4 files changed, 47 insertions, 4 deletions
diff --git a/core/os/main_loop.cpp b/core/os/main_loop.cpp
index dc68c2a9f9..434f6fa300 100644
--- a/core/os/main_loop.cpp
+++ b/core/os/main_loop.cpp
@@ -48,8 +48,10 @@ void MainLoop::_bind_methods() {
 	BIND_CONSTANT(NOTIFICATION_WM_ABOUT);
 	BIND_CONSTANT(NOTIFICATION_CRASH);
 	BIND_CONSTANT(NOTIFICATION_OS_IME_UPDATE);
-	BIND_CONSTANT(NOTIFICATION_APP_RESUMED);
-	BIND_CONSTANT(NOTIFICATION_APP_PAUSED);
+	BIND_CONSTANT(NOTIFICATION_APPLICATION_RESUMED);
+	BIND_CONSTANT(NOTIFICATION_APPLICATION_PAUSED);
+	BIND_CONSTANT(NOTIFICATION_APPLICATION_FOCUS_IN);
+	BIND_CONSTANT(NOTIFICATION_APPLICATION_FOCUS_OUT);
 
 	ADD_SIGNAL(MethodInfo("on_request_permissions_result", PropertyInfo(Variant::STRING, "permission"), PropertyInfo(Variant::BOOL, "granted")));
 };
diff --git a/core/os/main_loop.h b/core/os/main_loop.h
index 90790a45a1..2c34cf193c 100644
--- a/core/os/main_loop.h
+++ b/core/os/main_loop.h
@@ -52,8 +52,10 @@ public:
 		NOTIFICATION_WM_ABOUT = 2011,
 		NOTIFICATION_CRASH = 2012,
 		NOTIFICATION_OS_IME_UPDATE = 2013,
-		NOTIFICATION_APP_RESUMED = 2014,
-		NOTIFICATION_APP_PAUSED = 2015,
+		NOTIFICATION_APPLICATION_RESUMED = 2014,
+		NOTIFICATION_APPLICATION_PAUSED = 2015,
+		NOTIFICATION_APPLICATION_FOCUS_IN = 2016,
+		NOTIFICATION_APPLICATION_FOCUS_OUT = 2017,
 	};
 
 	virtual void init();
diff --git a/core/os/os.cpp b/core/os/os.cpp
index c842be333c..231069fcfb 100644
--- a/core/os/os.cpp
+++ b/core/os/os.cpp
@@ -41,6 +41,7 @@
 #include <stdarg.h>
 
 OS *OS::singleton = nullptr;
+uint64_t OS::target_ticks = 0;
 
 OS *OS::get_singleton() {
 	return singleton;
@@ -468,6 +469,41 @@ void OS::close_midi_inputs() {
 	}
 }
 
+void OS::add_frame_delay(bool p_can_draw) {
+	const uint32_t frame_delay = Engine::get_singleton()->get_frame_delay();
+	if (frame_delay) {
+		// Add fixed frame delay to decrease CPU/GPU usage. This doesn't take
+		// the actual frame time into account.
+		// Due to the high fluctuation of the actual sleep duration, it's not recommended
+		// to use this as a FPS limiter.
+		delay_usec(frame_delay * 1000);
+	}
+
+	// Add a dynamic frame delay to decrease CPU/GPU usage. This takes the
+	// previous frame time into account for a smoother result.
+	uint64_t dynamic_delay = 0;
+	if (is_in_low_processor_usage_mode() || !p_can_draw) {
+		dynamic_delay = get_low_processor_usage_mode_sleep_usec();
+	}
+	const int target_fps = Engine::get_singleton()->get_target_fps();
+	if (target_fps > 0 && !Engine::get_singleton()->is_editor_hint()) {
+		// Override the low processor usage mode sleep delay if the target FPS is lower.
+		dynamic_delay = MAX(dynamic_delay, (uint64_t)(1000000 / target_fps));
+	}
+
+	if (dynamic_delay > 0) {
+		target_ticks += dynamic_delay;
+		uint64_t current_ticks = get_ticks_usec();
+
+		if (current_ticks < target_ticks) {
+			delay_usec(target_ticks - current_ticks);
+		}
+
+		current_ticks = get_ticks_usec();
+		target_ticks = MIN(MAX(target_ticks, current_ticks - dynamic_delay), current_ticks + dynamic_delay);
+	}
+}
+
 OS::OS() {
 	void *volatile stack_bottom;
 
diff --git a/core/os/os.h b/core/os/os.h
index 04e10518dc..f21c0d4df7 100644
--- a/core/os/os.h
+++ b/core/os/os.h
@@ -43,6 +43,7 @@
 
 class OS {
 	static OS *singleton;
+	static uint64_t target_ticks;
 	String _execpath;
 	List<String> _cmdline;
 	bool _keep_screen_on = true; // set default value to true, because this had been true before godot 2.0.
@@ -212,6 +213,8 @@ public:
 	virtual double get_unix_time() const;
 
 	virtual void delay_usec(uint32_t p_usec) const = 0;
+	virtual void add_frame_delay(bool p_can_draw);
+
 	virtual uint64_t get_ticks_usec() const = 0;
 	uint32_t get_ticks_msec() const;
 	uint64_t get_splash_tick_msec() const;