Resettable.swift

@propertyWrapper
public struct Resettable<Wrapped> {
  private var defaultValue: Wrapped
  private var possibleValue: Wrapped?

  public var wrappedValue: Wrapped {
    get {
      return possibleValue ?? defaultValue
    }
    set {
      self.possibleValue = newValue
    }
  }

  public var projectedValue: Self {
    get { self }
    set { self = newValue }
  }

  public init(wrappedValue: Wrapped? = nil, default defaultValue: Wrapped) {
    self.defaultValue = defaultValue
    self.possibleValue = wrappedValue
  }

  public mutating func reset() {
    self.possibleValue = nil
  }
}

struct MyThing {
  @Resettable(default: 0.5) var factor: Double
  @Resettable(default: 0.5) var factor2: Double = 1.0
}

print(MyThing().factor)   // 0.5
print(MyThing().factor2)  // 1.0

var t = MyThing()
t.factor = 2.0
print(t.factor)  // 2.0
t.$factor.reset()
print(t.factor)  // 0.5

print(
  "size of MyThing:",
  MemoryLayout.size(ofValue: MyThing())
)  // 41 😱