Codechange: unroll the SDL2 main loop

This commit prepares for the next commit, as Emscripten needs to
have a way to trigger a single iteration of the main loop. To
keep the real changes more clear, this commit only unrolls the
loop, and makes no changes to the logic itself.

src/video/sdl2_v.cpp

@@ -664,55 +664,16 @@ void VideoDriver_SDL::Stop()
 	}
 }
 
-void VideoDriver_SDL::MainLoop()
+void VideoDriver_SDL::LoopOnce()
 {
-	uint32 cur_ticks = SDL_GetTicks();
-	uint32 last_cur_ticks = cur_ticks;
-	uint32 next_tick = cur_ticks + MILLISECONDS_PER_TICK;
 	uint32 mod;
 	int numkeys;
 	const Uint8 *keys;
-
-	CheckPaletteAnim();
-
-	std::thread draw_thread;
-	std::unique_lock<std::recursive_mutex> draw_lock;
-	if (_draw_threaded) {
-		/* Initialise the mutex first, because that's the thing we *need*
-		 * directly in the newly created thread. */
-		_draw_mutex = new std::recursive_mutex();
-		if (_draw_mutex == nullptr) {
-			_draw_threaded = false;
-		} else {
-			draw_lock = std::unique_lock<std::recursive_mutex>(*_draw_mutex);
-			_draw_signal = new std::condition_variable_any();
-			_draw_continue = true;
-
-			_draw_threaded = StartNewThread(&draw_thread, "ottd:draw-sdl", &DrawSurfaceToScreenThread);
-
-			/* Free the mutex if we won't be able to use it. */
-			if (!_draw_threaded) {
-				draw_lock.unlock();
-				draw_lock.release();
-				delete _draw_mutex;
-				delete _draw_signal;
-				_draw_mutex = nullptr;
-				_draw_signal = nullptr;
-			} else {
-				/* Wait till the draw mutex has started itself. */
-				_draw_signal->wait(*_draw_mutex);
-			}
-		}
-	}
-
-	DEBUG(driver, 1, "SDL2: using %sthreads", _draw_threaded ? "" : "no ");
-
-	for (;;) {
 	uint32 prev_cur_ticks = cur_ticks; // to check for wrapping
 	InteractiveRandom(); // randomness
 
 	while (PollEvent() == -1) {}
-		if (_exit_game) break;
+	if (_exit_game) return;
 
 	mod = SDL_GetModState();
 	keys = SDL_GetKeyboardState(&numkeys);
@@ -777,8 +738,56 @@ void VideoDriver_SDL::MainLoop()
 		CheckPaletteAnim();
 		DrawSurfaceToScreen();
 	}
+}
+
+void VideoDriver_SDL::MainLoop()
+{
+	cur_ticks = SDL_GetTicks();
+	last_cur_ticks = cur_ticks;
+	next_tick = cur_ticks + MILLISECONDS_PER_TICK;
+
+	CheckPaletteAnim();
+
+	if (_draw_threaded) {
+		/* Initialise the mutex first, because that's the thing we *need*
+		 * directly in the newly created thread. */
+		_draw_mutex = new std::recursive_mutex();
+		if (_draw_mutex == nullptr) {
+			_draw_threaded = false;
+		} else {
+			draw_lock = std::unique_lock<std::recursive_mutex>(*_draw_mutex);
+			_draw_signal = new std::condition_variable_any();
+			_draw_continue = true;
+
+			_draw_threaded = StartNewThread(&draw_thread, "ottd:draw-sdl", &DrawSurfaceToScreenThread);
+
+			/* Free the mutex if we won't be able to use it. */
+			if (!_draw_threaded) {
+				draw_lock.unlock();
+				draw_lock.release();
+				delete _draw_mutex;
+				delete _draw_signal;
+				_draw_mutex = nullptr;
+				_draw_signal = nullptr;
+			} else {
+				/* Wait till the draw mutex has started itself. */
+				_draw_signal->wait(*_draw_mutex);
+			}
+		}
 	}
 
+	DEBUG(driver, 1, "SDL2: using %sthreads", _draw_threaded ? "" : "no ");
+
+	while (!_exit_game) {
+		LoopOnce();
+	}
+
+	MainLoopCleanup();
+#endif
+}
+
+void VideoDriver_SDL::MainLoopCleanup()
+{
 	if (_draw_mutex != nullptr) {
 		_draw_continue = false;
 		/* Sending signal if there is no thread blocked

src/video/sdl2_v.h

@@ -42,12 +42,21 @@ public:
 	const char *GetName() const override { return "sdl"; }
 private:
 	int PollEvent();
+	void LoopOnce();
+	void MainLoopCleanup();
 	bool CreateMainSurface(uint w, uint h, bool resize);
 
 	/**
 	 * This is true to indicate that keyboard input is in text input mode, and SDL_TEXTINPUT events are enabled.
 	 */
 	bool edit_box_focused;
+
+	uint32 cur_ticks;
+	uint32 last_cur_ticks;
+	uint32 next_tick;
+
+	std::thread draw_thread;
+	std::unique_lock<std::recursive_mutex> draw_lock;
 };
 
 /** Factory for the SDL video driver. */