Fabric: Introducing ShadowNode::shallowSourceNode()

Summary:
The method replaces the current source node with its source node.
It's useful when we have to clone some node but don't want to change a source node pointer.

Reviewed By: fkgozali

Differential Revision: D7503384

fbshipit-source-id: 81ec64079c7e99cb9abdda2af10d85281a94e1b1

ReactCommon/fabric/core/shadownode/ShadowNode.cpp

@@ -108,6 +108,10 @@ void ShadowNode::clearSourceNode() {
   sourceNode_.reset();
 }
 
+void ShadowNode::shallowSourceNode() {
+  sourceNode_ = sourceNode_.lock()->getSourceNode();
+}
+
 #pragma mark - Equality
 
 bool ShadowNode::operator==(const ShadowNode& rhs) const {

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -67,6 +67,14 @@ public:
   void replaceChild(const SharedShadowNode &oldChild, const SharedShadowNode &newChild);
   void clearSourceNode();
 
+  /*
+   * Replaces the current source node with its source node.
+   * This method might be used for illuminating side-effects caused by the last
+   * cloning operation which are not desirable from the diffing algorithm
+   * perspective.
+   */
+  void shallowSourceNode();
+
 #pragma mark - Equality
 
   /*