stopwatch_formula_updates.kt

override fun evaluate(
    input: Unit,
    state: State,
    context: FormulaContext<State>
): Evaluation<StopwatchRenderModel> {
    return Evaluation(
        renderModel = ...,
        updates = context.updates {
            if (state.isRunning) {
                val incrementTimePassedEvents = RxStream.fromObservable {
                    Observable
                        .interval(1, TimeUnit.MILLISECONDS)
                        .observeOn(AndroidSchedulers.mainThread())
                }

                events(incrementTimePassedEvents) {
                    transition(state.copy(
                        timePassedInMillis = state.timePassedInMillis + 1
                    ))
                }
            }
        }
    )
}