TimerPerMinute.kt

import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.runBlocking
import kotlinx.datetime.*

fun main(args: Array<String>) {
    runBlocking {
        timerPerMinute().collect {
            println("collect: $it")
        }
    }
}

fun timerPerMinute() = flow {
    val currentMillis = Clock.System.now().toEpochMilliseconds()
    val nextOneMinuteDateTime = Clock.System.now().plus(1, DateTimeUnit.MINUTE)
        .toLocalDateTime(TimeZone.currentSystemDefault())
    val nextOneMinuteInMillis = nextOneMinuteDateTime.truncateToMinute().toEpochMilliseconds()

    // subtract to find first time difference in milis for initial delay
    val timeDifferenceMilis = nextOneMinuteInMillis - currentMillis
    var firstTime = true
    
    println("nextDateTimeSecond: $nextOneMinuteInMillis")
    println("dateTime: $currentMillis")
    println("oneMinuteLater: $timeDifferenceMilis")

    while (true) {
        val now = Clock.System.now()
        emit(getHourAndMinute(now))
        val delayAmount = if (firstTime) (timeDifferenceMilis) else 60000
        println("delayAmount: $delayAmount")
        firstTime = false
        delay(delayAmount)
    }
}

fun LocalDateTime.truncateToMinute(): LocalDateTime = LocalDateTime(
    year = year,
    month = month,
    dayOfMonth = dayOfMonth,
    hour = hour,
    minute = minute,
    second = 0,
)

fun LocalDateTime.toEpochMilliseconds(
    timeZone: TimeZone = TimeZone.currentSystemDefault()
) = this.toInstant(timeZone).toEpochMilliseconds()

fun getHourAndMinute(
    instant: Instant,
    timeZone: TimeZone = TimeZone.currentSystemDefault(),
): String {
    val time = instant.toLocalDateTime(timeZone).time
    val hour = formatDoubleDigitTime(time.hour)
    val minute = formatDoubleDigitTime(time.minute)
    return "$hour:$minute"
}

private fun formatDoubleDigitTime(time: Int) = if (time > 9) time.toString() else "0$time"