🔴Protocol Extensions

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.

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)

define requirements for conforming types. (💈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" }
}

👉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

//                   ╭───── 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

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Last updated 3 years ago

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