly braces containing the requirements. For example:

protocol MyProtocol {
    func doSomething()
}

intermediate

Can protocols declare properties in Swift? If yes, how?

Yes, protocols can declare properties. They specify whether the property must be gettable or gettable and settable. For example:

protocol Example {
    var name: String { get set }
    var id: Int { get }
}

beginner

What does it mean when a protocol property is declared with { get } only?

It means the property must be readable (gettable) but does not have to be writable (settable).

intermediate

How do you specify that a protocol inherits from other protocols?

List the protocols separated by commas after the protocol name. For example:

protocol Combined: ProtocolA, ProtocolB {
    // requirements
}

Which keyword is used to declare a protocol in Swift?

A class

B interface

C struct

D protocol

In Swift, the keyword protocol is used to declare a protocol.

How do you declare a read-only property requirement in a protocol?

A var propertyName: Type { get }

B var propertyName: Type

C let propertyName: Type

D var propertyName: Type { get set }

Using { get } means the property must be readable but not necessarily writable.

Can a struct adopt a protocol in Swift?

A Yes, structs can adopt protocols.

B No, only classes can adopt protocols.

C Only enums can adopt protocols.

D Protocols cannot be adopted by any type.

In Swift, classes, structs, and enums can all adopt protocols.

How do you declare a protocol that inherits from two other protocols named A and B?

A protocol NewProtocol inherits A, B

B protocol NewProtocol extends A, B

C protocol NewProtocol: A, B

D protocol NewProtocol implements A, B

Use a colon and list the protocols separated by commas after the protocol name.

Which of the following is NOT a valid protocol requirement?

A var count: Int { get set }

B private func helper()

C init(value: Int)

D func doWork()

Protocol requirements cannot have access modifiers like private.

Explain how to declare a protocol in Swift and include property and method requirements.

Think about the blueprint idea and how you list requirements.

Describe how protocol inheritance works in Swift and why it might be useful.

Consider how you can build on existing blueprints.

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