This LiveData provides the time in hh:mm:ss format, updating the value every second

ClockLiveData.kt

package io.github.yaraki.dependencyinjection

import android.arch.lifecycle.LiveData
import android.os.Handler
import android.os.Looper
import android.os.SystemClock
import android.text.format.DateFormat
import java.util.*

class ClockLiveData : LiveData<CharSequence>() {

    private var handler = Handler(Looper.getMainLooper())
    private val calendar = Calendar.getInstance()

    private val ticker: Runnable = object : Runnable {
        override fun run() {
            if (!hasActiveObservers()) {
                return
            }
            calendar.timeInMillis = System.currentTimeMillis()
            value = DateFormat.format("hh:mm:ss", calendar)
            val now = SystemClock.uptimeMillis()
            val next = now + (1000 - now % 1000)
            handler.postAtTime(this, next)
        }
    }

    override fun onActive() {
        super.onActive()
        ticker.run()
    }

}

With a Kotlin coroutine

package io.github.yaraki.coroutineex

import android.arch.lifecycle.LiveData
import android.text.format.DateFormat
import kotlinx.coroutines.experimental.android.UI
import kotlinx.coroutines.experimental.delay
import kotlinx.coroutines.experimental.launch
import java.util.*

class ClockLiveData : LiveData<CharSequence>() {

    private val calendar = Calendar.getInstance()

    override fun onActive() {
        super.onActive()
        launch(UI) {
            while (isActive && hasActiveObservers()) {
                val now = System.currentTimeMillis()
                calendar.timeInMillis = now
                value = DateFormat.format("hh:mm:ss", calendar)
                delay(1000L - (now % 1000L))
            }
        }
    }

}