🔴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

📘 手冊

📗 參考

🗣 討論

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

💈例一

💈例二

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