Mirror.handleChildren()

recursive reflections: inspect/manipulate on children's children.

🌀 Mirror

💾 程式： replit

//     *: born,  +: new,  /: update
// ─────────────────────────────────────
// 2022.01.27 * (v.1) + handleChildren
// 2022.01.28 / handleChildren: global func -> Mirror method

extension Mirror {
    /// ⭐ handle children of type T
    public static func handleChildren<T>(         // T: child's type ⭐ 
        of subject  : Any,          // parent (⭐ of type `Any`)
        type        : T.Type,       // child type
        recursive   : Bool = false, // first level only or all levels of children
        with handler: (T)->Void)    // child handler
    {
        // ⭐ mirror of `subject`
        let mirror = Mirror(reflecting: subject)

        // ⭐ do somethingfor each child.value of type `T`
        // ⭐ for case let ... where 
        //    ----------------------
        //    (pattern-match tuples + data binding + condition)
        //            ↱ ⭐ child.label ignored     ╭── ⭐ where ───╮
        for case let (_, value) in mirror.children where value is T {
            // handle T value
            handler(value as! T)     // ⭐ force type casting (Any -> T)
            // ⭐ handle children's T value recursively
            if recursive {
                handleChildren(of: value, type: T.self, recursive: true, with: handler)
            }
        }
    }
}

💈範例

⬆️ 需要： Logger

// supporting types (Credentials, ...) not listed here.
struct Session {
    let credentials = Credentials()
    let favorites   = Favorites()
    let settings    = Settings()
}

extension Session {

    // 1. ⭐ reset everything one by one
    func logOut1() {
        Logger.box("# 1: manual reset")
        credentials.reset()
        favorites.reset()
        settings.reset()
    }

    // 2. ⭐ use `Mirror` to reset every `Resettable` (first level) child
    func logOut2() {

        Logger.box("# 2: use 'Mirror' directly")

        // 2.1: mirror self
        let mirror = Mirror(reflecting: self)

        // 2.2: inspect children
        // ⭐ `for case let ... where` (pattern matching + binding + condition)
        // ---------------------------------------------------------------------
        //            ↱ ⭐ child.label ignored     ╭─── ⭐ where clause ────╮
        for case let (_, value) in mirror.children where value is Resettable {

            // ⭐ 2.2.1: `if let ... as?` (optional binding)
            // ----------------------------------------------
            // if let resettable = child.value as? Resettable {
            //     resettable.reset()
            // }

            // ⭐ 2.2.2: `(... as? ...)?.method()` (optional chaining)
            // -------------------------------------------------------
            // for child in mirror.children {
            //     (child.value as? Resettable)?.reset()
            // }

            // ⭐ 2.2.3: `as!` force type casting (from Any to Resettable)
            // -----------------------------------------------------------
            // doing this is safe because `value` IS `Resettable`!
            (value as! Resettable).reset()
        }
    }

    // 3. ⭐ wrap "method 2" in `Mirror.handleChildren` meth
    func logOut3(){
        Logger.box("# 3: Mirror.handleChildren() (level 1)")
        // ⭐ use `handleChildren` on level 1 children
        Mirror.handleChildren(of: self, type: Resettable.self) { child in
            child.reset()
        }
    }

    // 4. ⭐ use `handleChildren` recursively
    func logOut4(){
        Logger.box("# 4: Mirror.handleChildren() (recursive)")
        Mirror.handleChildren(of: self, type: Resettable.self, recursive: true) { child in
            child.reset()
        }
        Logger.topline("⭐ this method has recursive mode.")
    }
}

執行結果：

// ┌─────────────────────────┐
// │    # 1: manual reset    │
// └─────────────────────────┘
// 🔸 resetting `Credentials` ...
// 🔸 resetting `Favorites` ...
// 🔸 resetting `Settings` ...
// ┌──────────────────────────────────┐
// │    # 2: use 'Mirror' directly    │
// └──────────────────────────────────┘
// 🔸 resetting `Credentials` ...
// 🔸 resetting `Favorites` ...
// 🔸 resetting `Settings` ...
// ┌──────────────────────────────────────────────┐
// │    # 3: Mirror.handleChildren() (level 1)    │
// └──────────────────────────────────────────────┘
// 🔸 resetting `Credentials` ...
// 🔸 resetting `Favorites` ...
// 🔸 resetting `Settings` ...
// ┌────────────────────────────────────────────────┐
// │    # 4: Mirror.handleChildren() (recursive)    │
// └────────────────────────────────────────────────┘
// 🔸 resetting `Credentials` ...
// 🔸 resetting `Favorites` ...
// 🔸 resetting `Settings` ...
// 🔸 resetting `PreferredLanguages` ...
// ─────────────────────────────────
// ⭐ this method has recursive mode.\

📗 參考

• Sundell ⟩ Reflection in Swift

  • Recursive reflections ⭐️ (💈範例)

📘 手冊

• Swift ⟩

  • Debugging and Reflection ⟩ Mirror (struct)

    • Mirror.Child

    • Mirror.Children

    • Mirror.DisplayStyle

👥 相關

• Pattern Matching - pattern-match on mirror's children.

• for case let ... where - (pattern matching + binding + condition)

• if let ... as? - optional binding.

• (... as? ...)?.method() - optional chaining.

• ... as! ... - force type casting.

• Logger is used in [💈範例].

History

contents

1. (2022.01.27) - first version. (global function)

1

這個程式碼應該要寫成 global function，如果寫成 protocol method 會出現許多問題 (例如：children of children 可能根本不遵循這個 protocol，導致無法使用此 method recursively)

👉 💾 程式： attemp 1 (with lots of errors)

/// ⭐ handle children of type T
func handleChildren<T>(         // T: child's type ⭐ 
    of subject  : Any,          // parent (⭐ of type `Any`)
    type        : T.Type,       // child type
    recursive   : Bool = false, // first level only or all levels of children
    with handler: (T)->Void)    // child handler
{
    // ⭐ mirror of `subject`
    let mirror = Mirror(reflecting: subject)

    // ⭐ do somethingfor each child.value of type `T`
    // ⭐ for case let ... where 
    //    ----------------------
    //    (pattern-match tuples + data binding + condition)
    //            ↱ ⭐ child.label ignored     ╭── ⭐ where ───╮
    for case let (_, value) in mirror.children where value is T {
        // handle T value
        handler(value as! T)     // ⭐ force type casting (Any -> T)
        // ⭐ handle children's T value recursively
        if recursive {
            handleChildren(of: value, type: T.self, recursive: true, with: handler)
        }
    }
}