Fabric: Restoring the original purpose of StateData class

Summary:
This diff restores the original role of StateData as a dummy class that has only one purpose - designate that there is no state associated with the node.

I believe having an abstract StateData class for all data types is not necessary and actually slightly harmful. And here the reasons:
* Having virtual dispatch enabled for classes introduces some overhead, and it should be used only if it is absolutely necessary. In this case, we don't need to have virtual dispatch enabled for Data classes because it's already enabled for State classes; therefore all virtual resolution is done there and that's sufficient. No need to pay for what we don't need.
* Continuing the previous point, yes, we expect some API being exposed for Data classes (such as `getDynamic`), but introducing a base abstract class for that is *not* an idiomatic C++ solution; in C++ it's by design that most of the template contracts are actually duck-typed. This is a well-known design issue/concern that will be addressed with an effort called "Concepts" in C++20. Anyway, we should not use abstract classes as *only*  indication of conformance to some interface.
* StateData does not introduce any convenience. As it is clear from several subclass implementations, we don't really use base methods from it.
* The previous issue actually happens especially because of the interface of StateData class. Consider constructor that accepts `folly::dynamic` and does nothing, it actually useless because it's impossible to use it unless it's called inside superclass's constructor. That's the case because C++ does not support virtual dispatch for constructors (for good).
* Avoiding subclassing StateData counter-intuitively enforces the desired conformance to imaginary interface: it does not compile otherwise.

Reviewed By: JoshuaGross

Differential Revision: D15342338

fbshipit-source-id: 1f04f7fc5247499e7cfc09cd4b3cccffdc0b6b06

Author: shergin

ReactCommon/fabric/components/modal/ModalHostViewState.cpp

@@ -6,17 +6,12 @@
  */
 
 #include "ModalHostViewState.h"
-#include <glog/logging.h>
-
-#ifdef ANDROID
-#include <folly/dynamic.h>
-#endif
 
 namespace facebook {
 namespace react {
 
 #ifdef ANDROID
-const folly::dynamic ModalHostViewState::getDynamic() const {
+folly::dynamic ModalHostViewState::getDynamic() const {
   return folly::dynamic::object("screenWidth", screenSize.width)(
       "screenHeight", screenSize.height);
 }

ReactCommon/fabric/components/modal/ModalHostViewState.h

@@ -7,24 +7,26 @@
 
 #pragma once
 
-#include <react/core/StateData.h>
 #include <react/graphics/Float.h>
 #include <react/graphics/Geometry.h>
 #include <react/graphics/conversions.h>
 
+#ifdef ANDROID
+#include <folly/dynamic.h>
+#endif
+
 namespace facebook {
 namespace react {
 
 /*
  * State for <BottomSheetView> component.
  */
-class ModalHostViewState : public StateData {
+class ModalHostViewState final {
  public:
   using Shared = std::shared_ptr<const ModalHostViewState>;
 
   ModalHostViewState(){};
   ModalHostViewState(Size screenSize_) : screenSize(screenSize_){};
-  virtual ~ModalHostViewState() = default;
 
 #ifdef ANDROID
   ModalHostViewState(folly::dynamic data)
@@ -35,7 +37,7 @@ class ModalHostViewState : public StateData {
   const Size screenSize{};
 
 #ifdef ANDROID
-  virtual const folly::dynamic getDynamic() const override;
+  folly::dynamic getDynamic() const;
 #endif
 
 #pragma mark - Getters

ReactCommon/fabric/core/state/StateData.cpp

@@ -17,23 +17,16 @@ namespace facebook {
 namespace react {
 
 /*
- * Base class for state data.
- * Must be used to provide getDynamic for Android.
+ * Dummy type that is used as a placeholder for state data for nodes that
+ * don't have a state.
  */
-class StateData {
- public:
+struct StateData final {
   using Shared = std::shared_ptr<void>;
 
-  StateData() {}
-
 #ifdef ANDROID
-  StateData(folly::dynamic data) {}
-
-  // Destructor must either be virtual or protected if we have any
-  // virtual methods
-  virtual ~StateData();
-
-  virtual const folly::dynamic getDynamic() const;
+  StateData() = default;
+  StateData(folly::dynamic data){};
+  folly::dynamic getDynamic() const;
 #endif
 };