mpd: revamped to event-driven, single-threaded

Fix MPD connection issues by converting/rewriting module into a
state-machine driven system. It is fully single-threaded and uses
events for transitioning between states. It supports all features
and functionality of the previous MPD module.

Signed-off-by: Joseph Benden <joe@benden.us>

include/modules/mpd/mpd.hpp

@@ -0,0 +1,66 @@
+#pragma once
+
+#include <mpd/client.h>
+#include <spdlog/fmt/bundled/format.h>
+#include <spdlog/spdlog.h>
+
+#include <condition_variable>
+#include <thread>
+
+#include "ALabel.hpp"
+#include "modules/mpd/state.hpp"
+
+namespace waybar::modules {
+
+class MPD : public ALabel {
+  friend class detail::Context;
+
+  // State machine
+  detail::Context context_{this};
+
+  const std::string module_name_;
+
+  // Not using unique_ptr since we don't manage the pointer
+  // (It's either nullptr, or from the config)
+  const char*    server_;
+  const unsigned port_;
+
+  unsigned timeout_;
+
+  detail::unique_connection connection_;
+
+  detail::unique_status status_;
+  mpd_state             state_;
+  detail::unique_song   song_;
+
+ public:
+  MPD(const std::string&, const Json::Value&);
+  virtual ~MPD() noexcept = default;
+  auto update() -> void;
+
+ private:
+  std::string getTag(mpd_tag_type type, unsigned idx = 0) const;
+  void        setLabel();
+  std::string getStateIcon() const;
+  std::string getOptionIcon(std::string optionName, bool activated) const;
+
+  // GUI-side methods
+  bool handlePlayPause(GdkEventButton* const&);
+  void emit() { dp.emit(); }
+
+  // MPD-side, Non-GUI methods.
+  void tryConnect();
+  void checkErrors(mpd_connection* conn);
+  void fetchState();
+  void queryMPD();
+
+  inline bool stopped() const { return connection_ && state_ == MPD_STATE_STOP; }
+  inline bool playing() const { return connection_ && state_ == MPD_STATE_PLAY; }
+  inline bool paused() const { return connection_ && state_ == MPD_STATE_PAUSE; }
+};
+
+#if !defined(MPD_NOINLINE)
+#include "modules/mpd/state.inl.hpp"
+#endif
+
+}  // namespace waybar::modules