monix.reactive.internal.operators.ThrottleLastObservable.scala Maven / Gradle / Ivy

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Sub-module of Monix, exposing the Observable pattern for modeling of reactive streams. See: https://monix.io
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/*
 * Copyright (c) 2014-2021 by The Monix Project Developers.
 * See the project homepage at: https://monix.io
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package monix.reactive.internal.operators

import monix.execution.Ack.{Continue, Stop}
import monix.execution.cancelables.{CompositeCancelable, SingleAssignCancelable}
import monix.execution.{Ack, Cancelable}
import monix.reactive.Observable
import monix.reactive.observers.Subscriber
import scala.concurrent.Future

private[reactive] final class ThrottleLastObservable[+A, S](
  source: Observable[A],
  sampler: Observable[S],
  shouldRepeatOnSilence: Boolean)
  extends Observable[A] {

  def unsafeSubscribeFn(downstream: Subscriber[A]): Cancelable = {
    val upstreamSubscription = SingleAssignCancelable()
    val samplerSubscription = SingleAssignCancelable()
    val composite = CompositeCancelable(upstreamSubscription, samplerSubscription)

    upstreamSubscription := source.unsafeSubscribeFn(new Subscriber.Sync[A] { upstreamSubscriber =>
      implicit val scheduler = downstream.scheduler

      // Value is volatile to keep write to lastValue visible
      // after this one is seen as being true
      @volatile private[this] var hasValue = false
      // MUST BE written before `hasValue = true`
      private[this] var lastValue: A = _
      // To be written in onComplete/onError, to be read from tick
      private[this] var upstreamIsDone = false
      // MUST BE synchronized by `upstreamSubscriber`.
      private[this] var downstreamIsDone = false

      def onNext(elem: A): Ack =
        if (downstreamIsDone) Stop
        else {
          lastValue = elem
          hasValue = true
          Continue
        }

      def onError(ex: Throwable): Unit =
        upstreamSubscriber.synchronized {
          if (!downstreamIsDone) {
            downstreamIsDone = true
            samplerSubscription.cancel()
            downstream.onError(ex)
          }
        }

      def onComplete(): Unit =
        upstreamSubscriber.synchronized {
          upstreamIsDone = true
        }

      samplerSubscription := sampler.unsafeSubscribeFn(new Subscriber[S] { self =>
        implicit val scheduler = downstream.scheduler

        def onNext(elem: S): Future[Ack] =
          upstreamSubscriber.synchronized(signalNext())

        def onError(ex: Throwable): Unit =
          upstreamSubscriber.onError(ex)

        def onComplete(): Unit =
          upstreamSubscriber.synchronized {
            upstreamIsDone = true
            signalNext()
            ()
          }

        def signalNext(): Future[Ack] =
          if (downstreamIsDone) Stop
          else {
            val next =
              if (!hasValue) Continue
              else {
                hasValue = shouldRepeatOnSilence
                val ack = downstream.onNext(lastValue)
                ack.syncOnStopOrFailure { _ =>
                  downstreamIsDone = true
                  upstreamSubscription.cancel()
                }
              }

            if (!upstreamIsDone) next
            else {
              downstreamIsDone = true
              upstreamSubscription.cancel()
              if (next ne Stop) downstream.onComplete()
              Stop
            }
          }
      })
    })

    composite
  }
}