I want to make a property wrapper that stores 2 values: the current value and an old value. This is what I have so far:

@propertyWrapper struct DelayedValue<Value> {
    var currentValue: Value
    var oldValue: Value {
        didSet {
            currentValue = oldValue
        }
    }

    var wrappedValue: Value {
        get { return currentValue }
        set { currentValue = newValue }
    }

    var projectedValue: DelayedValue<Value> { self }

    init(wrappedValue: Value) {
        currentValue = wrappedValue
        oldValue = wrappedValue
    }
}

The problem that I'm having is that I can't set the oldValue on the wrapped property.

If I have a @DelayedValue @State private var offset = 0.0 and I try to do _offset.oldValue = 1 I get both "Cannot assign to property: 'self' is immutable" and "Cannot assign value of type 'Int' to type 'State<Double>'"

If I now do:

@propertyWrapper struct DelayedState<Value> {
    @State var currentValue: Value
    @State var oldValue: Value {
        mutating didSet {
            currentValue = oldValue
        }
    }

    var wrappedValue: Value {
        get { return currentValue }
        mutating set { currentValue = newValue }
    }

    var projectedValue: DelayedState<Value> { self }

    init(wrappedValue: Value) {
        _currentValue = State(initialValue: wrappedValue)
        _oldValue = State(initialValue: wrappedValue)
    }
}

And try to do:

@DelayedState private var offset = 0.0
...
offset = 100

I now get this new error: "Cannot assign to property: 'self' is immutable"

Some background: I am trying to "freeze" a view as described here and I think this kind of property wrapper could help me do just that.