Executor implementation for PagedList.Builder's setFetchExecutor and setNotifyExecutor.

Executor.kt

object Executor {
    val uiExecutor = Handler(Looper.getMainLooper()).toExecutor()
    val ioExecutor = LooperThread().also { it.start() }.getHandler().toExecutor()

    private fun Handler.toExecutor(): Executor = Executor {
        while (!post(it)) {
            Thread.sleep(0)
        }
    }

    private class LooperThread : Thread("sub looper") {
        private var handler: Handler? = null
        private val lock = ReentrantLock()
        private val cond = lock.newCondition()

        override fun run() {
            Looper.prepare()
            lock.withLock {
                handler = Handler()
                cond.signalAll()
            }
            Looper.loop()
        }

        fun getHandler() = lock.withLock {
            if (handler == null) {
                cond.await()
            }
            handler!!
        }
    }
}