Key Path Expressions as Functions

able to use the key path expression \Root.value wherever functions of (Root) -> Value are allowed. Swift 5.2

💈範例

👉 paiza.io

struct User {
    let email: String
    let isActive: Bool
}

let users = [
    User(email: "[e-mail]", isActive: true),
    User(email: "[e-mail]", isActive: false)
]

// ⭐️ key path as functions
// 1. \.email    = { $0.email    }
// 2. \.isActive = { $0.isActive }
//                     ╭──1──╮
let emails = users.map(\.email)    // (User) -> String
// ["[e-mail]", "[e-mail]"]

//                             ╭───2────╮
let activeUsers = users.filter(\.isActive)    // (User) -> Bool
// [User(email: "[e-mail]", isActive: true)]

// ⭐️ subscript <---> key path 
// user[keyPath: \User.isActive] == user.isActive 
// let userStates = users.map { user in user[keyPath: \User.isActive] } // == users.map { $0.isActive }
     

📗 參考

• InfoQ ⟩ Swift 5.2 Brings callAsFunction, Subscript with Default Arguments, and More

• Swift Evolution ⟩ SE-0249: Key Path Expressions as Functions

• 彼得潘 ⟩ Swift 方便存取 property 的 KeyPath & KeyPath as Functions

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