Scala example source code file (DynamicVariable.scala)

This example Scala source code file (DynamicVariable.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

anyref, dynamicvariable, inheritablethreadlocal, s, string, t

The DynamicVariable.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2006-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package util

import java.lang.InheritableThreadLocal

/** `DynamicVariables` provide a binding mechanism where the current
 *  value is found through dynamic scope, but where access to the
 *  variable itself is resolved through static scope.
 *
 *  The current value can be retrieved with the value method. New values
 *  should be pushed using the `withValue` method. Values pushed via
 *  `withValue` only stay valid while the `withValue`'s second argument, a
 *  parameterless closure, executes. When the second argument finishes,
 *  the variable reverts to the previous value.
 *
 *  {{{
 *  someDynamicVariable.withValue(newValue) {
 *    // ... code called in here that calls value ...
 *    // ... will be given back the newValue ...
 *  }
 *  }}}
 *
 *  Each thread gets its own stack of bindings.  When a
 *  new thread is created, the `DynamicVariable` 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.
 *
 *  @author  Lex Spoon
 *  @version 1.1, 2007-5-21
 */
class DynamicVariable[T](init: T) {
  private val tl = new InheritableThreadLocal[T] {
    override def initialValue = init.asInstanceOf[T with AnyRef]
  }

  /** Retrieve the current value */
  def value: T = tl.get.asInstanceOf[T]

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

    try thunk
    finally tl set oldval
  }

  /** Change the currently bound value, discarding the old value.
    * Usually withValue() gives better semantics.
    */
  def value_=(newval: T) = tl set newval

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