Copy-On-Write Swift Property Wrapper

🐮.swift

final private class Ref<T> {
    var value: T
    init(_ v: T) { self.value = v }
}

@propertyWrapper
public struct CopyOnWrite<T> {
    private var ref: Ref<T>
    
    public init(_ value: T) {
        self.ref = Ref(value)
    }
    
    public init(wrappedValue: T) {
        self = Self(wrappedValue)
    }
    
    public var wrappedValue: T {
        @inline(__always)
        get { self.ref.value }
        @inline(__always)
        set {
            if isKnownUniquelyReferenced(&self.ref) {
                self.ref.value = newValue
            } else {
                self.ref = Ref(newValue)
            }
        }
    }
}