Use feature flags in Fabric C++ core, in particular for paragraph measurement caching

Summary: The goal is to be able to use MobileConfig params inside of core React Native C++ code. This works on Catalyst iOS now and Wilde; need to add support for FB4A and Catalyst Android.

Reviewed By: fkgozali

Differential Revision: D13883007

fbshipit-source-id: 115fe6cc884d2a0b9ca26dadf867a5f0ae99f262

ReactCommon/fabric/components/text/paragraph/ParagraphComponentDescriptor.h

@@ -34,7 +34,24 @@ class ParagraphComponentDescriptor final
     // Every single `ParagraphShadowNode` will have a reference to
     // a shared `EvictingCacheMap`, a simple LRU cache for Paragraph
     // measurements.
+#ifdef ANDROID
+    auto paramName = "react_fabric:enabled_paragraph_measure_cache_android";
+#else
+    auto paramName = "react_fabric:enabled_paragraph_measure_cache_ios";
+#endif
+    // TODO: T39927960 - get rid of this if statement
+    bool enableCache =
+        (contextContainer != nullptr
+             ? contextContainer
+                   ->getInstance<std::shared_ptr<const ReactNativeConfig>>(
+                       "ReactNativeConfig")
+                   ->getBool(paramName)
+             : false);
+    if (enableCache) {
       measureCache_ = std::make_unique<ParagraphMeasurementCache>();
+    } else {
+      measureCache_ = nullptr;
+    }
   }
 
   void adopt(UnsharedShadowNode shadowNode) const override {
@@ -50,7 +67,8 @@ class ParagraphComponentDescriptor final
 
     // `ParagraphShadowNode` uses this to cache the results of text rendering
     // measurements.
-    paragraphShadowNode->setMeasureCache(measureCache_.get());
+    paragraphShadowNode->setMeasureCache(
+        measureCache_ ? measureCache_.get() : nullptr);
 
     // All `ParagraphShadowNode`s must have leaf Yoga nodes with properly
     // setup measure function.

ReactCommon/fabric/components/text/paragraph/ParagraphShadowNode.cpp

@@ -61,22 +61,29 @@ Size ParagraphShadowNode::measure(LayoutConstraints layoutConstraints) const {
   AttributedString attributedString = getAttributedString();
   const ParagraphAttributes attributes = getProps()->paragraphAttributes;
 
+  auto makeMeasurements = [&] {
+    return textLayoutManager_->measure(
+        attributedString, getProps()->paragraphAttributes, layoutConstraints);
+  };
+
   // Cache results of this function so we don't need to call measure()
   // repeatedly
+  if (measureCache_ != nullptr) {
     ParagraphMeasurementCacheKey cacheKey =
         std::make_tuple(attributedString, attributes, layoutConstraints);
     if (measureCache_->exists(cacheKey)) {
       return measureCache_->get(cacheKey);
     }
 
-  auto measuredSize = textLayoutManager_->measure(
-      attributedString, getProps()->paragraphAttributes, layoutConstraints);
-
+    auto measuredSize = makeMeasurements();
     measureCache_->set(cacheKey, measuredSize);
 
     return measuredSize;
   }
 
+  return makeMeasurements();
+}
+
 void ParagraphShadowNode::layout(LayoutContext layoutContext) {
   updateLocalDataIfNeeded();
   ConcreteViewShadowNode::layout(layoutContext);