Use a mutex to guard access to lastLayoutMetrics_ in ViewEventEmitter

Summary:
Of course, compare_exchange_strong didn't actually do what I wanted.

Using a mutex is simpler and actually has the semantics we want: atomically get the current value, compare, and bail if the value is the same, or swap and continue.

Changelog: [Internal]

Reviewed By: shergin

Differential Revision: D19754451

fbshipit-source-id: 6b0aef217b235959af683ec5e31b07a0dd7bb040

ReactCommon/fabric/components/view/ViewEventEmitter.cpp

@@ -38,12 +38,12 @@ void ViewEventEmitter::onLayout(const LayoutMetrics &layoutMetrics) const {
   // Due to State Reconciliation, `onLayout` can be called potentially many
   // times with identical layoutMetrics. Ensure that the JS event is only
   // dispatched when the value changes.
-  auto lastLayoutMetricsValue = lastLayoutMetrics_.load();
-  // compare_exchange_strong atomically swap the values if they're different, or
-  // return false if the values are the same.
-  if (!lastLayoutMetrics_.compare_exchange_strong(
-          lastLayoutMetricsValue, layoutMetrics)) {
-    return;
+  {
+    std::lock_guard<std::mutex> guard(layoutMetricsMutex_);
+    if (lastLayoutMetrics_ == layoutMetrics) {
+      return;
+    }
+    lastLayoutMetrics_ = layoutMetrics;
   }
 
   dispatchEvent("layout", [frame = layoutMetrics.frame](jsi::Runtime &runtime) {

ReactCommon/fabric/components/view/ViewEventEmitter.h

@@ -7,8 +7,8 @@
 
 #pragma once
 
-#include <atomic>
 #include <memory>
+#include <mutex>
 
 #include <react/core/LayoutMetrics.h>
 #include <react/core/ReactPrimitives.h>
@@ -38,7 +38,8 @@ class ViewEventEmitter : public TouchEventEmitter {
   void onLayout(const LayoutMetrics &layoutMetrics) const;
 
  private:
-  mutable std::atomic<LayoutMetrics> lastLayoutMetrics_;
+  mutable std::mutex layoutMetricsMutex_;
+  mutable LayoutMetrics lastLayoutMetrics_;
 };
 
 } // namespace react