add EventSourceObservable factory

jsdom/src/main/scala/colibri/jsdom/EventSourceObservable.scala

@@ -0,0 +1,22 @@
+package colibri.jsdom
+
+import colibri._
+import org.scalajs.dom
+
+object EventSourceObservable {
+  def apply(url: String): Observable[dom.MessageEvent] =
+    from(() => new dom.EventSource(url))
+
+  def apply(url: String, eventSourceInit: dom.EventSourceInit): Observable[dom.MessageEvent] =
+    from(() => new dom.EventSource(url, eventSourceInit))
+
+  def from(createSource: () => dom.EventSource): Observable[dom.MessageEvent] = Observable.create { observer =>
+    val source = createSource()
+    source.onerror = { ev =>
+      observer.unsafeOnError(new Exception(s"Failed EventSource (${ev.filename}:${ev.lineno}:${ev.colno}): ${ev.message}"))
+    }
+    source.onmessage = observer.unsafeOnNext
+
+    Cancelable(source.close)
+  }
+}