fabric/debug module and DebugStringConvertible class

Summary:
This is a very first diff in a series of undefined length implementing React Native Shadow Tree infra in C++.
All Shadow Nodes, Props and etc. implements `DebugStringConvertible`.

Authored by shergin

Reviewed By: shergin

Differential Revision: D7163868

fbshipit-source-id: 9c001aa5bd0723f709a07b1833f512c51e3bec11

ReactCommon/fabric/debug/DebugStringConvertible.h

@@ -0,0 +1,59 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace facebook {
+namespace react {
+
+class DebugStringConvertible;
+
+using SharedDebugStringConvertible = std::shared_ptr<const DebugStringConvertible>;
+using SharedDebugStringConvertibleList = std::vector<const SharedDebugStringConvertible>;
+
+// Abstract class describes conformance to DebugStringConvertible concept
+// and implements basic recursive debug string assembly algorithm.
+// Use this as a base class for providing a debugging textual representation
+// of your class.
+// TODO (#26770211): Clear up the naming.
+class DebugStringConvertible {
+
+public:
+  // Returns a name of the object.
+  // Default implementation returns "Node".
+  virtual std::string getDebugName() const;
+
+  // Returns a value assosiate with the object.
+  // Default implementation returns an empty string.
+  virtual std::string getDebugValue() const;
+
+  // Returns a list of `DebugStringConvertible` objects which can be considered
+  // as *children* of the object.
+  // Default implementation returns an empty list.
+  virtual SharedDebugStringConvertibleList getDebugChildren() const;
+
+  // Returns a list of `DebugStringConvertible` objects which can be considered
+  // as *properties* of the object.
+  // Default implementation returns an empty list.
+  virtual SharedDebugStringConvertibleList getDebugProps() const;
+
+  // Returns a string which represents the object in a human-readable way.
+  // Default implementation returns a description of the subtree
+  // rooted at this node, represented in XML-like format.
+  virtual std::string getDebugDescription(int level = 0) const;
+
+  // Do same as `getDebugDescription` but return only *children* and
+  // *properties* parts (which are used in `getDebugDescription`).
+  virtual std::string getDebugPropsDescription(int level = 0) const;
+  virtual std::string getDebugChildrenDescription(int level = 0) const;
+};
+
+} // namespace react
+} // namespace facebook