Fix: Draft fix of SwitchEventStream for raquo#103

src/main/scala/com/raquo/airstream/flatten/SwitchEventStream.scala

@@ -10,7 +10,7 @@ import scala.util.{ Failure, Success, Try }
   *
   * This stream emits the events from the last such stream created this way.
   *
-  * Events are emitted at the same time as the currently tracked stream emits them.
+  * Events are emitted at the same time as the currently tracked stream emits them (but in a new transaction).
   *
   * When `parent` emits a nextValue, this stream switches to emitting events from `makeStream(nextValue)` (which is a stream).
   *
@@ -35,10 +35,9 @@ class SwitchEventStream[I, O](
 
   override protected val topoRank: Int = 1
 
-  private[this] var maybeCurrentEventStream: js.UndefOr[Try[EventStream[O]]] = parent match {
-    case signal: Signal[I @unchecked] => signal.tryNow().map(makeStream)
-    case _ => js.undefined
-  }
+  private[this] var firstStart: Boolean = true
+
+  private[this] var maybeCurrentEventStream: js.UndefOr[Try[EventStream[O]]] = js.undefined
 
   // @TODO[Elegance] Maybe we should abstract away this kind of internal observer
   private[this] val internalEventObserver: InternalObserver[O] = InternalObserver[O](
@@ -71,11 +70,18 @@ class SwitchEventStream[I, O](
   }
 
   override protected[this] def onStart(): Unit = {
+    super.onStart()
+    if (firstStart) {
+      firstStart = false
+      maybeCurrentEventStream = parent match {
+        case signal: Signal[I @unchecked] => signal.tryNow().map(makeStream)
+        case _ => js.undefined
+      }
+    }
     maybeCurrentEventStream.foreach { streamTry =>
       val initialStream = streamTry.fold(err => EventStream.fromTry(Failure(err), emitOnce = true), identity)
       initialStream.addInternalObserver(internalEventObserver)
     }
-    super.onStart()
   }
 
   override protected[this] def onStop(): Unit = {