src/Pure/System/event_bus.scala

 
 
   /* event invocation */
 
   def event(x: Event) { synchronized { receivers.foreach(_ ! x) } }
+
+
+  /* await global condition -- triggered via bus events */
+
+  def await(cond: => Boolean)
+  {
+    case object Wait
+    val a = new Actor {
+      def act {
+        if (cond) react { case Wait => reply(()); exit(Wait) }
+        else {
+          loop {
+            react {
+              case trigger if trigger != Wait =>
+                if (cond) { react { case Wait => reply(()); exit(Wait) } }
+            }
+          }
+        }
+      }
+    }
+    this += a
+    a.start
+    a !? Wait
+    this -= a
+  }
 }