Make layout event asynchronous unbatched

Summary:
Changelog: [internal]

Making "layout" event asynchronous unbatched will bring the behaviour closer to Paper.
Paper puts "layout" event in the event queue from ShadowQueue and schedules a flush of event queue on JavaScript queue.

Before this, Fabric puts "layout" event in the event queue from JavaScript queue, waits until main run loop is about to go to sleep and then schedules a flush of event queue on JavaScript thread.

Now Fabric no longer waits for main run loop to go to sleep but induces a flush of event queue on JavaScript thread right away.

Reviewed By: JoshuaGross, shergin

Differential Revision: D25849468

fbshipit-source-id: 366e99364507f7bc1d09325c04a604bcb2043e3d

Author: sammy-SC

ReactCommon/react/renderer/components/view/ViewEventEmitter.cpp

@@ -79,37 +79,40 @@ void ViewEventEmitter::onLayout(const LayoutMetrics &layoutMetrics) const {
     layoutEventState->isDispatching = true;
   }
 
-  dispatchEvent("layout", [layoutEventState](jsi::Runtime &runtime) {
-    auto frame = Rect{};
-
-    {
-      std::lock_guard<std::mutex> guard(layoutEventState->mutex);
-
-      layoutEventState->isDispatching = false;
-
-      // If some *particular* `frame` was already dispatched before,
-      // and since then there were no other new values of the `frame` observed,
-      // do nothing.
-      if (layoutEventState->wasDispatched) {
-        return jsi::Value::null();
-      }
-
-      frame = layoutEventState->frame;
-
-      // If some *particular* `frame` was *not* already dispatched before,
-      // it's time to dispatch it and mark as dispatched.
-      layoutEventState->wasDispatched = true;
-    }
-
-    auto layout = jsi::Object(runtime);
-    layout.setProperty(runtime, "x", frame.origin.x);
-    layout.setProperty(runtime, "y", frame.origin.y);
-    layout.setProperty(runtime, "width", frame.size.width);
-    layout.setProperty(runtime, "height", frame.size.height);
-    auto payload = jsi::Object(runtime);
-    payload.setProperty(runtime, "layout", std::move(layout));
-    return jsi::Value(std::move(payload));
-  });
+  dispatchEvent(
+      "layout",
+      [layoutEventState](jsi::Runtime &runtime) {
+        auto frame = Rect{};
+
+        {
+          std::lock_guard<std::mutex> guard(layoutEventState->mutex);
+
+          layoutEventState->isDispatching = false;
+
+          // If some *particular* `frame` was already dispatched before,
+          // and since then there were no other new values of the `frame`
+          // observed, do nothing.
+          if (layoutEventState->wasDispatched) {
+            return jsi::Value::null();
+          }
+
+          frame = layoutEventState->frame;
+
+          // If some *particular* `frame` was *not* already dispatched before,
+          // it's time to dispatch it and mark as dispatched.
+          layoutEventState->wasDispatched = true;
+        }
+
+        auto layout = jsi::Object(runtime);
+        layout.setProperty(runtime, "x", frame.origin.x);
+        layout.setProperty(runtime, "y", frame.origin.y);
+        layout.setProperty(runtime, "width", frame.size.width);
+        layout.setProperty(runtime, "height", frame.size.height);
+        auto payload = jsi::Object(runtime);
+        payload.setProperty(runtime, "layout", std::move(layout));
+        return jsi::Value(std::move(payload));
+      },
+      EventPriority::AsynchronousUnbatched);
 }
 
 } // namespace react