Protocol declaration syntax in Swift - Step-by-Step Execution

Concept Flow - Protocol declaration syntax

Start Protocol Declaration

↓

Write 'protocol' keyword

↓

Name the Protocol

↓

Define Requirements (properties, methods)

↓

Close Protocol Declaration

↓

End

This flow shows how to declare a protocol: start with the keyword, name it, add required properties and methods, then close.

Execution Sample

Swift

protocol Vehicle {
    var numberOfWheels: Int { get }
    func drive()
}

Declares a protocol named Vehicle with a read-only property and a method requirement.

Execution Table

Step	Action	Code Part	Result
1	Start protocol declaration	protocol	Begin defining a protocol
2	Name the protocol	Vehicle	Protocol named Vehicle created
3	Declare property requirement	var numberOfWheels: Int { get }	Read-only property required
4	Declare method requirement	func drive()	Method drive() required
5	Close protocol declaration	}	Protocol Vehicle fully declared

💡 Protocol declaration ends after closing brace '}'

Variable Tracker

Element	Start	After Step 2	After Step 3	After Step 4	Final
Protocol Name	null	Vehicle	Vehicle	Vehicle	Vehicle
Properties	null	null	numberOfWheels (read-only)	numberOfWheels (read-only)	numberOfWheels (read-only)
Methods	null	null	null	drive()	drive()

Key Moments - 2 Insights

Why do we use { get } after the property declaration?

The { get } means the property must be readable (a getter), but it does not have to be writable. See execution_table step 3.

Can protocols contain code implementations inside?

No, protocols only declare requirements (like properties and methods), but do not provide code. This is shown in execution_table steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is declared at step 3?

A A read-only property requirement

B A method implementation

C The protocol name

D Closing the protocol

💡 Hint

Check the 'Code Part' and 'Result' columns at step 3 in execution_table

At which step is the protocol name defined?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Action' and 'Code Part' columns in execution_table

If we add { get set } to the property, how would variable_tracker change?

A Protocol name would change

B Methods would change

C Properties would be writable and readable

D No change at all

💡 Hint

Refer to variable_tracker row 'Properties' and understand { get set } means read and write

Concept Snapshot

protocol ProtocolName {
    var propertyName: Type { get / get set }
    func methodName()
}
- Use 'protocol' keyword to declare
- Define required properties and methods
- Properties can be read-only ({ get }) or read-write ({ get set })

Full Transcript

This visual trace shows how to declare a protocol in Swift. First, write the keyword 'protocol', then name your protocol. Next, declare required properties with { get } to indicate read-only or { get set } for read-write. Then declare required methods without implementation. Finally, close the declaration with a brace. Protocols only specify what is required, not how it works.

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