Added support for custom reader classes

extras/tests/Helpers/CustomReader.hpp

@@ -0,0 +1,26 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2019
+// MIT License
+
+#pragma once
+
+#include <sstream>
+
+class CustomReader {
+  std::stringstream _stream;
+
+ public:
+  CustomReader(const char* input) : _stream(input) {}
+
+  int read() {
+    return _stream.get();
+  }
+
+  size_t readBytes(char* buffer, size_t length) {
+    _stream.read(buffer, static_cast<std::streamsize>(length));
+    return static_cast<size_t>(_stream.gcount());
+  }
+
+ private:
+  CustomReader(const CustomReader&);
+};