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/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent


/** A `DelayedLazyVal` is a wrapper for lengthy computations which have a
 *  valid partially computed result.
 *
 *  The first argument is a function for obtaining the result at any given
 *  point in time, and the second is the lengthy computation.  Once the
 *  computation is complete, the `apply` method will stop recalculating it
 *  and return a fixed value from that point forward.
 *
 *  @param  f      the function to obtain the current value at any point in time
 *  @param  body   the computation to run to completion in another thread
 *
 *  @author  Paul Phillips
 *  @version 2.8
 */
class DelayedLazyVal[T](f: () => T, body: => Unit)(implicit exec: ExecutionContext){
  @volatile private[this] var _isDone = false
  private[this] lazy val complete = f()

  /** Whether the computation is complete.
   *
   *  @return true if the computation is complete.
   */
  def isDone = _isDone

  /** The current result of f(), or the final result if complete.
   *
   *  @return the current value
   */
  def apply(): T = if (isDone) complete else f()

  exec.execute(new Runnable { def run = { body; _isDone = true } })
}