การนำกลับมาใช้ใหม่และข้อจำกัด

เหตุใดจึงต้องมีข้อจำกัด

แบบทั่วไป + เปรียบเทียบได้

@propertyWrapper
struct Bounded<Value: Comparable> {
    private var value: Value
    private let range: ClosedRange<Value>

    var wrappedValue: Value {
        get { value }
        set {
            // clamp using Comparable
            if newValue < range.lowerBound { value = range.lowerBound }
            else if newValue > range.upperBound { value = range.upperBound }
            else { value = newValue }
        }
    }

    init(wrappedValue: Value, _ range: ClosedRange<Value>) {
        self.range = range
        self.value = min(max(wrappedValue, range.lowerBound), range.upperBound)
    }
}

struct Stats {
    @Bounded(0...100) var score: Int = 120
    @Bounded("a"..."z") var letter: String = "Swift"
}
var st = Stats()
print(st.score)   // 100
st.letter = "m"
print(st.letter)

ส่วนขยายที่มีข้อจำกัด

extension Bounded where Value: BinaryInteger {
    var isMaxed: Bool { wrappedValue == range.upperBound }
}
extension Bounded where Value: FloatingPoint {
    mutating func bump(by delta: Value) { wrappedValue = wrappedValue + delta }
}

struct Meter {
    @Bounded(0...10) var steps: Int = 9
    @Bounded(0.0...1.0) var progress: Double = 0.25
}
var m = Meter()
print(m.$steps)          // projectedValue not defined; accessing wrapper is not allowed directly here
print(m._steps)          // compiler creates backing storage name; shown for illustration only
print(m.steps)           // 9
print(m._progress)       // backing storage (illustrative)
m._progress.bump(by: 0.6)
print(m.progress)        // 0.85 (clamped if exceeded)

ตัวห่อหุ้มสำหรับคอลเลกชันเท่านั้น

@propertyWrapper
struct MaxLength<Value: RangeReplaceableCollection> where Value.Element: Sendable {
    private var storage: Value
    private let limit: Int

    var wrappedValue: Value {
        get { storage }
        set {
            var v = newValue
            if v.count > limit { v.removeLast(v.count - limit) }
            storage = v
        }
    }

    init(wrappedValue: Value, _ limit: Int) {
        self.limit = limit
        self.storage = wrappedValue
        if storage.count > limit { storage.removeLast(storage.count - limit) }
    }
}

struct Post {
    @MaxLength(10) var title: String = "hello swift learners"
    @MaxLength(5) var tags: [String] = ["swift","ios","spm"]
}
var p = Post()
print(p.title)  // "hello swif"
print(p.tags)   // ["swift","ios","spm"]

การประกอบตัวห่อหุ้ม

@propertyWrapper
struct Trimmed {
    private var s: String = ""
    var wrappedValue: String {
        get { s }
        set { s = newValue.trimmingCharacters(in: .whitespacesAndNewlines) }
    }
    init(wrappedValue: String) { self.wrappedValue = wrappedValue }
}

@propertyWrapper
struct NonEmpty {
    private var s: String = ""
    var wrappedValue: String {
        get { s }
        set { s = newValue.isEmpty ? "N/A" : newValue }
    }
    init(wrappedValue: String) { self.wrappedValue = wrappedValue }
}

struct Profile {
    @NonEmpty @Trimmed var displayName: String = "  "
}
var prof = Profile()
print(prof.displayName)  // "N/A"

ตัวแปรภายในและการนำกลับมาใช้

@propertyWrapper
struct Default<Value> {
    private var value: Value
    private let make: () -> Value
    var wrappedValue: Value {
        get { value }
        set { value = newValue }
    }
    init(wrappedValue: Value, _ factory: @escaping () -> Value) {
        self.value = wrappedValue
        self.make = factory
    }
    mutating func reset() { value = make() }
}

// Local variable usage
do {
    @Default({ 0 }) var counter: Int = 5
    print(counter) // 5
    _counter.reset()
    print(counter) // 0
}

ข้อจำกัดขณะคอมไพล์ของตัวห่อหุ้ม

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