การสร้างตัวห่อหุ้มและ projectedValue

เหตุผลที่ใช้ตัวห่อหุ้มคุณสมบัติ

Clamped และแฟล็กที่ส่งออก

@propertyWrapper
struct Clamped {
    private var value: Int
    private let range: ClosedRange<Int>
    private(set) var wasClampedLastSet = false

    var wrappedValue: Int {
        get { value }
        set {
            let clamped = min(max(newValue, range.lowerBound), range.upperBound)
            wasClampedLastSet = (clamped != newValue)
            value = clamped
        }
    }

    var projectedValue: Bool { wasClampedLastSet } // $prop -> was it clamped?

    init(wrappedValue: Int, range: ClosedRange<Int>) {
        self.range = range
        let clamped = min(max(wrappedValue, range.lowerBound), range.upperBound)
        self.value = clamped
        self.wasClampedLastSet = (clamped != wrappedValue)
    }
}

struct Player {
    @Clamped(range: 0...100) var hp: Int = 120  // init clamps to 100
}

var p = Player()
print(p.hp)     // 100
p.hp = -5
print(p.hp, p.$hp)  // 0 true (was clamped)

ปรับมาตรฐานและมุมมองทางเลือก

@propertyWrapper
struct Normalized {
    private var storage: String = ""
    var wrappedValue: String {
        get { storage }
        set { storage = newValue.trimmingCharacters(in: .whitespacesAndNewlines).lowercased() }
    }
    var projectedValue: String { storage.uppercased() } // alternate view

    init(wrappedValue: String) { self.wrappedValue = wrappedValue }
}

struct User {
    @Normalized var username: String = "  Alice  "
}
let u = User()
print(u.username)   // "alice"
print(u.$username)  // "ALICE" (projected)

ตัวห่อหุ้มที่กำหนดค่าได้

@propertyWrapper
struct Padded {
    private var value: String
    private let length: Int
    var wrappedValue: String {
        get { value }
        set {
            let s = newValue
            value = s.count >= length ? String(s.prefix(length))
                                      : s + String(repeating: " ", count: length - s.count)
        }
    }
    var projectedValue: Int { value.count } // current width
    init(wrappedValue: String, length: Int) {
        self.length = length
        self.value = ""
        self.wrappedValue = wrappedValue
    }
}

struct Row {
    @Padded(length: 6) var code: String = "AB"
}
var r = Row()
print("|\(r.code)|")   // |AB    |
print(r.$code)        // 6

ประวัติผ่าน $value

@propertyWrapper
struct History<Value> {
    private var value: Value
    private(set) var changes: [Value] = []
    var wrappedValue: Value {
        get { value }
        set { changes.append(value); value = newValue }
    }
    var projectedValue: [Value] { changes } // $prop -> previous values
    init(wrappedValue: Value) { self.value = wrappedValue }
}

struct Settings {
    @History var level: Int = 1
}
var s = Settings()
s.level = 2
s.level = 3
print(s.level)    // 3
print(s.$level)   // [1, 2]

แนวทางการออกแบบ

จุดประสงค์ของค่าที่ส่งออก

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