stopwatch_formula_state.kt

class StopwatchFormula : Formula<Unit, StopwatchFormula.State, StopwatchRenderModel> {

    data class State(
        val timePassedInMillis: Long,
        val isRunning: Boolean
    )

    override fun initialState(input: Unit): State = State(
        timePassedInMillis = 0,
        isRunning = false
    )
    
    ....
}