Fabric: Using shared pointer for event counter in ViewEventEmitter

Summary:
The callback `dispatchEvent` is called asynchronously on the JavaScript thread, so all data it uses must be copied to the lambda. To work around this constraint we use a shared pointer for the counter.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D24598840

fbshipit-source-id: fb5581858d54dc806863caf0c7c4f612ed6046e2

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

@@ -46,17 +46,17 @@ void ViewEventEmitter::onLayout(const LayoutMetrics &layoutMetrics) const {
     lastLayoutMetrics_ = layoutMetrics;
   }
 
-  std::atomic_uint_fast8_t *eventCounter = &eventCounter_;
-  uint_fast8_t expectedEventCount = ++*eventCounter;
+  auto expectedEventCount = ++*eventCounter_;
 
   // dispatchUniqueEvent only drops consecutive onLayout events to the same
   // node. We want to drop *any* unprocessed onLayout events when there's a
   // newer one.
   dispatchEvent(
       "layout",
-      [frame = layoutMetrics.frame, expectedEventCount, eventCounter](
-          jsi::Runtime &runtime) {
-        uint_fast8_t actualEventCount = eventCounter->load();
+      [frame = layoutMetrics.frame,
+       expectedEventCount,
+       eventCounter = eventCounter_](jsi::Runtime &runtime) {
+        auto actualEventCount = eventCounter->load();
         if (expectedEventCount != actualEventCount) {
           // Drop stale events
           return jsi::Value::null();

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

@@ -40,7 +40,9 @@ class ViewEventEmitter : public TouchEventEmitter {
  private:
   mutable std::mutex layoutMetricsMutex_;
   mutable LayoutMetrics lastLayoutMetrics_;
-  mutable std::atomic_uint_fast8_t eventCounter_{0};
+
+  mutable std::shared_ptr<std::atomic_uint_fast8_t> eventCounter_{
+      std::make_shared<std::atomic_uint_fast8_t>(0)};
 };
 
 } // namespace react