Fabric: Making EventEmitter::setEnabled additive

Summary: In the previous approach, when event emitter got disabled for split second, we could lose the EventTarget because JS GC can collect it before we re-enable this. Now we "over-enable" this first, and "under-disable" later.

Reviewed By: mdvacca

Differential Revision: D12990112

fbshipit-source-id: 4e3c0c0e05f03509ec72ca570f59ce16597353f0

ReactCommon/fabric/events/EventEmitter.cpp

@@ -63,13 +63,24 @@ void EventEmitter::dispatchEvent(
       priority);
 }
 
-void EventEmitter::setEnabled(bool enabled) const {
-  bool alreadyEnabled = eventTarget_ != nullptr;
-  if (enabled == alreadyEnabled) {
+void EventEmitter::enable() const {
+  enableCounter_++;
+  toggleEventTargetOwnership_();
+}
+
+void EventEmitter::disable() const {
+  enableCounter_--;
+  toggleEventTargetOwnership_();
+}
+
+void EventEmitter::toggleEventTargetOwnership_() const {
+  bool shouldBeRetained = enableCounter_ > 0;
+  bool alreadyBeRetained = eventTarget_ != nullptr;
+  if (shouldBeRetained == alreadyBeRetained) {
     return;
   }
 
-  if (enabled) {
+  if (shouldBeRetained) {
     eventTarget_ = weakEventTarget_.lock();
     weakEventTarget_.reset();
   } else {
@@ -78,9 +89,5 @@ void EventEmitter::setEnabled(bool enabled) const {
   }
 }
 
-bool EventEmitter::getEnabled() const {
-  return eventTarget_ != nullptr;
-}
-
 } // namespace react
 } // namespace facebook

ReactCommon/fabric/events/EventEmitter.h

@@ -53,13 +53,15 @@ class EventEmitter {
   virtual ~EventEmitter() = default;
 
   /*
-   * Indicates that an event can be delivered to `eventTarget`.
-   * Callsite must acquire `DispatchMutex` to access those methods.
-   * The `setEnabled` operation is not guaranteed: sometimes `EventEmitter`
-   * can be re-enabled after disabling, sometimes not.
+   * `DispatchMutex` must be acquired before calling.
+   * Enables/disables event emitter.
+   * Enabled event emitter retains a pointer to `eventTarget` strongly (as
+   * `std::shared_ptr`) whereas disabled one weakly (as `std::weak_ptr`).
+   * The enable state is additive; a number of `enable` calls should be equal to
+   * a number of `disable` calls to release the event target.
    */
-  void setEnabled(bool enabled) const;
-  bool getEnabled() const;
+  void enable() const;
+  void disable() const;
 
  protected:
 #ifdef ANDROID
@@ -78,10 +80,13 @@ class EventEmitter {
       const EventPriority &priority = EventPriority::AsynchronousBatched) const;
 
  private:
+  void toggleEventTargetOwnership_() const;
+
   mutable SharedEventTarget eventTarget_;
   mutable WeakEventTarget weakEventTarget_;
   Tag tag_;
   WeakEventDispatcher eventDispatcher_;
+  mutable int enableCounter_{0};
 };
 
 } // namespace react

ReactCommon/fabric/uimanager/ShadowTree.cpp

@@ -181,14 +181,14 @@ void ShadowTree::toggleEventEmitters(
   std::lock_guard<std::recursive_mutex> lock(EventEmitter::DispatchMutex());
 
   for (const auto &mutation : mutations) {
-    if (mutation.type == ShadowViewMutation::Delete) {
-      mutation.oldChildShadowView.eventEmitter->setEnabled(false);
+    if (mutation.type == ShadowViewMutation::Create) {
+      mutation.newChildShadowView.eventEmitter->enable();
     }
   }
 
   for (const auto &mutation : mutations) {
-    if (mutation.type == ShadowViewMutation::Create) {
-      mutation.newChildShadowView.eventEmitter->setEnabled(true);
+    if (mutation.type == ShadowViewMutation::Delete) {
+      mutation.oldChildShadowView.eventEmitter->disable();
     }
   }
 }