Fabric: Using `const &` type for `ShadowNodeFragment`'s fields

Summary:
@public
To avoid unnecessary copying of `shared_ptr`s inside ShadowNodeFragment, now we store them as `const &` references.

Reviewed By: mdvacca

Differential Revision: D8988388

fbshipit-source-id: 0b3582e57ce7577b8fa819392bf33f34e1a60b59

ReactCommon/fabric/components/view/ConcreteViewShadowNode.h

@@ -14,6 +14,7 @@
 #include <fabric/core/ConcreteShadowNode.h>
 #include <fabric/core/LayoutableShadowNode.h>
 #include <fabric/core/ShadowNode.h>
+#include <fabric/core/ShadowNodeFragment.h>
 #include <fabric/debug/DebugStringConvertibleItem.h>
 
 namespace facebook {

ReactCommon/fabric/core/componentdescriptor/ConcreteComponentDescriptor.h

@@ -13,6 +13,8 @@
 #include <fabric/core/ComponentDescriptor.h>
 #include <fabric/core/Props.h>
 #include <fabric/core/ShadowNode.h>
+#include <fabric/core/ShadowNodeFragment.h>
+#include <fabric/events/EventDispatcher.h>
 
 namespace facebook {
 namespace react {

ReactCommon/fabric/core/shadownode/ShadowNode.cpp

@@ -9,6 +9,7 @@
 
 #include <string>
 
+#include <fabric/core/ShadowNodeFragment.h>
 #include <fabric/debug/DebugStringConvertible.h>
 #include <fabric/debug/debugStringConvertibleUtils.h>
 

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -21,6 +21,8 @@
 namespace facebook {
 namespace react {
 
+struct ShadowNodeFragment;
+
 class ShadowNode;
 
 using SharedShadowNode = std::shared_ptr<const ShadowNode>;
@@ -29,15 +31,6 @@ using SharedShadowNodeList = std::vector<std::shared_ptr<const ShadowNode>>;
 using SharedShadowNodeSharedList = std::shared_ptr<const SharedShadowNodeList>;
 using SharedShadowNodeUnsharedList = std::shared_ptr<SharedShadowNodeList>;
 
-struct ShadowNodeFragment {
-  Tag tag;
-  Tag rootTag;
-  SharedProps props;
-  SharedEventEmitter eventEmitter;
-  SharedShadowNodeSharedList children;
-  SharedLocalData localData;
-};
-
 using ShadowNodeCloneFunction = std::function<UnsharedShadowNode(
   const SharedShadowNode &sourceShadowNode,
   const ShadowNodeFragment &fragment

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.cpp

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include "ShadowNodeFragment.h"
+
+namespace facebook {
+namespace react {
+
+SharedProps &ShadowNodeFragment::nullSharedProps() {
+  static auto &instance = *new SharedProps();
+  return instance;
+}
+
+SharedEventEmitter &ShadowNodeFragment::nullSharedEventEmitter() {
+  static auto &instance = *new SharedEventEmitter();
+  return instance;
+}
+
+SharedShadowNodeSharedList &ShadowNodeFragment::nullSharedChildren() {
+  static auto &instance = *new SharedShadowNodeSharedList();
+  return instance;
+}
+
+SharedLocalData &ShadowNodeFragment::nullLocalData() {
+  static auto &instance = *new SharedLocalData();
+  return instance;
+}
+
+} // namespace react
+} // namespace facebook

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.h

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * 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 <fabric/core/LocalData.h>
+#include <fabric/core/Props.h>
+#include <fabric/core/ReactPrimitives.h>
+#include <fabric/core/ShadowNode.h>
+#include <fabric/events/EventEmitter.h>
+
+namespace facebook {
+namespace react {
+
+/*
+ * An object which supposed to be used as a parameter specifying a shape
+ * of created or cloned ShadowNode.
+ * Note: Most of the fields are `const &` references (essentially just raw
+ * pointers) which means that the Fragment does not copy/store them or
+ * retain ownership of them.
+ */
+struct ShadowNodeFragment {
+  Tag tag = 0;
+  Tag rootTag = 0;
+  const SharedProps &props = nullSharedProps();
+  const SharedEventEmitter &eventEmitter = nullSharedEventEmitter();
+  const SharedShadowNodeSharedList &children = nullSharedChildren();
+  const SharedLocalData &localData = nullLocalData();
+
+private:
+  static SharedProps &nullSharedProps();
+  static SharedEventEmitter &nullSharedEventEmitter();
+  static SharedShadowNodeSharedList &nullSharedChildren();
+  static SharedLocalData &nullLocalData();
+};
+
+} // namespace react
+} // namespace facebook

ReactCommon/fabric/uimanager/FabricUIManager.cpp

@@ -14,6 +14,7 @@
 #include <fabric/components/view/ViewShadowNode.h>
 #include <fabric/core/componentDescriptor.h>
 #include <fabric/core/LayoutContext.h>
+#include <fabric/core/ShadowNodeFragment.h>
 #include <fabric/debug/DebugStringConvertible.h>
 #include <fabric/debug/DebugStringConvertibleItem.h>