Fabric: YogaLayoutableShadowNode::isLeaf_ and using that in ParagraphShadowNode

Summary:
As soon as we have traits infra, it's pretty straight-forward to implement this kinda flag. Without the traits, it's challenging to build that in a performant and elegant way. The challenges of the dynamic_cast-involved approach are:
 * How to check for that feature in YogaLayoutableShadowNode?
 * Even if we use `dynamic_cast`, to which class to cast?
 * We also need to do that in constructor... and dynamic_cast-like checks for `this` don't work in constructors (C++ design limitation).
 * How to scale that if we have more classes like Paragraph (and we will have it for sure)?
 * Performance.
 * Relying on RTTI.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D18525347

fbshipit-source-id: b1915f43ff3fe4364ab6345fb9d1becc591b5a35

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

@@ -39,6 +39,12 @@ class ParagraphShadowNode : public ConcreteViewShadowNode<
  public:
   using ConcreteViewShadowNode::ConcreteViewShadowNode;
 
+  static ShadowNodeTraits BaseTraits() {
+    auto traits = ConcreteViewShadowNode::BaseTraits();
+    traits.set(ShadowNodeTraits::Trait::LeafYogaNode);
+    return traits;
+  }
+
   /*
    * Returns a `AttributedString` which represents text content of the node.
    */

ReactCommon/fabric/components/view/ConcreteViewShadowNode.h

@@ -58,7 +58,8 @@ class ConcreteViewShadowNode : public ConcreteShadowNode<
       ComponentDescriptor const &componentDescriptor,
       ShadowNodeTraits traits)
       : BaseShadowNode(fragment, componentDescriptor, traits),
-        YogaLayoutableShadowNode() {
+        YogaLayoutableShadowNode(
+            traits.check(ShadowNodeTraits::Trait::LeafYogaNode)) {
     YogaLayoutableShadowNode::setProps(
         *std::static_pointer_cast<const ConcreteViewProps>(fragment.props));
     YogaLayoutableShadowNode::setChildren(

ReactCommon/fabric/components/view/yoga/YogaLayoutableShadowNode.cpp

@@ -21,18 +21,21 @@
 namespace facebook {
 namespace react {
 
-YogaLayoutableShadowNode::YogaLayoutableShadowNode()
+YogaLayoutableShadowNode::YogaLayoutableShadowNode(bool isLeaf)
-    : yogaConfig_(nullptr), yogaNode_(&initializeYogaConfig(yogaConfig_)) {
+    : yogaConfig_(nullptr),
+      yogaNode_(&initializeYogaConfig(yogaConfig_)),
+      isLeaf_(isLeaf) {
   yogaNode_.setContext(this);
 }
 
 YogaLayoutableShadowNode::YogaLayoutableShadowNode(
-    const YogaLayoutableShadowNode &layoutableShadowNode)
+    YogaLayoutableShadowNode const &layoutableShadowNode)
     : LayoutableShadowNode(layoutableShadowNode),
       yogaConfig_(nullptr),
       yogaNode_(
           layoutableShadowNode.yogaNode_,
-          &initializeYogaConfig(yogaConfig_)) {
+          &initializeYogaConfig(yogaConfig_)),
+      isLeaf_(layoutableShadowNode.isLeaf_) {
   yogaNode_.setContext(this);
   yogaNode_.setOwner(nullptr);
 
@@ -70,6 +73,10 @@ void YogaLayoutableShadowNode::enableMeasurement() {
 }
 
 void YogaLayoutableShadowNode::appendChild(YogaLayoutableShadowNode *child) {
+  if (isLeaf_) {
+    return;
+  }
+
   ensureUnsealed();
 
   yogaNode_.setDirty(true);
@@ -95,6 +102,10 @@ void YogaLayoutableShadowNode::appendChild(YogaLayoutableShadowNode *child) {
 
 void YogaLayoutableShadowNode::setChildren(
     YogaLayoutableShadowNode::UnsharedList children) {
+  if (isLeaf_) {
+    return;
+  }
+
   ensureUnsealed();
 
   // Optimization:

ReactCommon/fabric/components/view/yoga/YogaLayoutableShadowNode.h

@@ -32,10 +32,10 @@ class YogaLayoutableShadowNode : public LayoutableShadowNode,
 
 #pragma mark - Constructors
 
-  YogaLayoutableShadowNode();
+  YogaLayoutableShadowNode(bool isLeaf);
 
   YogaLayoutableShadowNode(
-      const YogaLayoutableShadowNode &layoutableShadowNode);
+      YogaLayoutableShadowNode const &layoutableShadowNode);
 
 #pragma mark - Mutating Methods
 
@@ -64,14 +64,14 @@ class YogaLayoutableShadowNode : public LayoutableShadowNode,
    */
   void setProps(const YogaStylableProps &props);
 
-  /**
+  /*
    * Sets layoutable size of node.
    */
   void setSize(Size size) const;
 
   void setPadding(RectangleEdges<Float> padding) const;
 
-  /**
+  /*
    * Sets position type of Yoga node (relative, absolute).
    */
   void setPositionType(YGPositionType positionType) const;
@@ -108,6 +108,13 @@ class YogaLayoutableShadowNode : public LayoutableShadowNode,
    */
   mutable YGNode yogaNode_;
 
+  /*
+   * Forces associated YGNode to be a leaf.
+   * Adding a child `ShadowNode` will not add `YGNode` associated with it as a
+   * child to the stored `YGNode`.
+   */
+  bool const isLeaf_;
+
  private:
   static YGConfig &initializeYogaConfig(YGConfig &config);
   static YGNode *yogaNodeCloneCallbackConnector(