Introducing `LayoutableShadowNode::isLayoutOnly` and (theoretical) view-flattening

Summary:
@public
Voalá, this small change actually implements view flattening. Obviously, it does not work right now because there are no `ShadowNode` classes which implement `isLayoutOnly`.
Surprisingly, correct implementing of `isLayoutOnly` is quite tricky, we will work on this in coming diffs.

Reviewed By: mdvacca

Differential Revision: D9403565

fbshipit-source-id: 1f16f912cb5c6841405a1fc3cf36aec28698c11f

ReactCommon/fabric/core/layout/LayoutableShadowNode.cpp

@@ -31,6 +31,10 @@ bool LayoutableShadowNode::setLayoutMetrics(LayoutMetrics layoutMetrics) {
   return true;
 }
 
+bool LayoutableShadowNode::LayoutableShadowNode::isLayoutOnly() const {
+  return false;
+}
+
 void LayoutableShadowNode::cleanLayout() {
   isLayoutClean_ = true;
 }

ReactCommon/fabric/core/layout/LayoutableShadowNode.h

@@ -52,6 +52,13 @@ public:
    */
   virtual LayoutMetrics getLayoutMetrics() const;
 
+  /*
+   * Returns `true` if the node represents only information necessary for
+   * layout computation and can be safely removed from view hierarchy.
+   * Default implementation returns `false`.
+   */
+  virtual bool isLayoutOnly() const;
+
 protected:
 
   /*

ReactCommon/fabric/uimanager/Differentiator.cpp

@@ -11,13 +11,27 @@
 namespace facebook {
 namespace react {
 
+static void sliceChildShadowNodeViewPairsRecursively(ShadowViewNodePairList &pairList, Point layoutOffset, const ShadowNode &shadowNode) {
+  for (const auto &childShadowNode : shadowNode.getChildren()) {
+    auto shadowView = ShadowView(*childShadowNode);
+
+    const auto layoutableShadowNode = dynamic_cast<const LayoutableShadowNode *>(childShadowNode.get());
+    if (layoutableShadowNode && layoutableShadowNode->isLayoutOnly()) {
+      sliceChildShadowNodeViewPairsRecursively(
+        pairList,
+        layoutOffset + shadowView.layoutMetrics.frame.origin,
+        *childShadowNode
+      );
+    } else {
+      shadowView.layoutMetrics.frame.origin += layoutOffset;
+      pairList.push_back({shadowView, *childShadowNode});
+    }
+  }
+}
+
 static ShadowViewNodePairList sliceChildShadowNodeViewPairs(const ShadowNode &shadowNode) {
   ShadowViewNodePairList pairList;
-
+  sliceChildShadowNodeViewPairsRecursively(pairList, {0, 0}, shadowNode);
-  for (const auto &childShadowNode : shadowNode.getChildren()) {
-    pairList.push_back({ShadowView(*childShadowNode), *childShadowNode});
-  }
-
   return pairList;
 }