Fabric: Now `EventEmitter::setEnabled()` relies on stored `eventTarget` is not being nullptr

Summary:
This diff implements a new model of managing `enabled` flag in EventEmitter.
Now we simply rely on `eventTarget_` is not being `nullptr` (and we reset the pointer to "disable" the event emitter).

Reviewed By: mdvacca

Differential Revision: D9764857

fbshipit-source-id: 1dd3ce0c8589048babbf2dbac9f8359358b31a34

ReactCommon/fabric/events/EventEmitter.cpp

@@ -76,14 +76,18 @@ void EventEmitter::dispatchEvent(
 }
 
 void EventEmitter::setEnabled(bool enabled) const {
-  enabled_ = enabled;
+  bool alreadyEnabled = eventTarget_ != nullptr;
+  if (enabled == alreadyEnabled) {
+    return;
+  }
+
   if (!enabled) {
-    eventTarget_ = nullptr;
+    eventTarget_.reset();
   }
 }
 
 bool EventEmitter::getEnabled() const {
-  return enabled_;
+  return eventTarget_ != nullptr;
 }
 
 } // namespace react

ReactCommon/fabric/events/EventEmitter.h

@@ -56,6 +56,8 @@ public:
   /*
    * Indicates that an event can be delivered to `eventTarget`.
    * Callsite must acquire `DispatchMutex` to access those methods.
+   * The `setEnabled` operation is not guaranteed: the `EventEmitter` cannot
+   * be re-enabled after disabling; in this case, the method does nothing.
    */
   void setEnabled(bool enabled) const;
   bool getEnabled() const;
@@ -75,7 +77,6 @@ private:
   mutable SharedEventTarget eventTarget_;
   Tag tag_;
   WeakEventDispatcher eventDispatcher_;
-  mutable bool enabled_; // Protected by `DispatchMutex`.
 };
 
 } // namespace react