Slow Javascript touch events on Android

Posted by oneself on Stack Overflow See other posts from Stack Overflow or by oneself
Published on 2012-03-29T16:24:22Z Indexed on 2012/03/30 17:29 UTC
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Filed under:

JavaScript

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android

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mobile

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touch

I'm trying to write a simple html based drawing application (standalone simplified code attached bellow). I've tested this on the following devices:

iPad 1 and 2: Works great
ASUS T101 running Windows: Works great
Samsung Galaxy Tab: Extremely slow and patchy -- unusable.
Lenovo IdeaPad K1: Extremely slow and patchy -- unusable.
Asus Transformer Prime: Noticeable lag compare with the iPad -- close to usable.

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.

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?

multi.html:

<html>
<body>

<style media="screen">
  canvas { border: 1px solid #CCC; }
</style>

<canvas style="" id="draw" height="450" width="922"></canvas>

<script class="jsbin" src="jquery.js"></script>
<script src="multi.js"></script>

</body>
</html>

multi.js:

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 });
});

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