Fabric: Computed attributed string caching inside <Paragraph>

Summary:
@public
Quite trivial. We had to have this from the day one.
We don't need cache invalidation policy because all subtree is immutable.

Reviewed By: mdvacca

Differential Revision: D8709973

fbshipit-source-id: bd7fcf0ae1dcb23894321cb5d16da18cb1ab788f

ReactCommon/fabric/attributedstring/AttributedString.h

@@ -72,4 +72,3 @@ private:
 
 } // namespace react
 } // namespace facebook
-

ReactCommon/fabric/text/basetext/BaseTextShadowNode.cpp

@@ -20,8 +20,6 @@ AttributedString BaseTextShadowNode::getAttributedString(
   const TextAttributes &textAttributes,
   const SharedShadowNodeSharedList &childNodes
 ) const {
-  // TODO: Implement caching.
-
   AttributedString attributedString;
 
   for (const auto &childNode : *childNodes) {

ReactCommon/fabric/text/paragraph/ParagraphShadowNode.cpp

@@ -19,7 +19,12 @@ ComponentName ParagraphShadowNode::getComponentName() const {
 }
 
 AttributedString ParagraphShadowNode::getAttributedString() const {
-  return BaseTextShadowNode::getAttributedString(getProps()->textAttributes, getChildren());
+  if (!cachedAttributedString_.has_value()) {
+    cachedAttributedString_ =
+      BaseTextShadowNode::getAttributedString(getProps()->textAttributes, getChildren());
+  }
+
+  return cachedAttributedString_.value();
 }
 
 void ParagraphShadowNode::setTextLayoutManager(SharedTextLayoutManager textLayoutManager) {

ReactCommon/fabric/text/paragraph/ParagraphShadowNode.h

@@ -14,6 +14,7 @@
 #include <fabric/text/TextShadowNode.h>
 #include <fabric/textlayoutmanager/TextLayoutManager.h>
 #include <fabric/view/ConcreteViewShadowNode.h>
+#include <folly/Optional.h>
 
 namespace facebook {
 namespace react {
@@ -63,6 +64,12 @@ private:
   void updateLocalData();
 
   SharedTextLayoutManager textLayoutManager_;
+
+  /*
+   * Cached attributed string that represents the content of the subtree started
+   * from the node.
+   */
+  mutable folly::Optional<AttributedString> cachedAttributedString_ {};
 };
 
 } // namespace react