RequestAnimationFrame in Processing.js

In the weeks since the Github Game Off, I have been working on making my game, PolyBranch, playable in mobile browsers. A huge part of this has been optimization. PolyBranch was written using slightly non-conventional, although hugely beneficial methods in that the core game was written in Processing (Java) and then exported to JavaScript via Processing.js from the Processing 2.0 IDE. This is the standard way of exporting your Processing sketches to the web, however, in addition to this I followed Pomax's guide to bind my Processing.js code and JavaScript code to each other, thus allowing myself to make calls to Processing with the site's JavaScript. This is how the majority of the event binding in the game was done, including the entire menu and any GUI elements. The big advantage of this is being able to write your GUI in straight HTML5. Being designed for making interfaces (websites), it is much easier to write your GUI this way than directly in Processing!

Now, back to my original point, what makes this so important is optimization. When developing HTML5 canvas for mobile, it has been widely accepted that JavaScripts requestAnimationFrame() function offers huge advantages over traditional JS timing functions like setInterval() or setTimeout (see Paul Irish's post for more info as well as the polyfill). Unfortunately, because rAF wasn't around when PJS was first written, it uses setInterval() instead. The good people on the PJS team are aware of this and hard at work on a fix, but just isn't there yet. So, because of this, I found a work around! Because I had already bound PJS and JS together so I could do the interface, I also handed control of the looping to JS entirely! This way, I was able to write a rAF loop which calls pjs.draw(), as I will demonstrate below:

First, binding JavaScript and Processing.js together (see Pomax's guide for more details)

	//at the top of your .pde or .pjs
	void bindJavascript(JavaScript js) {
	javascript = js;
	}
	JavaScript javascript;

view raw sketch.pjs hosted with ❤ by GitHub

	<!-- Insert this script after processing.js is included -->
	<script type="text/javascript">
	var bound = false;
	var pjs;
	function bindJavascript() {
	//reference the ID of your canvas that will be rendering your processing sketch
	pjs = Processing.getInstanceById('polybranch');
	if(pjs!=null) {
	pjs.bindJavascript(this);
	bound = true;
	}
	if(!bound) setTimeout(bindJavascript, 250);
	}
	bindJavascript();
	</script>

view raw bind_pjs.html hosted with ❤ by GitHub

I have had better luck with renaming my .pde file to a .pjs file at this point. This is a practice I have copied from examples around the web, although I do not know why it works. If anyone has an explanation, I would love to know! :)

Now that Processing and JavaScript can communicate with each other, we can use rAF to call Processing's draw function!

	var playing = true;
	//call this function to start the loop
	function startRaf(){
	rafGo();
	}
	function rafGo() {
	//loop stops when playing is set to false
	if(playing){
	//be sure to include Paul Irish's rAF shim!
	requestAnimationFrame(rafGo);
	pjs.draw();
	}
	}
	startRaf();

view raw pjs_raf.js hosted with ❤ by GitHub

This is a very bare-bones demonstration of the draw loop. Be sure to check out Kushagra Agarwal's article on time based animations for best results!

As a disclaimer, I should note that I had moved over primarily to JavaScript development at this point in the process and was using this method so I could retain the logic I had written in Processing. I was no longer working within the Processing IDE! If you are using Processing.js as a JavaScript library, you can also check out this discussion from the PJS google group.