Replacement for didSet when using SwiftUI

TrackedValue.swift

/// TrackedValue is a property wrapper that allows you to work around the
/// lack of didSet when working with SwiftUI.
/// Explanation and example usage at: https://stackoverflow.com/a/60145397/467209
@propertyWrapper
struct TrackedValue<Tracked, Value>: DynamicProperty {
	var trackedHolder: State<ValueHolder<Tracked>>
	var valueHolder: State<ValueHolder<Value>>

	init(wrappedValue value: Value, tracked: Tracked) {
		self.trackedHolder = State(initialValue: ValueHolder(tracked))
		self.valueHolder = State(initialValue: ValueHolder(value))
	}

	var wrappedValue: Value {
		get { self.valueHolder.wrappedValue.value }
		nonmutating set { self.valueHolder.wrappedValue = ValueHolder(newValue) }
	}

	var projectedValue: Self { return self }

	@discardableResult
	public func update(tracked: Tracked, with block:(Tracked, Value) -> Value) -> EmptyView {
		self.valueHolder.wrappedValue.value = block(self.trackedHolder.wrappedValue.value, self.valueHolder.wrappedValue.value)
		self.trackedHolder.wrappedValue.value = tracked
		return EmptyView()
	}

}

class ValueHolder<Value> {
	init(_ value: Value) { self.value = value }
	var value: Value
}