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/DebugStringConvertibleItem.h

@@ -0,0 +1,45 @@
+/**
+ * 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 <fabric/debug/DebugStringConvertible.h>
+
+namespace facebook {
+namespace react {
+
+// Trivial implementation of `DebugStringConvertible` abstract class
+// with a stored output; useful for assembling `DebugStringConvertible` values
+// in custom implementations of `getDebugChildren` and `getDebugProps`.
+class DebugStringConvertibleItem:
+  public DebugStringConvertible {
+
+public:
+  DebugStringConvertibleItem() = default;
+  DebugStringConvertibleItem(const DebugStringConvertibleItem &item) = default;
+
+  DebugStringConvertibleItem(
+    const std::string &name = "",
+    const std::string &value = "",
+    const SharedDebugStringConvertibleList &props = {},
+    const SharedDebugStringConvertibleList &children = {}
+  );
+
+  std::string getDebugName() const override;
+  std::string getDebugValue() const override;
+  SharedDebugStringConvertibleList getDebugChildren() const override;
+  SharedDebugStringConvertibleList getDebugProps() const override;
+
+private:
+  std::string name_;
+  std::string value_;
+  SharedDebugStringConvertibleList props_;
+  SharedDebugStringConvertibleList children_;
+};
+
+} // namespace react
+} // namespace facebook