🔴Protocol Extensions

✅ 可以做

• provide default implementations. (dynamic method dispatch) 👉 📗 參考

• add new functionalities: (static method dispatch)

  • add methods

  • add (convenience) initializers (but not convenience override init)

  • add subscripts

  • add computed properties

• make existing types conform to protocol.

⛔ 不能做

• cannot extend/inherit from another protocol. (protocol inheritance is always specified in the protocol definition. 👉 Swift ⟩ Protocol Extensions)

• cannot add stored properties.

• cannot add property observers.

• cannot add designated init or deinit to a class.

• cannot override. (⛔ overriding declarations in extensions is not supported)

📘 手冊

• Swift ⟩

  • Extensions

  • Protocol Extensions

  • Nested Types

  • Nested Types in Extensions

📗 參考

• Hacking with Swift ⟩ Swift Protocols: Tips and Tricks

• Dmitrii Ivanov ⟩ Protocol extensions

• RightPoint ⟩ Method Dispatch in Swift ⭐️

• Ole Begemann ⟩ Method Dispatch in Protocol Extensions

🗣 討論

• Nested types in protocols (and nesting protocols in types)

問：protocol extension 裡面可以加 nested types 嗎？

答：目前看來不行❗️ (👉 🗣 討論)

Extensions can add new nested types to existing class, struct, and enum. (but not protocol extensions❓) 👉Nested Types in Extensions

Protocol Definition vs. Protocol Extension

• Declaring a method inside the protocol (definition) creates an extension point – a method that we encourage conforming types to override.

• If we put methods into the protocol extension but not the protocol definition, all conforming types still get the method, except now we’re making it harder for them to override. (💈例二)

🔸 definition

• define requirements for conforming types. (💈例一)

🌀 extension

• provide default implementations for protocol requirements. (💈例一)

• provide default values. (💈例一)

💈例一

// 🔸 protocol definition
protocol Logging {
    func log(_ message: String)     // method requirement
    var filename: String { get }    // property requirement
}

// 🌀 protocol extension
extension Logging {
    // ⭐ default implementation
    func log(_ message: String) {
        print("\(Date()): \(message).")
    }
    // ⭐ default value
    var filename: String { return "app.log" }
}

💈例二

👉replit

// 🔸 protocol definition
protocol Developer {}

// 🌀 protocol extension
extension Developer {
    // ⭐ added in extension but not in definition
    func attendMeeting() { print("OK, let's go!") }
}

// 📦 conforming type
struct SeniorDeveloper: Developer {
    func attendMeeting() { print("No way!") }
}

// ⭐ protocol extension method is called
let dev: Developer = SeniorDeveloper()
dev.attendMeeting()       // ⭐ "OK, let's go!"❗

// ⭐ instance method is called
let david = SeniorDeveloper()
david.attendMeeting()           // ⭐ "No way!"

Conditional Protocol Extensions

💈範例

//                   ╭───── condition ──────╮
extension Collection where Element: Equatable {
    func allEqual() -> Bool {
        for element in self {
            if element != self.first { return false }
        }
        return true
    }
}

// test run
let numbers = [100, 100, 100, 100, 100]
numbers.allEqual()    // true

📘 手冊

• Swift ⟩ Protocol Extensions ⟩ Adding Constraints to Protocol Extensions