ui: Introduce IsRunningSlowly (#168)

examples/blocks/blocks/game.go

@@ -46,6 +46,8 @@ func (game *Game) Update(r *ebiten.Image) error {
 		SceneManager: game.sceneManager,
 		Input:        &game.input,
 	})
+	if !ebiten.IsRunningSlowly() {
 		game.sceneManager.Draw(r)
+	}
 	return nil
 }

graphicscontext.go

@@ -39,6 +39,9 @@ func (c *graphicsContext) update(f func(*Image) error) error {
 	if err := f(c.screen); err != nil {
 		return err
 	}
+	if IsRunningSlowly() {
+		return nil
+	}
 	// TODO: In WebGL, we don't need to clear the image here.
 	if err := c.defaultRenderTarget.Clear(); err != nil {
 		return err

run.go

@@ -26,6 +26,7 @@ var runContext = &struct {
 	newScreenWidth  int
 	newScreenHeight int
 	newScreenScale  int
+	isRunningSlowly bool
 }{}
 
 // CurrentFPS returns the current number of frames per second.
@@ -33,6 +34,13 @@ func CurrentFPS() float64 {
 	return runContext.fps
 }
 
+// IsRunningSlowly returns true if the game is running too slowly to keep 60 FPS.
+// The game screen is not updated when IsRunningSlowly is true.
+// It is recommended to skip heavy processing, especially drawing, when IsRunningSlowly is true.
+func IsRunningSlowly() bool {
+	return runContext.isRunningSlowly
+}
+
 // Run runs the game.
 // f is a function which is called at every frame.
 // The argument (*Image) is the render target that represents the screen.
@@ -94,10 +102,12 @@ func Run(f func(*Image) error, width, height, scale int, title string) error {
 		}
 		now := ui.Now()
 		// If beforeForUpdate is too old, we assume that screen is not shown.
+		runContext.isRunningSlowly = false
 		if int64(5*time.Second/60) < now-beforeForUpdate {
 			beforeForUpdate = now
 		} else {
 			c := int((now - beforeForUpdate) * 60 / int64(time.Second))
+			runContext.isRunningSlowly = c >= 2
 			for i := 0; i < c; i++ {
 				if err := graphicsContext.update(f); err != nil {
 					return err