Add foundations for event system

Summary:
@public

We want to enable tooling, instrumentation, and statistics within Yoga without coupling these functionalities to our core code.

This commit introduces the foundations of a simple, global event system.
For the time being, we will only support a single subscriber. Should we require more than one, we can add support for it later.

Reviewed By: SidharthGuglani

Differential Revision: D15153678

fbshipit-source-id: 7d96f4c8def646a6a1b3908f946e7f81a6dba9c3

ReactCommon/yoga/yoga/events.cpp

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#include "events.h"
+#include <memory>
+#include <stdexcept>
+
+namespace facebook {
+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;
+}
+
+} // namespace
+
+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);
+}
+
+void Event::publish(const YGNode& node, Type eventType, const Data& eventData) {
+  auto& subscriber = globalEventSubscriber();
+  if (subscriber) {
+    subscriber(node, eventType, eventData);
+  }
+}
+
+} // namespace yoga
+} // namespace facebook

ReactCommon/yoga/yoga/events.h

@@ -0,0 +1,55 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#pragma once
+
+#include <functional>
+
+struct YGConfig;
+struct YGNode;
+
+namespace facebook {
+namespace yoga {
+
+struct Event {
+  enum Type {};
+  class Data;
+  using Subscriber = void(const YGNode&, Type, Data);
+
+  template <Type E>
+  struct TypedData {};
+
+  class Data {
+    const void* data_;
+
+  public:
+    template <Type E>
+    Data(const TypedData<E>& data) : data_{&data} {}
+
+    template <Type E>
+    const TypedData<E>& get() const {
+      return *static_cast<const TypedData<E>*>(data_);
+    };
+  };
+
+  static void subscribe(std::function<Subscriber>&& subscriber);
+
+  template <Type E>
+  static void publish(const YGNode& node, const TypedData<E>& eventData = {}) {
+    publish(node, E, Data{eventData});
+  }
+
+  template <Type E>
+  static void publish(const YGNode* node, const TypedData<E>& eventData = {}) {
+    publish<E>(*node, eventData);
+  }
+
+private:
+  static void publish(const YGNode&, Type, const Data&);
+};
+
+} // namespace yoga
+} // namespace facebook