dynamicvariable.scala

JAVA 语言的函数式编程扩展

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2006-2007, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

// $Id: DynamicVariable.scala 11974 2007-06-11 14:08:21Z mihaylov $


package scala.util


import Predef._
import System.Threading.Thread
import System.LocalDataStoreSlot

/** <p>
 *    DynamicVariables provide a binding mechanism where the current
 *    value is found through <em>dynamic scope</em>, but where
 *    access to the variable itself is resolved through <em>static
 *    scope</em>.
 *  </p>
 *  <p>
 *    The current value can be retrieved with the
 *    <code>value</code> method.  New values should be
 *    pushed using the <code>withValue</code> method.
 *    Values pushed via <code>withValue</code> only
 *    stay valid while the <code>withValue</code>'s
 *    <em>second</em> argument, a parameterless closure,
 *    executes.  When the second argument finishes,
 *    the variable reverts to the previous value.
 *  </p>
 *  <p>
 *    Usage of <code>withValue</code> looks like this:
 *  </p>
 *  <blockquote><pre>
 *  someDynamicVariable.withValue(newValue) {
 *    // ... code called in here that calls value ...
 *    // ... will be given back the newValue ...
 *  }
 *  </pre></blockquote>
 *  <p>
 *    Each thread gets its own stack of bindings.  When a 
 *    new thread is created, the fluid gets a copy of
 *    the stack of bindings from the parent thread, and 
 *    from then on the bindings for the new thread
 *    are independent of those for the original thread.
 *  </p>
 *
 *  @author  Lex Spoon
 *  @version 1.1, 2007-5-21
 */
class DynamicVariable[T](init: T) {
  private val slot: LocalDataStoreSlot = Thread.AllocateDataSlot()
  value = init

  /** Retrieve the current value */
  def value: T = Thread.GetData(slot).asInstanceOf[T]

  /** Set the value of the fluid while executing the specified
    * thunk.
    *
    * @param newval The value to which to set the fluid
    * @param thunk The code to evaluate under the new setting
    */
  def withValue[S](newval: T)(thunk: =>S): S = {
    val oldval = value
    value = newval

    try { thunk } finally {
      value = oldval
    }
  }

  /** Change the currently bound value, discarding the old value.
    * Usually withValue() gives better semantics.
    */
  def value_=(newval: T) = { Thread.SetData(slot, newval.asInstanceOf[AnyRef]) }

  override def toString: String = "DynamicVariable(" + value  +")"
}