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POSlow Javascript touch events on Android
 

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    <p>I'm trying to write a simple html based drawing application (standalone simplified code attached bellow). I've tested this on the following devices:</p> <ul> <li><strong>iPad 1 and 2</strong>: Works great</li> <li><strong>ASUS T101 running Windows</strong>: Works great</li> <li><strong>Samsung Galaxy Tab</strong>: Extremely slow and patchy -- unusable.</li> <li><strong>Lenovo IdeaPad K1</strong>: Extremely slow and patchy -- unusable.</li> <li><strong>Asus Transformer Prime</strong>: Noticeable lag compare with the iPad -- close to usable.</li> </ul> <p>The Asus tablet is running ICS, the other android tablets are running 3.1 and 3.2. I tested using the stock Android browser. I also tried the Android Chrome Beta, but that was even worse.</p> <p>Here's a video which demonstrates the issue: <a href="http://www.youtube.com/watch?v=Wlh94FBNVEQ" rel="noreferrer">http://www.youtube.com/watch?v=Wlh94FBNVEQ</a></p> <p>My questions is why are the Android tablets so slow? Am I doing something wrong or is it an inherit problem with Android OS or browser, or is there anything I can do about it in my code?</p> <p>multi.html:</p> <pre><code>&lt;html&gt; &lt;body&gt; &lt;style media="screen"&gt; canvas { border: 1px solid #CCC; } &lt;/style&gt; &lt;canvas style="" id="draw" height="450" width="922"&gt;&lt;/canvas&gt; &lt;script class="jsbin" src="jquery.js"&gt;&lt;/script&gt; &lt;script src="multi.js"&gt;&lt;/script&gt; &lt;/body&gt; &lt;/html&gt; </code></pre> <p>multi.js:</p> <pre><code>var CanvasDrawr = function(options) { // grab canvas element var canvas = document.getElementById(options.id), ctxt = canvas.getContext("2d"); canvas.style.width = '100%' canvas.width = canvas.offsetWidth; canvas.style.width = ''; // set props from options, but the defaults are for the cool kids ctxt.lineWidth = options.size || Math.ceil(Math.random() * 35); ctxt.lineCap = options.lineCap || "round"; ctxt.pX = undefined; ctxt.pY = undefined; var lines = [,,]; var offset = $(canvas).offset(); var eventCount = 0; var self = { // Bind click events init: function() { // Set pX and pY from first click canvas.addEventListener('touchstart', self.preDraw, false); canvas.addEventListener('touchmove', self.draw, false); }, preDraw: function(event) { $.each(event.touches, function(i, touch) { var id = touch.identifier; lines[id] = { x : this.pageX - offset.left, y : this.pageY - offset.top, color : 'black' }; }); event.preventDefault(); }, draw: function(event) { var e = event, hmm = {}; eventCount += 1; $.each(event.touches, function(i, touch) { var id = touch.identifier, moveX = this.pageX - offset.left - lines[id].x, moveY = this.pageY - offset.top - lines[id].y; var ret = self.move(id, moveX, moveY); lines[id].x = ret.x; lines[id].y = ret.y; }); event.preventDefault(); }, move: function(i, changeX, changeY) { ctxt.strokeStyle = lines[i].color; ctxt.beginPath(); ctxt.moveTo(lines[i].x, lines[i].y); ctxt.lineTo(lines[i].x + changeX, lines[i].y + changeY); ctxt.stroke(); ctxt.closePath(); return { x: lines[i].x + changeX, y: lines[i].y + changeY }; }, }; return self.init(); }; $(function(){ var drawr = new CanvasDrawr({ id: "draw", size: 5 }); }); </code></pre>
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