Introduce support for asynchronous server-side methods (#12)

* Add preliminary changes for async server methods

* Refactor the Message concept and break it into distinctive types

* Continue working on async server methods (high-level API mainly)

* Continue developing support for async server

* Finishing async server methods

* Finishing async server methods (fixing tests & cleaning up)

* A little code cleaning

* Add unit tests for type traits of free functions

* Support for generating async server methods in stub headers

* Update ChangeLog for v0.3.0

* Update the tutorial with how to use async server-side methods

* Update the TOC in sdbus-c++ tutorial

* Update numbering in TOC

* Remove unnecessary code

* Final cleanups

include/sdbus-c++/MethodResult.h

@@ -0,0 +1,126 @@
+/**
+ * (C) 2017 KISTLER INSTRUMENTE AG, Winterthur, Switzerland
+ *
+ * @file ConvenienceClasses.h
+ *
+ * Created on: Nov 8, 2016
+ * Project: sdbus-c++
+ * Description: High-level D-Bus IPC C++ library based on sd-bus
+ *
+ * This file is part of sdbus-c++.
+ *
+ * sdbus-c++ is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * sdbus-c++ is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with sdbus-c++. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef SDBUS_CXX_METHODRESULT_H_
+#define SDBUS_CXX_METHODRESULT_H_
+
+#include <sdbus-c++/Message.h>
+#include <cassert>
+
+// Forward declaration
+namespace sdbus {
+    namespace internal {
+        class Object;
+    }
+    class Error;
+}
+
+namespace sdbus {
+
+    /********************************************//**
+     * @class MethodResult
+     *
+     * Represents result of an asynchronous server-side method.
+     * An instance is provided to the method and shall be set
+     * by the method to either method return value or an error.
+     *
+     ***********************************************/
+    class MethodResult
+    {
+    protected:
+        friend sdbus::internal::Object;
+        MethodResult() = default;
+        MethodResult(const MethodCall& msg, sdbus::internal::Object& object);
+
+        template <typename... _Results> void returnResults(const _Results&... results) const;
+        void returnError(const Error& error) const;
+
+    private:
+        void send(const MethodReply& reply) const;
+
+    private:
+        MethodCall call_;
+        sdbus::internal::Object* object_{};
+    };
+
+    template <typename... _Results>
+    inline void MethodResult::returnResults(const _Results&... results) const
+    {
+        assert(call_.isValid());
+        auto reply = call_.createReply();
+#ifdef __cpp_fold_expressions
+        (reply << ... << results);
+#else
+        using _ = std::initializer_list<int>;
+        (void)_{(void(reply << results), 0)...};
+#endif
+        send(reply);
+    }
+
+    inline void MethodResult::returnError(const Error& error) const
+    {
+        auto reply = call_.createErrorReply(error);
+        send(reply);
+    }
+
+    /********************************************//**
+     * @class Result
+     *
+     * Represents result of an asynchronous server-side method.
+     * An instance is provided to the method and shall be set
+     * by the method to either method return value or an error.
+     *
+     ***********************************************/
+    template <typename... _Results>
+    class Result : protected MethodResult
+    {
+    public:
+        Result() = default;
+        Result(MethodResult result);
+        void returnResults(const _Results&... results) const;
+        void returnError(const Error& error) const;
+    };
+
+    template <typename... _Results>
+    inline Result<_Results...>::Result(MethodResult result)
+        : MethodResult(std::move(result))
+    {
+    }
+
+    template <typename... _Results>
+    inline void Result<_Results...>::returnResults(const _Results&... results) const
+    {
+        MethodResult::returnResults(results...);
+    }
+
+    template <typename... _Results>
+    inline void Result<_Results...>::returnError(const Error& error) const
+    {
+        MethodResult::returnError(error);
+    }
+
+}
+
+#endif /* SDBUS_CXX_METHODRESULT_H_ */