Add `observableSource` converter

src/main/scala/org/gerweck/scalafx/akka/AkkaFX.scala

@@ -1,12 +1,15 @@
 package org.gerweck.scalafx.akka
 
-import scala.concurrent.Future
+import scala.collection.mutable
+import scala.concurrent._
 
 import akka.Done
+import akka.stream.OverflowStrategy
 import akka.stream.scaladsl._
 
 import scalafx.application.Platform.runLater
 import scalafx.beans.property.Property
+import scalafx.beans.value.ObservableValue
 
 /** A master object that exposes all the Akka-ScalaFX bridges.
   *
@@ -28,4 +31,45 @@ trait AkkaStreamFX {
       }
     }
   }
+
+  /** A [[akka.stream.scaladsl.Source]] that generates an event for each
+    * change of an [[scalafx.beans.value.ObservableValue]].
+    *
+    * This source adds an `onChange` handler to the given `ObservableValue`.
+    * Each time it observes a change, the new value is pushed from the
+    * `Source`. The change handler is registered as soon as the source is
+    * materialized into a graph. It should be safe to use a single source
+    * in several graphs, as each will register its own change listener upon
+    * materialization.
+    *
+    * @param prop The value to observe.
+    *
+    * @param queueSize The maximum number of values to queue while waiting for
+    * the downstream flow to consume more data.
+    *
+    * @param overflow What to do when the queue is full because the downstream
+    * flow cannot keep up. The default behavior is to block, slowing the UI's
+    * main thread until some events are consumed, freeing space in the queue.
+    */
+  def observableSource[A](prop: ObservableValue[_, A],
+                          queueSize: Int = 10,
+                          overflow: OverflowStrategy = OverflowStrategy.backpressure
+                        )(implicit ec: ExecutionContext) = {
+    val src = Source
+      .queue[A](queueSize, overflow)
+      .mapMaterializedValue { m =>
+        val sub = {
+          prop.onChange { (dta, oldV, newV) =>
+            m.offer(newV)
+          }
+        }
+        m.watchCompletion foreach { c =>
+          runLater {
+            sub.cancel()
+          }
+        }
+        m
+      }
+    src
+  }
 }