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Move objects around the <canvas> using the keyboard and jQuery

August 12th, 2010

One of the things that Flash is great at is making games and it’s one of the things that people are hyping the <canvas> tag being able to take over. Personally, I think it’s going to be a couple of years before that happens and one of the reasons is how much easier it is to build games using Flash. In fact, I was trying to figure out how to allow a user to move a block around a <canvas> using the arrow keys and I was getting pretty frustrated. Until I realized I could just use jQuery.

I’m not going to go through the whole thing because it’s just the next step from an earlier <canvas> post I did. But I’ll go over the changes and the great thing is, there isn’t too many. Plus, if you’ve ever made something similar in Flash, this will seem pretty familiar.

The HTML:

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<body onload="init();">
<canvas id="canvas" width="500" height="400"></canvas>
</body>

Just setting the <canvas> height and width and setting the init() function to fire after the page loads.

The CSS:

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canvas {
  display:block;
  margin:auto;
  border:1px dashed black;
}

Nothing interesting here, a dashed border so we can see the edges of the <canvas> and the margin: auto to center it.

The JavaScript:

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var context;
var rightKey = false;
var leftKey = false;
var upKey = false;
var downKey = false;
var block_x;
var block_y;
var block_h = 30;
var block_w = 30;

function init() {
  context = $('#canvas')[0].getContext('2d');
  WIDTH = $('#canvas').width();
  HEIGHT = $('#canvas').height();
  block_x = WIDTH / 2 - 15;
  block_y = HEIGHT /2 - 15;
  setInterval('draw()', 25);
}
function clearCanvas() {
  context.clearRect(0,0,WIDTH,HEIGHT);
}

function draw() {
  clearCanvas();
  if (rightKey) block_x += 5;
  else if (leftKey) block_x -= 5;
  if (upKey) block_y -= 5;
  else if (downKey) block_y += 5;
  if (block_x <= 0) block_x = 0;
  if ((block_x + block_w) >= WIDTH) block_x = WIDTH - block_w;
  if (block_y <= 0) block_y = 0;
  if ((block_y + block_h) >= HEIGHT) block_y = HEIGHT - block_h;
  context.fillRect(block_x,block_y,block_w,block_h);
}

function onKeyDown(evt) {
  if (evt.keyCode == 39) rightKey = true;
  else if (evt.keyCode == 37) leftKey = true;
  if (evt.keyCode == 38) upKey = true;
  else if (evt.keyCode == 40) downKey = true;
}

function onKeyUp(evt) {
  if (evt.keyCode == 39) rightKey = false;
  else if (evt.keyCode == 37) leftKey = false;
  if (evt.keyCode == 38) upKey = false;
  else if (evt.keyCode == 40) downKey = false;
}

$(document).keydown(onKeyDown);
$(document).keyup(onKeyUp);

So the first thing we do here is find the <canvas> using jQuery and get the height and width:

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context = $('#canvas')[0].getContext('2d');
WIDTH = $('#canvas').width();
HEIGHT = $('#canvas').height();

And instead of drawing a ball and animating it bouncing, we use the arrow keys to move it around

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function draw() {
  clearCanvas();
  if (rightKey) block_x += 5;
  else if (leftKey) block_x -= 5;
  if (upKey) block_y -= 5;
  else if (downKey) block_y += 5;
  if (block_x <= 0) block_x = 0;
  if ((block_x + block_w) >= WIDTH) block_x = WIDTH - block_w;
  if (block_y <= 0) block_y = 0;
  if ((block_y + block_h) >= HEIGHT) block_y = HEIGHT - block_h;
  context.fillRect(block_x,block_y,block_w,block_h);
}

If you’ve ever made a game with Flash or something similar, you should be able to see what’s going on here, we’re just saying, if the this key is pressed then move it this direction. Also, it checks to see if the block is at the edge, then don’t let it go any farther in that direction.

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function onKeyDown(evt) {
  if (evt.keyCode == 39) rightKey = true;
  else if (evt.keyCode == 37) leftKey = true;
  if (evt.keyCode == 38) upKey = true;
  else if (evt.keyCode == 40) downKey = true;
}

function onKeyUp(evt) {
  if (evt.keyCode == 39) rightKey = false;
  else if (evt.keyCode == 37) leftKey = false;
  if (evt.keyCode == 38) upKey = false;
  else if (evt.keyCode == 40) downKey = false;
}

With these two functions, we’re just checking if the one of the arrow keys has been pressed, then we change the corresponding variable to true and it will move. Likewise, we then check to see if it’s been released and then the variable will become false and it will stop moving.

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$(document).keydown(onKeyDown);
$(document).keyup(onKeyUp);

Finally, we use the jQuery events keydown() and keyup() to simplify the process of checking if one of the arrow keys are being pressed.

And here’s a demo of what we get.

It’s nothing amazing right now, but this is the first step towards developing a game using <canvas>. Hopefully, as the technology evolves, we get events and methods like AS3’s hitTest to make game development a lot easier. As I’ve said before, right now the <canvas> is fun to fool around with, but I think we’re still a few years and an pretty good IDE away from it being something the average developer can really work with.

2 Responses to Move objects around the <canvas> using the keyboard and jQuery

  1. Mehul says:

    can u give me example of only selected image and text move from multiple images and text on keyup and keydown event……

  2. Thanks from Spain dude!
    You helped me a lot, I was having some problems with keyboard lag (when I pressed a key, my character started moving, then stopped for 1 second, and then it start moving normaly) but with this I could solve the problem!

    http://www.fixitmedia.es/trabajos/canvas

    You can see my example here, I will improve it with some new features

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