use vector for subscribers in event system

Summary:
Replaced global event subscriber with a vector of subscriber functions
Further changes in commit stack support the mutiple subscribers in event system

Reviewed By: davidaurelio

Differential Revision: D15352451

fbshipit-source-id: 7ca6f0943735bf1f76a906c23e15e14ae3c5f42c

yoga/event/event.cpp

@@ -15,31 +15,26 @@ namespace yoga {
 
 namespace {
 
-// For now, a single subscriber is enough.
-// This can be changed as soon as the need for more than one subscriber arises.
-std::function<Event::Subscriber>& globalEventSubscriber() {
-  static std::function<Event::Subscriber> subscriber = nullptr;
-  return subscriber;
+Event::Subscribers& eventSubscribers() {
+  static Event::Subscribers subscribers = {};
+  return subscribers;
 }
 
 } // namespace
 
 void Event::reset() {
-  globalEventSubscriber() = nullptr;
+  eventSubscribers() = {};
 }
 
 void Event::subscribe(std::function<Subscriber>&& subscriber) {
-  if (globalEventSubscriber() != nullptr) {
-    throw std::logic_error(
-        "Yoga currently supports only one global event subscriber");
-  }
-  globalEventSubscriber() = std::move(subscriber);
+  eventSubscribers().push_back(subscriber);
 }
 
 void Event::publish(const YGNode& node, Type eventType, const Data& eventData) {
-  auto& subscriber = globalEventSubscriber();
-  if (subscriber) {
-    subscriber(node, eventType, eventData);
+  for (auto& subscriber : eventSubscribers()) {
+    if (subscriber) {
+      subscriber(node, eventType, eventData);
+    }
   }
 }
 

yoga/event/event.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <functional>
+#include <vector>
 
 struct YGConfig;
 struct YGNode;
@@ -24,6 +25,7 @@ struct Event {
   };
   class Data;
   using Subscriber = void(const YGNode&, Type, Data);
+  using Subscribers = std::vector<std::function<Subscriber>>;
 
   template <Type E>
   struct TypedData {};