Rearrange the dialog event loop a little

This patch is contributed by @x-qq
and should improve CPU usage on Linux.

src/dialogxml/dialogs/dialog.cpp

@@ -387,13 +387,9 @@ void cDialog::run(std::function<void(cDialog&)> onopen){
 	// TODO: The introduction of the static topWindow means I may be able to use this instead of parent->win; do I still need parent?
 	sf::RenderWindow* parentWin = &(parent ? parent->win : mainPtr);
 	cursor_type former_curs = Cursor::current;
-	set_cursor(sword_curs);
-	using kb = sf::Keyboard;
-	kb::Key k;
-	cKey key, pendingKey = {true};
-	sf::Event currentEvent;
-	std::string itemHit = "";
 	dialogNotToast = true;
+	set_cursor(sword_curs);
+	sf::Event currentEvent;
 	// Focus the first text field, if there is one
 	if(!tabOrder.empty()) {
 		auto iter = std::find_if(tabOrder.begin(), tabOrder.end(), [](std::pair<std::string,cTextField*> ctrl){
@@ -421,14 +417,49 @@ void cDialog::run(std::function<void(cDialog&)> onopen){
 	ModalSession dlog(win, *parentWin);
 	if(onopen) onopen(*this);
 	animTimer.restart();
-	while(dialogNotToast){
+
+	handle_events();
+
+	win.setVisible(false);
+	while(parentWin->pollEvent(currentEvent));
+	set_cursor(former_curs);
+	topWindow = formerTop;
+	makeFrontWindow(*parentWin);
+}
+
+// This method is a main event event loop of the dialog.
+void cDialog::handle_events() {
+	sf::Event currentEvent;
+
+	sf::Clock framerate_clock;
+	const sf::Int64 desired_microseconds_per_frame { 1000000 / 60 }; // us / FPS
+
+	while(dialogNotToast) {
+		while(win.pollEvent(currentEvent)) handle_one_event(currentEvent);
+
+		// Ideally, this should be the only draw call that is done in a cycle.
 		draw();
-		if(!win.pollEvent(currentEvent)) continue;
+
+		// Prevent the loop from executing too fast.
+		const sf::Int64 remaining_time_budget = desired_microseconds_per_frame - framerate_clock.getElapsedTime().asMicroseconds();
+		if(remaining_time_budget > 0) sf::sleep(sf::microseconds(remaining_time_budget));
+		framerate_clock.restart();
+	}
+}
+
+// This method handles one event received by the dialog.
+void cDialog::handle_one_event(const sf::Event& currentEvent) {
+	using kb = sf::Keyboard;
+
+	cKey key;
+	// HACK: This needs to be stored between consecutive invocations of this function
+	static cKey pendingKey = {true};
+	std::string itemHit = "";
 	location where;
-		switch(currentEvent.type){
+
+	switch(currentEvent.type) {
 		case sf::Event::KeyPressed:
-				k = currentEvent.key.code;
-				switch(k){
+			switch(currentEvent.key.code){
 				case kb::Up:
 					key.spec = true;
 					key.k = key_up;
@@ -497,10 +528,10 @@ void cDialog::run(std::function<void(cDialog&)> onopen){
 				case kb::RControl:
 				case kb::LSystem:
 				case kb::RSystem:
-						continue;
+					return;
 				default:
 					key.spec = false;
-						key.c = keyToChar(k, false);
+					key.c = keyToChar(currentEvent.key.code, false);
 					break;
 			}
 			key.mod = mod_none;
@@ -512,11 +543,13 @@ void cDialog::run(std::function<void(cDialog&)> onopen){
 			// Now check for focused fields.
 			if(currentFocus.empty()) break;
 			// If it's a tab, handle tab order
-				if(key.spec && key.k == key_tab){
+			if(key.spec && key.k == key_tab) {
 				// Could use key.mod, but this is slightly easier.
-					if(currentEvent.key.shift)
+				if(currentEvent.key.shift) {
 					handleTabOrder(currentFocus, tabOrder.rbegin(), tabOrder.rend());
-					else handleTabOrder(currentFocus, tabOrder.begin(), tabOrder.end());
+				} else {
+					handleTabOrder(currentFocus, tabOrder.begin(), tabOrder.end());
+				}
 			} else {
 				// If it's a character key, and the system key (control/command) is not pressed,
 				// we have an upcoming TextEntered event which contains more information.
@@ -562,12 +595,6 @@ void cDialog::run(std::function<void(cDialog&)> onopen){
 			if(!inField) set_cursor(sword_curs);
 			break;
 	}
-	}
-	win.setVisible(false);
-	while(parentWin->pollEvent(currentEvent));
-	set_cursor(former_curs);
-	topWindow = formerTop;
-	makeFrontWindow(*parentWin);
 }
 
 template<typename Iter> void cDialog::handleTabOrder(string& itemHit, Iter begin, Iter end) {

src/dialogxml/dialogs/dialog.hpp

@@ -214,6 +214,8 @@ public:
 	cDialog(cDialog& other) = delete;
 private:
 	void draw();
+	void handle_events();
+	void handle_one_event(const sf::Event&);
 	void process_keystroke(cKey keyHit);
 	void process_click(location where, eKeyMod mods);
 	bool dialogNotToast, didAccept;