monifu.reactive.subjects.AsyncSubject.scala Maven / Gradle / Ivy

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/*
 * Copyright (c) 2014-2016 by its authors. Some rights reserved.
 * 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 monifu.reactive.subjects

import monifu.concurrent.atomic.padded.Atomic
import monifu.reactive.Ack.{Cancel, Continue}
import monifu.reactive.subjects.PublishSubject.State
import monifu.reactive.internals._
import monifu.reactive.{Ack, Subject, Subscriber}
import scala.annotation.tailrec
import scala.concurrent.Future

/**
 * An `AsyncSubject` emits the last value (and only the last value) emitted by
 * the source Observable, and only after that source Observable completes.
 *
 * If the source terminates with an error, the `AsyncSubject` will not emit any
 * items to subsequent subscribers, but will simply pass along the error
 * notification from the source Observable.
 */
final class AsyncSubject[T] extends Subject[T,T] { self =>
  /*
   * NOTE: the stored vector value can be null and if it is, then
   * that means our subject has been terminated.
   */
  private[this] val stateRef = Atomic(State[T]())

  private[this] var onNextHappened = false
  private[this] var cachedElem: T = _

  @tailrec
  def onSubscribe(subscriber: Subscriber[T]): Unit = {
    val state = stateRef.get
    val subscribers = state.subscribers

    if (subscribers eq null) {
      // our subject was completed, taking fast path
      val errorThrown = state.errorThrown
      if (errorThrown != null)
        subscriber.onError(errorThrown)
      else if (onNextHappened) {
        import subscriber.scheduler
        subscriber.onNext(cachedElem)
          .onContinueSignalComplete(subscriber)
      }
      else {
        subscriber.onComplete()
      }
    }
    else {
      val update = State(subscribers :+ subscriber)

      if (!stateRef.compareAndSet(state, update))
        onSubscribe(subscriber) // repeat
    }
  }

  def onNext(elem: T): Future[Ack] = {
    if (stateRef.get.isDone) Cancel else {
      if (!onNextHappened) onNextHappened = true
      cachedElem = elem
      Continue
    }
  }

  def onError(ex: Throwable): Unit = {
    onCompleteOrError(ex)
  }

  def onComplete(): Unit = {
    onCompleteOrError(null)
  }

  @tailrec
  private def onCompleteOrError(ex: Throwable): Unit = {
    val state = stateRef.get
    val subscribers = state.subscribers
    val isDone = subscribers eq null

    if (!isDone) {
      if (!stateRef.compareAndSet(state, state.complete(ex)))
        onCompleteOrError(ex)
      else {
        val iterator = subscribers.iterator
        while (iterator.hasNext) {
          val ref = iterator.next()
          import ref.scheduler

          if (ex != null)
            ref.onError(ex)
          else if (onNextHappened)
            ref.onNext(cachedElem).onContinueSignalComplete(ref)
          else
            ref.onComplete()
        }
      }
    }
  }
}

object AsyncSubject {
  def apply[T](): AsyncSubject[T] =
    new AsyncSubject[T]()
}