From 5408f4c2dd6c353e4e826c295a6a2af730b5871e Mon Sep 17 00:00:00 2001
From: Sotiri Bakagiannis <sotirios.bakagiannis@gmail.com>
Date: Fri, 29 Jan 2021 12:07:27 +0000
Subject: [PATCH] core/Window: implement getActualFrameRate, update
 monitorFramePeriod accordingly

---
 js/core/Window.js | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/js/core/Window.js b/js/core/Window.js
index 01c9959..931868b 100644
--- a/js/core/Window.js
+++ b/js/core/Window.js
@@ -41,7 +41,7 @@ export class Window extends PsychObject
 	 */
 	get monitorFramePeriod()
 	{
-		return this._monitorFramePeriod;
+		return 1.0 / this.getActualFrameRate();
 	}
 
 	constructor({
@@ -69,9 +69,6 @@ export class Window extends PsychObject
 		// setup PIXI:
 		this._setupPixi();
 
-		// monitor frame period:
-		this._monitorFramePeriod = 1.0 / this.getActualFrameRate();
-
 		this._frameCount = 0;
 
 		this._flipCallbacks = [];
@@ -145,14 +142,18 @@ export class Window extends PsychObject
 	 * @name module:core.Window#getActualFrameRate
 	 * @function
 	 * @public
-	 * @return {number} always returns 60.0 at the moment
-	 *
-	 * @todo estimate the actual frame rate.
+	 * @return {number} rAF based delta time based approximation, 60.0 by default
 	 */
 	getActualFrameRate()
 	{
-		// TODO
-		return 60.0;
+		// gets updated frame by frame
+		const lastDelta = this.psychoJS.scheduler._lastDelta;
+		const fpsMaybe = 1000 / lastDelta;
+		// NB: calling `Number.isFinite()` might skip the implicit to number conversion, but
+		// would also need polyfilling for IE
+		const fps = isFinite(fpsMaybe) ? fpsMaybe : 60.0;
+
+		return fps;
 	}