stopwatch_formula.kt

class StopwatchFormula : Formula<Unit, StopwatchState, StopwatchRenderModel> {

    override fun initialState(input: Unit) = StopwatchState(
        timePassedInMillis = 0,
        isRunning = false
    )

    override fun evaluate(
        input: Unit,
        state: StopwatchState,
        context: FormulaContext<StopwatchState>
    ): Evaluation<StopwatchRenderModel> {
        return Evaluation(
            renderModel = StopwatchRenderModel(
                timePassed = formatTimePassed(state.timePassedInMillis),
                startStopButton = startStopButton(state, context),
                resetButton = resetButton(state, context)
            )
        )
    }

    private fun formatTimePassed(timePassedInMillis: Long): String {
        // TODO: actually format it.
        return "$timePassedInMillis ms"
    }

    private fun startStopButton(state: StopwatchState, context: FormulaContext<StopwatchState>): ButtonRenderModel {
        return ButtonRenderModel(
            text = if (state.isRunning) "Stop" else "Start",
            onSelected = context.callback {
                transition(state.copy(isRunning = !state.isRunning))
            }
        )
    }

    private fun resetButton(state: StopwatchState, context: FormulaContext<StopwatchState>): ButtonRenderModel {
        return ButtonRenderModel(
            text = "Reset",
            onSelected = context.callback {
                transition(state.copy(timePassedInMillis = 0, isRunning = false))
            }
        )
    }
}