Fabric: Fixing const correctness in ShadowNodeFragment (and some prettification)

Summary:
Previously, all placeholders methods have return type `SomeType &` which is not correct because it allows the called to modify enclosed `static` value of the placeholders; the type should be `SomeType const &`.
Besides that this diff migrates some type aliases to the new style (which makes evething much prettier and readable).

Reviewed By: mdvacca

Differential Revision: D14819076

fbshipit-source-id: 87e68d546f16d7a9ad1fb65e1b238859f9381eb7

ReactCommon/fabric/core/events/EventEmitter.h

@@ -35,6 +35,8 @@ class EventEmitter {
   using Tag = int32_t;
 
  public:
+  using Shared = std::shared_ptr<EventEmitter const>;
+
   static std::mutex &DispatchMutex();
 
   static ValueFactory defaultPayloadFactory();

ReactCommon/fabric/core/shadownode/LocalData.h

@@ -28,6 +28,8 @@ using SharedLocalData = std::shared_ptr<const LocalData>;
  */
 class LocalData : public Sealable, public DebugStringConvertible {
  public:
+  using Shared = std::shared_ptr<LocalData const>;
+
   virtual ~LocalData() = default;
 
   virtual folly::dynamic getDynamic() const {

ReactCommon/fabric/core/shadownode/Props.h

@@ -18,18 +18,20 @@ namespace react {
 
 class Props;
 
-using SharedProps = std::shared_ptr<const Props>;
+using SharedProps = std::shared_ptr<Props const>;
 
 /*
  * Represents the most generic props object.
  */
 class Props : public virtual Sealable, public virtual DebugStringConvertible {
  public:
+  using Shared = std::shared_ptr<Props const>;
+
   Props() = default;
-  Props(const Props &sourceProps, const RawProps &rawProps);
+  Props(Props const &sourceProps, RawProps const &rawProps);
   virtual ~Props() = default;
 
-  const std::string nativeId;
+  std::string const nativeId;
 
   /*
    * Special value that represents generation number of `Props` object, which
@@ -38,10 +40,10 @@ class Props : public virtual Sealable, public virtual DebugStringConvertible {
    * revision equals `0`.
    * The value might be used for optimization purposes.
    */
-  const int revision{0};
+  int const revision{0};
 
 #ifdef ANDROID
-  const folly::dynamic rawProps = folly::dynamic::object();
+  folly::dynamic const rawProps = folly::dynamic::object();
 #endif
 };
 

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -43,8 +43,14 @@ class ShadowNode : public virtual Sealable,
                    public virtual DebugStringConvertible,
                    public std::enable_shared_from_this<ShadowNode> {
  public:
-  using Shared = std::shared_ptr<const ShadowNode>;
-  using Weak = std::weak_ptr<const ShadowNode>;
+  using Shared = std::shared_ptr<ShadowNode const>;
+  using Weak = std::weak_ptr<ShadowNode const>;
+  using Unshared = std::shared_ptr<ShadowNode>;
+  using ListOfShared =
+      better::small_vector<Shared, kShadowNodeChildrenSmallVectorSize>;
+  using SharedListOfShared = std::shared_ptr<ListOfShared const>;
+  using UnsharedListOfShared = std::shared_ptr<ListOfShared>;
+
   using AncestorList = better::small_vector<
       std::pair<
           std::reference_wrapper<ShadowNode const> /* parentNode */,

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.cpp

@@ -10,35 +10,36 @@
 namespace facebook {
 namespace react {
 
-Tag ShadowNodeFragment::tagPlaceholder() {
+Tag const ShadowNodeFragment::tagPlaceholder() {
   return 0;
 }
 
-Tag ShadowNodeFragment::surfaceIdPlaceholder() {
+SurfaceId const ShadowNodeFragment::surfaceIdPlaceholder() {
   return 0;
 }
 
-SharedProps &ShadowNodeFragment::propsPlaceholder() {
-  static auto &instance = *new SharedProps();
+Props::Shared const &ShadowNodeFragment::propsPlaceholder() {
+  static auto &instance = *new Props::Shared();
   return instance;
 }
 
-SharedEventEmitter &ShadowNodeFragment::eventEmitterPlaceholder() {
-  static auto &instance = *new SharedEventEmitter();
+EventEmitter::Shared const &ShadowNodeFragment::eventEmitterPlaceholder() {
+  static auto &instance = *new EventEmitter::Shared();
   return instance;
 }
 
-SharedShadowNodeSharedList &ShadowNodeFragment::childrenPlaceholder() {
-  static auto &instance = *new SharedShadowNodeSharedList();
+ShadowNode::SharedListOfShared const &
+ShadowNodeFragment::childrenPlaceholder() {
+  static auto &instance = *new ShadowNode::SharedListOfShared();
   return instance;
 }
 
-SharedLocalData &ShadowNodeFragment::localDataPlaceholder() {
-  static auto &instance = *new SharedLocalData();
+LocalData::Shared const &ShadowNodeFragment::localDataPlaceholder() {
+  static auto &instance = *new LocalData::Shared();
   return instance;
 }
 
-State::Shared &ShadowNodeFragment::statePlaceholder() {
+State::Shared const &ShadowNodeFragment::statePlaceholder() {
   static auto &instance = *new State::Shared();
   return instance;
 }

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.h

@@ -25,21 +25,26 @@ namespace react {
  * retain ownership of them.
  */
 struct ShadowNodeFragment {
-  Tag tag = 0;
-  Tag rootTag = 0;
-  const SharedProps &props = propsPlaceholder();
-  const SharedEventEmitter &eventEmitter = eventEmitterPlaceholder();
-  const SharedShadowNodeSharedList &children = childrenPlaceholder();
-  const SharedLocalData &localData = localDataPlaceholder();
-  const State::Shared &state = statePlaceholder();
+  Tag const tag = tagPlaceholder();
+  SurfaceId const rootTag = surfaceIdPlaceholder();
+  Props::Shared const &props = propsPlaceholder();
+  EventEmitter::Shared const &eventEmitter = eventEmitterPlaceholder();
+  ShadowNode::SharedListOfShared const &children = childrenPlaceholder();
+  LocalData::Shared const &localData = localDataPlaceholder();
+  State::Shared const &state = statePlaceholder();
 
-  static Tag tagPlaceholder();
-  static Tag surfaceIdPlaceholder();
-  static SharedProps &propsPlaceholder();
-  static SharedEventEmitter &eventEmitterPlaceholder();
-  static SharedShadowNodeSharedList &childrenPlaceholder();
-  static SharedLocalData &localDataPlaceholder();
-  static State::Shared &statePlaceholder();
+  /*
+   * Placeholders.
+   * Use as default arguments as an indication that the field does not need to
+   * be changed.
+   */
+  static Tag const tagPlaceholder();
+  static SurfaceId const surfaceIdPlaceholder();
+  static Props::Shared const &propsPlaceholder();
+  static EventEmitter::Shared const &eventEmitterPlaceholder();
+  static ShadowNode::SharedListOfShared const &childrenPlaceholder();
+  static LocalData::Shared const &localDataPlaceholder();
+  static State::Shared const &statePlaceholder();
 };
 
 } // namespace react