Implement findNodeAtPoint method

Summary:
This diff implements the findNodeAtPoint method to return the ShadowNode that is positioned into a Point of the screen.

What's not supported:
- Scroll position
- Transform
- return layoutable nodes that are contained inside a non-layoutable node

Changelog: [internal]

Reviewed By: shergin

Differential Revision: D19190285

fbshipit-source-id: fdc0358dc21312e9950a4eb16c36020e9e43e33f

Author: mdvacca

ReactCommon/fabric/core/layout/LayoutableShadowNode.cpp

@@ -114,6 +114,32 @@ void LayoutableShadowNode::layout(LayoutContext layoutContext) {
   }
 }
 
+ShadowNode::Shared LayoutableShadowNode::findNodeAtPoint(
+    ShadowNode::Shared node,
+    Point point) {
+  auto layoutableShadowNode =
+      dynamic_cast<const LayoutableShadowNode *>(node.get());
+
+  if (!layoutableShadowNode) {
+    return nullptr;
+  }
+  auto frame = layoutableShadowNode->getLayoutMetrics().frame;
+  auto isPointInside = frame.containsPoint(point);
+
+  if (!isPointInside) {
+    return nullptr;
+  }
+
+  auto newPoint = point - frame.origin;
+  for (const auto &childShadowNode : node->getChildren()) {
+    auto hitView = findNodeAtPoint(childShadowNode, newPoint);
+    if (hitView) {
+      return hitView;
+    }
+  }
+  return isPointInside ? node : nullptr;
+}
+
 void LayoutableShadowNode::layoutChildren(LayoutContext layoutContext) {
   // Default implementation does nothing.
 }

ReactCommon/fabric/core/layout/LayoutableShadowNode.h

@@ -17,6 +17,7 @@
 #include <react/core/Sealable.h>
 #include <react/core/ShadowNode.h>
 #include <react/debug/DebugStringConvertible.h>
+#include <react/graphics/Geometry.h>
 #include <react/graphics/Transform.h>
 
 namespace facebook {
@@ -86,6 +87,14 @@ class LayoutableShadowNode : public virtual Sealable {
       LayoutableShadowNode const &ancestorLayoutableShadowNode,
       LayoutInspectingPolicy policy) const;
 
+  /*
+   * Returns the ShadowNode that is rendered at the Point received as a
+   * parameter.
+   */
+  static ShadowNode::Shared findNodeAtPoint(
+      ShadowNode::Shared node,
+      Point point);
+
  protected:
   /*
    * Clean or Dirty layout state:

ReactCommon/fabric/uimanager/UIManager.cpp

@@ -9,6 +9,7 @@
 
 #include <react/core/ShadowNodeFragment.h>
 #include <react/debug/SystraceSection.h>
+#include <react/graphics/Geometry.h>
 
 #include <glog/logging.h>
 
@@ -134,6 +135,12 @@ void UIManager::clearJSResponder() const {
   }
 }
 
+ShadowNode::Shared UIManager::findNodeAtPoint(
+    const ShadowNode::Shared &node,
+    Point point) const {
+  return LayoutableShadowNode::findNodeAtPoint(node, point);
+}
+
 void UIManager::setNativeProps(
     ShadowNode const &shadowNode,
     RawProps const &rawProps) const {

ReactCommon/fabric/uimanager/UIManager.h

@@ -88,6 +88,10 @@ class UIManager final : public ShadowTreeDelegate {
 
   void clearJSResponder() const;
 
+  ShadowNode::Shared findNodeAtPoint(
+      const ShadowNode::Shared &shadowNode,
+      Point point) const;
+
   /*
    * Returns layout metrics of given `shadowNode` relative to
    * `ancestorShadowNode` (relative to the root node in case if provided