A framework for registering data sources

This is needed so that we know which sources to export to lua, and a huge switch or something is
just too ugly.

src/CMakeLists.txt

@@ -38,7 +38,7 @@ set(conky_sources colours.cc combine.cc common.cc conky.cc core.cc
 	diskio.cc entropy.cc exec.cc fs.cc mail.cc mixer.cc net_stat.cc template.cc
 	mboxscan.cc read_tcp.cc scroll.cc specials.cc tailhead.cc
 	temphelper.cc text_object.cc timeinfo.cc top.cc algebra.cc prioqueue.c proc.cc
-	user.cc luamm.cc)
+	user.cc luamm.cc data-source.cc)
 
 # add timed thread library
 add_library(timed-thread timed-thread.cc)

src/data-source.cc

@@ -0,0 +1,106 @@
+/* -*- mode: c++; c-basic-offset: 4; tab-width: 4; indent-tabs-mode: t -*-
+ * vim: ts=4 sw=4 noet ai cindent syntax=cpp
+ *
+ * Conky, a system monitor, based on torsmo
+ *
+ * Please see COPYING for details
+ *
+ * Copyright (C) 2010 Pavel Labath et al.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <config.h>
+
+#include "data-source.hh"
+
+#include <iostream>
+#include <limits>
+#include <sstream>
+#include <unordered_map>
+
+namespace conky {
+	namespace {
+		/*
+		 * Returned when there is no data available.
+		 * An alternative would be to throw an exception, but if we don't want to react too
+		 * aggresively when the user e.g. uses a nonexisting variable, then returning NaN will do
+		 * just fine.
+		 */
+		float NaN = std::numeric_limits<float>::quiet_NaN();
+
+		/*
+		 * We cannot construct this object statically, because order of object construction in
+		 * different modules is not defined, so register_source could be called before this
+		 * object is constructed. Therefore, we create it on the first call to register_source.
+		 */
+		typedef std::unordered_map<std::string, data_source_factory> data_sources_t;
+		data_sources_t *data_sources;
+
+		void register_data_source_(const std::string &name, const data_source_factory &factory_func)
+		{
+			struct data_source_constructor {
+				data_source_constructor()  { data_sources = new data_sources_t(); }
+				~data_source_constructor() { delete data_sources; data_sources = NULL; }
+			};
+			static data_source_constructor constructor;
+
+			bool inserted = data_sources->insert({name, factory_func}).second;
+			if(not inserted)
+				throw std::logic_error("Data source with name '" + name + "' already registered");
+		}
+
+		static std::shared_ptr<data_source_base>
+		disabled_source_factory(lua::state &l, const std::string &name, const std::string &setting)
+		{
+			// XXX some generic way of reporting errors? NORM_ERR?
+			std::cerr << "Support for setting '" << name
+					  << "' has been disabled during compilation. Please recompile with '"
+					  << setting << "'" << std::endl;
+
+			return simple_numeric_source<float>::factory(l, name, &NaN);
+		}
+	}
+
+	double data_source_base::get_number() const
+	{ return NaN; }
+
+	std::string data_source_base::get_text() const
+	{
+		std::ostringstream s;
+		s << get_number();
+		return s.str();
+	}
+
+	template<typename T>
+	std::shared_ptr<data_source_base>
+	simple_numeric_source<T>::factory(lua::state &l, const std::string &name, const T *source)
+	{
+		l.pop();
+		return std::shared_ptr<simple_numeric_source>(new simple_numeric_source(name, source));
+	}
+
+	register_data_source::register_data_source(const std::string &name, 
+			const data_source_factory &factory_func)
+	{ register_data_source_(name, factory_func); }
+
+	register_disabled_data_source::register_disabled_data_source(const std::string &name, 
+			const std::string &setting)
+	{
+		register_data_source_(name,
+				std::bind(disabled_source_factory,
+							std::placeholders::_1, std::placeholders::_2, setting)
+			);
+	}
+}

src/data-source.hh

@@ -0,0 +1,107 @@
+/* -*- mode: c++; c-basic-offset: 4; tab-width: 4; indent-tabs-mode: t -*-
+ * vim: ts=4 sw=4 noet ai cindent syntax=cpp
+ *
+ * Conky, a system monitor, based on torsmo
+ *
+ * Please see COPYING for details
+ *
+ * Copyright (C) 2010 Pavel Labath et al.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef DATA_SOURCE_HH
+#define DATA_SOURCE_HH
+
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+
+#include "luamm.hh"
+
+namespace conky {
+
+    class data_source_base;
+
+	/*
+	 * Recieves a lua table on the stack and the name the object was registered with. It should
+	 * pop the table after consuming it and return the data source.
+	 */
+    typedef std::function<std::shared_ptr<data_source_base> (lua::state &l, const std::string &name)> data_source_factory;
+
+	/*
+	 * A base class for all data sources.
+	 * API consists of two functions:
+	 * - get_number should return numeric representation of the data (if available). This can
+	 *   then be used when drawing graphs, bars, ... The default implementation returns NaN.
+	 * - get_text should return textual representation of the data. This is used when simple
+	 *   displaying the value of the data source. The default implementation converts
+	 *   get_number() to a string, but you can override to return anything (e.g. add units)
+	 */
+    class data_source_base {
+    public:
+        const std::string name;
+
+        data_source_base(const std::string &name_)
+            : name(name_)
+        {}
+
+        virtual ~data_source_base() {}
+        virtual double get_number() const;
+        virtual std::string get_text() const;
+    };
+
+	/*
+	 * A simple data source that returns the value of some variable.
+	 * It ignores the lua table, but one can create a wrapper for the factory function that uses
+	 * data in the table to decide which variable to return.
+	 */
+	template<typename T>
+	class simple_numeric_source: public data_source_base {
+		static_assert(std::is_convertible<T, double>::value, "T must be convertible to double");
+
+		const T *source;
+
+		simple_numeric_source(const std::string &name_, const T *source_)
+			: data_source_base(name_), source(source_)
+		{}
+	public:
+		static std::shared_ptr<data_source_base>
+		factory(lua::state &l, const std::string &name, const T *source);
+
+		virtual double get_number() const
+		{ return *source; }
+	};
+
+	/*
+	 * Declaring an object of this type at global scope will register a data source with the give
+	 * name and factory function.
+	 */
+	class register_data_source {
+	public:
+		register_data_source(const std::string &name, const data_source_factory &factory_func);
+	};
+
+	/*
+	 * Use this to declare a data source that has been disabled during compilation. We can then
+	 * print a nice error message telling the used which setting to enable.
+	 */
+	class register_disabled_data_source {
+	public:
+		register_disabled_data_source(const std::string &name, const std::string &setting);
+	};
+}
+
+#endif /* DATA_SOURCE_HH */