GUI2/Loading Screen: made animation time-based instead of frame-based

Big thanks to @CelticMinstrel for help on this.

data/gui/window/loadscreen.cfg

@@ -133,15 +133,13 @@
 											#
 											# Uses a bezier in-out easing to calculate the position of the dot each frame.
 											#
-											# First, it gets a normalized [0, 1] value based on the value of `tick` in the range [0, steps], then uses
-											# this value multiplied by the width of the drawing canvas (sans the width of the text, since there still
-											# needs to be enough horizontal space to render the dot on the last step of the loop) to get this frame's
-											# x-coordinate. Note that `steps` *must* be a decimal or the formula won't work.
+											# `time` represents the elapsed time since the animation began, in milliseconds. This gets a normalized
+											# [0, 1] value based on the ellapsed time in the current iteration of the range [0, duration] (also in
+											# milliseconds). That value is then multiplied b the width of the drawing canvas (sans the width of the
+											# test, since there still needs to be enough horizontal space to render the dor on the last step of the
+											# loop) to get this frame's x-coordinate.)"
 											#
-											# For the record, the original linear formula I came up with was:
-											# "(floor(jump * (tick % steps)) where jump = (width / steps) where steps = 40)"
-											#
-											x = "(round(w * (t * t * (3.0 - 2.0 * t))) where w = (width - text_width) where t = ((tick % steps) / steps) where steps = 40.0)"
+											x = "(round(w * (t * t * (3.0 - 2.0 * t))) where w = (width - text_width), t = (time % duration) / as_decimal(duration) where duration = 1000)"
 											y = 0
 											w = "(text_width)"
 											h = "(height)"

src/gui/dialogs/loading_screen.cpp

@@ -32,7 +32,6 @@
 #include "preferences/general.hpp"
 #include "video.hpp"
 
-#include <chrono>
 #include <cstdlib>
 #include <functional>
 
@@ -75,12 +74,12 @@ REGISTER_DIALOG(loading_screen)
 loading_screen* loading_screen::singleton_ = nullptr;
 
 loading_screen::loading_screen(std::function<void()> f)
-	: animation_counter_(0)
-	, load_func_(f)
+	: load_func_(f)
 	, worker_result_()
 	, cursor_setter_()
 	, progress_stage_label_(nullptr)
 	, animation_(nullptr)
+	, animation_start_()
 	, current_stage_(loading_stage::none)
 	, visible_stages_()
 	, current_visible_stage_()
@@ -160,7 +159,15 @@ void loading_screen::draw_callback()
 		progress_stage_label_->set_label(iter->second);
 	}
 
-	animation_->get_drawing_canvas().set_variable("tick", wfl::variant(animation_counter_++));
+	using namespace std::chrono;
+	const auto now = steady_clock::now();
+
+	// We only need to set the start time once;
+	if(!animation_start_.has_value()) {
+		animation_start_ = now;
+	}
+
+	animation_->get_drawing_canvas().set_variable("time", wfl::variant(duration_cast<milliseconds>(now - *animation_start_).count()));
 	animation_->set_is_dirty(true);
 }
 

src/gui/dialogs/loading_screen.hpp

@@ -19,8 +19,10 @@
 #include "tstring.hpp"
 
 #include <atomic>
+#include <chrono>
 #include <future>
 #include <map>
+#include <optional>
 #include <vector>
 
 namespace cursor
@@ -99,14 +101,15 @@ class loading_screen : public modal_dialog, public events::pump_monitor
 
 	static loading_screen* singleton_;
 
-	int animation_counter_;
 	std::function<void()> load_func_;
 	std::future<void> worker_result_;
 	std::unique_ptr<cursor::setter> cursor_setter_;
 
 	label* progress_stage_label_;
 	drawing* animation_;
 
+	std::optional<decltype(std::chrono::steady_clock::now())> animation_start_;
+
 	std::atomic<loading_stage> current_stage_;
 
 	using stage_map = std::map<loading_stage, t_string>;