Fabric: Making EventEmitter reenableable

Summary:
The first implementation of EventEmitter's enable/disable feature didn't not provide a way to enable an object after it was disabled.
Apparently, we need this functionality due that fact that all nodes of the same family share same event emitter.

Reviewed By: mdvacca

Differential Revision: D10395849

fbshipit-source-id: 0eba54f0bb7ded35d64afb6559e6e27208c2b577

ReactCommon/fabric/events/EventEmitter.cpp

@@ -38,6 +38,7 @@ EventEmitter::EventEmitter(
     Tag tag,
     WeakEventDispatcher eventDispatcher)
     : eventTarget_(std::move(eventTarget)),
+      weakEventTarget_({}),
       tag_(tag),
       eventDispatcher_(std::move(eventDispatcher)) {}
 
@@ -66,7 +67,11 @@ void EventEmitter::setEnabled(bool enabled) const {
     return;
   }
 
-  if (!enabled) {
+  if (enabled) {
+    eventTarget_ = weakEventTarget_.lock();
+    weakEventTarget_.reset();
+  } else {
+    weakEventTarget_ = eventTarget_;
     eventTarget_.reset();
   }
 }

ReactCommon/fabric/events/EventEmitter.h

@@ -55,8 +55,8 @@ class EventEmitter {
   /*
    * 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.
+   * The `setEnabled` operation is not guaranteed: sometimes `EventEmitter`
+   * can be re-enabled after disabling, sometimes not.
    */
   void setEnabled(bool enabled) const;
   bool getEnabled() const;
@@ -79,6 +79,7 @@ class EventEmitter {
 
  private:
   mutable SharedEventTarget eventTarget_;
+  mutable WeakEventTarget weakEventTarget_;
   Tag tag_;
   WeakEventDispatcher eventDispatcher_;
 };