Protocol composition in Swift

Introduction

Protocol composition lets you combine multiple protocols into one requirement. It helps you say: "I want something that follows these rules together."

When a function needs a parameter that follows more than one protocol.

When you want to group behaviors without creating a new protocol.

When you want to write flexible code that works with different types sharing multiple features.

Syntax

Swift

func exampleFunction(param: ProtocolA & ProtocolB) {
    // code here
}

Use the & symbol to combine protocols.

The combined type must conform to all listed protocols.

Examples

This function accepts any object that can both drive and fly.

Swift

protocol Drivable {
    func drive()
}

protocol Flyable {
    func fly()
}

func travel(vehicle: Drivable & Flyable) {
    vehicle.drive()
    vehicle.fly()
}

This function requires a document that can be read and written.

Swift

protocol Readable {
    func read()
}

protocol Writable {
    func write()
}

func process(document: Readable & Writable) {
    document.read()
    document.write()
}

Sample Program

This program defines two protocols, Painter and Singer. The Artist struct follows both. The perform function uses protocol composition to accept anything that can paint and sing. It calls both methods.

Swift

protocol Painter {
    func paint()
}

protocol Singer {
    func sing()
}

struct Artist: Painter, Singer {
    func paint() {
        print("Painting a beautiful picture.")
    }
    func sing() {
        print("Singing a lovely song.")
    }
}

func perform(show: Painter & Singer) {
    show.paint()
    show.sing()
}

let artist = Artist()
perform(show: artist)

Important Notes

Protocol composition is a way to require multiple behaviors without making a new protocol.

It helps keep code flexible and reusable.

Summary

Protocol composition combines multiple protocols using &.

It lets you require multiple behaviors at once.

Use it to write flexible and clear code.

Practice

1. What does protocol composition in Swift allow you to do?

easy

A. Override methods from multiple protocols automatically

B. Create a new class that inherits from multiple classes

C. Define a protocol that inherits from a single class

D. Combine multiple protocols into one requirement using &

Solution

1. Step 1: Understand protocol composition concept

   Protocol composition uses the & symbol to combine multiple protocols into one requirement.

2. Step 2: Differentiate from class inheritance

   Swift does not support multiple class inheritance, but protocol composition allows combining multiple protocols.

3. Final Answer:

   Combine multiple protocols into one requirement using & -> Option D

4. Quick Check:

   Protocol composition = Combine protocols with & [OK]

Hint: Remember & joins protocols to require multiple behaviors [OK]

Common Mistakes:

• Confusing protocol composition with class inheritance
• Thinking protocols can inherit from classes
• Assuming protocol composition overrides methods

2. Which of the following is the correct syntax for protocol composition in Swift?

easy

A. protocol Combined = ProtocolA & ProtocolB

B. let combined: ProtocolA & ProtocolB

C. protocol Combined: ProtocolA & ProtocolB {}

D. class Combined: ProtocolA & ProtocolB {}

Solution

1. Step 1: Identify correct usage of protocol composition

   Protocol composition is used in type annotations like let variable: ProtocolA & ProtocolB.

2. Step 2: Check other options for syntax errors

   Options B and C misuse protocol inheritance or assignment syntax; D tries class inheritance from protocols incorrectly.

3. Final Answer:

   let combined: ProtocolA & ProtocolB -> Option B

4. Quick Check:

   Protocol composition syntax = type annotation with & [OK]

Hint: Use & in type declarations, not in protocol inheritance [OK]

Common Mistakes:

• Trying to inherit protocols with & in protocol declaration
• Using = instead of : or &
• Using class inheritance syntax with protocols

3. What will be the output of this Swift code?

protocol A { func foo() }
protocol B { func bar() }
struct S: A, B {
  func foo() { print("foo") }
  func bar() { print("bar") }
}
func test(x: A & B) {
  x.foo()
  x.bar()
}
let s = S()
test(x: s)

medium

A. bar

B. foo

C. foo
bar

D. Compilation error

Solution

1. Step 1: Understand protocol composition parameter

   The function test requires an argument conforming to both protocols A and B.

2. Step 2: Check struct S conformance and method calls

   S conforms to both A and B, so calling foo() and bar() prints "foo" and "bar" respectively.

3. Final Answer:

   foo bar -> Option C

4. Quick Check:

   Both protocol methods called = foo and bar printed [OK]

Hint: Both protocols' methods run when composed type passed [OK]

Common Mistakes:

• Assuming only one protocol's method runs
• Thinking protocol composition causes errors
• Confusing struct conformance with protocol composition

4. Identify the error in this Swift code using protocol composition:

protocol P1 { func p1() }
protocol P2 { func p2() }
func doSomething(x: P1 & P2) {
  x.p1()
  x.p2()
}
struct MyStruct: P1 {
  func p1() { print("p1") }
}
doSomething(x: MyStruct())

medium

A. MyStruct does not conform to P2, so cannot be passed to doSomething

B. Syntax error in protocol composition declaration

C. Missing implementation of p2() in doSomething function

D. doSomething should accept only one protocol, not composition

Solution

1. Step 1: Check struct conformance to protocols

   MyStruct conforms only to P1, missing P2 conformance.

2. Step 2: Understand function parameter requirement

   doSomething requires an argument conforming to both P1 and P2, so MyStruct instance is invalid.

3. Final Answer:

   MyStruct does not conform to P2, so cannot be passed to doSomething -> Option A

4. Quick Check:

   Protocol composition requires all protocols conformed [OK]

Hint: Check all protocols are conformed before passing composed type [OK]

Common Mistakes:

• Passing instance conforming to only one protocol
• Thinking protocol composition allows partial conformance
• Ignoring compiler errors about missing protocol methods

5. Given these protocols and structs:

protocol Drawable { func draw() }
protocol Colorful { var color: String { get } }
struct Circle: Drawable, Colorful {
  var color: String
  func draw() { print("Drawing a \(color) circle") }
}
struct Square: Drawable {
  func draw() { print("Drawing a square") }
}

Which code snippet correctly uses protocol composition to accept only Drawable and Colorful types?

hard

A. func render(item: Drawable & Colorful) { item.draw() }

B. func render(item: Drawable | Colorful) { item.draw() }

C. func render(item: Drawable) where item: Colorful { item.draw() }

D. func render(item: Drawable & Colorful) where item.color != "" { item.draw() }

Solution

1. Step 1: Identify correct protocol composition syntax

   func render(item: Drawable & Colorful) { item.draw() } uses Drawable & Colorful correctly in parameter type.

2. Step 2: Check other options for syntax or logic errors

   func render(item: Drawable | Colorful) { item.draw() } uses | which is invalid; C misuses where clause syntax; D uses where clause incorrectly on parameter.

3. Final Answer:

   func render(item: Drawable & Colorful) { item.draw() } -> Option A

4. Quick Check:

   Use & to combine protocols in function parameters [OK]

Hint: Use & to combine protocols in parameter types, not | or where [OK]

Common Mistakes:

• Using | instead of & for protocol composition
• Misusing where clauses in parameter declarations
• Trying to filter protocol properties in parameter signature