reactiveDate.kt

package net.aquadc.commonandroid

import android.content.res.Resources
import java.text.SimpleDateFormat
import java.util.*

/**
 * Returns text representation of `Date` and time (in millis)
 * after which this value should be updated;
 * time can be -1, which means that no update will ever be needed
 */
fun Date.relativeToNowWithTimeout(carrier: ReactiveDateCarrier) {
    val seconds = time / 1000
    val secondsNow = System.currentTimeMillis() / 1000

    val secondsDiff = (secondsNow - seconds).toInt()

    if (secondsDiff == 0) {
        carrier.text = carrier.resources.getString(R.string.rightNow)
        carrier.timeout = 1_000
        return
    }

    if (secondsDiff < 60) {
        carrier.text = secondsDiff.toPlural(carrier.resources, R.plurals.secondsAgo)
        carrier.timeout = 1_000
        return
    }

    val minutesDiff = secondsDiff / 60
    if (minutesDiff < 60) {
        carrier.text = minutesDiff.toPlural(carrier.resources, R.plurals.minutesAgo)
        carrier.timeout = 60_000 // minute
        return
    }

    val hoursDiff = secondsDiff / 3600
    if (hoursDiff < 3) {
        carrier.text = hoursDiff.toPlural(carrier.resources, R.plurals.hoursAgo)
        carrier.timeout = 1800_000 // half an hour
        return
    }

    val secondsFromMidnight = secondsFromMidnight(carrier.calendar)
    if (secondsDiff < secondsFromMidnight) { // today
        carrier.text = carrier.todayFormat.format(this)
        carrier.timeout = 1000L*(86_400 - secondsFromMidnight) // next midnight
        return
    }

    if (secondsDiff < secondsFromNewYear(carrier.calendar)) { // this year
        carrier.text = carrier.thisYearFormat.format(this)
        carrier.timeout = 1000L*secondsToNewYear(carrier.calendar) // update when year changes
        return
    }

    // previous years
    carrier.text = carrier.longTimeAgo.format(this)
    carrier.timeout = -1 // never update at all
}

class ReactiveDateCarrier(val resources: Resources) {
    internal val todayFormat: SimpleDateFormat
    internal val thisYearFormat: SimpleDateFormat
    internal val longTimeAgo: SimpleDateFormat

    init {
        val locale = resources.locale
        todayFormat = SimpleDateFormat(resources.getString(R.string.atTime), locale)
        thisYearFormat = SimpleDateFormat(resources.getString(R.string.atDate), locale)
        longTimeAgo = SimpleDateFormat(resources.getString(R.string.atFarDate), locale)
    }

    internal val calendar: Calendar = Calendar.getInstance()
    var text: String = ""
        internal set
    var timeout: Long = 0
        internal set
}

private fun secondsFromMidnight(c: Calendar): Int {
    c.timeInMillis = System.currentTimeMillis()

    val now = c.timeInMillis
    c.set(Calendar.HOUR_OF_DAY, 0)
    c.set(Calendar.MINUTE, 0)
    c.set(Calendar.SECOND, 0)
    c.set(Calendar.MILLISECOND, 0)
    val passed = now - c.timeInMillis
    return (passed / 1000).toInt()
}

private fun secondsFromNewYear(c: Calendar): Int {
    c.timeInMillis = System.currentTimeMillis()

    val now = c.timeInMillis
    c.set(Calendar.DAY_OF_YEAR, 1)
    c.set(Calendar.HOUR_OF_DAY, 0)
    c.set(Calendar.MINUTE, 0)
    c.set(Calendar.SECOND, 0)
    c.set(Calendar.MILLISECOND, 0)
    val passed = now - c.timeInMillis
    return (passed / 1000).toInt()
}

private fun secondsToNewYear(c: Calendar): Int {
    c.timeInMillis = System.currentTimeMillis()

    val now = c.timeInMillis
    c.set(Calendar.DAY_OF_YEAR, 1)
    c.set(Calendar.HOUR_OF_DAY, 0)
    c.set(Calendar.MINUTE, 0)
    c.set(Calendar.SECOND, 0)
    c.set(Calendar.MILLISECOND, 0)

    c.add(Calendar.YEAR, +1)
    val left = c.timeInMillis - now
    return (left / 1000).toInt()
}

strings.xml

<resources>
    <string name="rightNow">just now</string>
    <plurals name="secondsAgo">
        <item quantity="zero">%d seconds ago</item>
        <item quantity="one">%d second ago</item>
        <item quantity="other">%d seconds ago</item>
    </plurals>
    <plurals name="minutesAgo">
        <item quantity="zero">%d minutes ago</item>
        <item quantity="one">%d minute ago</item>
        <item quantity="other">%d minutes age</item>
    </plurals>
    <plurals name="hoursAgo">
        <item quantity="zero">%d hours ago</item>
        <item quantity="one">%d hour ago</item>
        <item quantity="other">%d hours ago</item>
    </plurals>
    <string name="atTime">в HH:mm</string>
    <string name="atDate">dd MMM</string>
    <string name="atFarDate">dd MMM yyyy г.</string>
</resources>