monix.reactive.internal.builders.EvalOnceObservable.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 monix.reactive.internal.builders

import monix.execution.Cancelable
import monix.reactive.Observable
import monix.reactive.observers.Subscriber
import scala.util.control.NonFatal

/** An observable that evaluates the given by-name argument,
  * and emits it.
  */
private[reactive] final class EvalOnceObservable[A](a: => A)
  extends Observable[A] {

  private[this] var result: A = _
  private[this] var errorThrown: Throwable = null
  @volatile private[this] var hasResult = false

  private def signalResult(out: Subscriber[A], value: A, ex: Throwable): Unit = {
    if (ex != null)
      try out.onError(ex) catch {
        case NonFatal(err) =>
          out.scheduler.reportFailure(err)
          out.scheduler.reportFailure(ex)
      }
    else try {
      out.onNext(value)
      out.onComplete()
    } catch {
      case NonFatal(err) =>
        out.scheduler.reportFailure(err)
    }
  }

  def unsafeSubscribeFn(subscriber: Subscriber[A]): Cancelable = {
    if (hasResult) signalResult(subscriber, result, errorThrown) else
      synchronized {
        if (hasResult) signalResult(subscriber, result, errorThrown) else {
          try result = a catch { case NonFatal(ex) => errorThrown = ex }
          hasResult = true
          signalResult(subscriber, result, errorThrown)
        }
      }

    Cancelable.empty
  }
}