Add RestoringWeakReference to utils.

RestoringWeakReference contains a scala.ref.WeakReference that, after it
has been nulled out, uses a restorer function to restore the value.
This can be used for data that can afford to be evicted by the garbage
collector, but will be needed later. One good example is Lift form
callbacks, which may need the value of an object, but where you don't
necessarily want to be retaining the object indefinitely while someone
is on a page in the face of GC contention.

core/util/src/main/scala/net/liftweb/util/RestoringWeakReference.scala

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2006-2011 WorldWide Conferencing, LLC
+ *
+ * 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 net.liftweb
+package util
+
+import scala.ref.WeakReference
+
+/**
+ * `RestoringWeakReference` contains a `scala.ref.WeakReference` that,
+ * after it has been nulled out, uses a restorer function to restore the
+ * value.  This can be used for data that can afford to be evicted by
+ * the garbage collector, but will be needed later. One good example is
+ * Lift form callbacks, which may need the value of an object, but where
+ * you don't necessarily want to be retaining the object indefinitely
+ * while someone is on a page in the face of GC contention.
+ *
+ * You can use `RestoringWeakReference` in a couple of basic ways:
+ *
+ * {{{
+ *   val ref = RestoringWeakReference(() => User.find(id))
+ * }}}
+ * In this situation, the `RestoringWeakReference` will immediately call
+ * the passed function to provide the starting value for the reference,
+ * and then the same function will be used if the reference is evicted.
+ *
+ * {{{
+ *   val ref = RestoringWeakReference(starter, () => User.find(id))
+ * }}}
+ * Here, starter is an `Option[User]` and `User.find` returns an
+ * `Option[User]`. The first parameter will be used to initialize the
+ * weak reference, while the function will be used to restore the value
+ * if the reference is evicted.
+ *
+ * {{{
+ *   val baseRef = new WeakReference(starter)
+ *   val ref = new RestoringWeakReference(baseRef, () => User.find(id))
+ * }}}
+ * If you already have a `WeakReference` instance, you can instantiate
+ * `RestoringWeakReference` directly and pass that reference as the
+ * starting value, as well.
+ *
+ * In all these cases, you use `ref.value` to get a hold of the value.
+ * That function will return the value if it is available, and, if not,
+ * it will restore the WeakReference and then return the value.
+ */
+class RestoringWeakReference[T <: AnyRef](private var reference:WeakReference[T], restorer:()=>T) {
+  def value : T = {
+	val existing = reference.get
+	if (! existing.isDefined) {
+	  restoreReference
+	  value
+	} else {
+	  existing.get
+	}
+  }
+
+  private def restoreReference = {
+	reference = new WeakReference(restorer())
+  }
+}
+object RestoringWeakReference {
+  def apply[T <: AnyRef](restorer:()=>T) = {
+	new RestoringWeakReference(new WeakReference(restorer()), restorer)
+  }
+  def apply[T <: AnyRef](starter:T, restorer:()=>T) = {
+	new RestoringWeakReference(new WeakReference(starter), restorer)
+  }
+}
+