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example.html
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<!doctype html>
<html>
<head>
<meta charset="UTF-8" />
<title>Canvas Test</title>
</head>
<body>
<section>
<div>
<canvas id="canvas" width="300" height="200">
This text is displayed if your browser does not support HTML5 Canvas.
</canvas>
</div>
<script type="text/javascript">
var canvas;
var ctx;
var dx = 5;
var dy = 5;
var x = 150;
var y = 100;
var WIDTH = 300;
var HEIGHT = 200;
function circle(x,y,r) {
ctx.beginPath();
ctx.arc(x, y, r, 0, Math.PI*2, true);
ctx.fill();
}
function rect(x,y,w,h) {
ctx.beginPath();
ctx.rect(x,y,w,h);
ctx.closePath();
ctx.fill();
ctx.stroke();
}
function clear() {
ctx.clearRect(0, 0, WIDTH, HEIGHT);
}
function init() {
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
return setInterval(draw, 10);
}
function doKeyDown(evt){
switch (evt.keyCode) {
case 38: /* Up arrow was pressed */
if (y - dy > 0){
y -= dy;
}
break;
case 40: /* Down arrow was pressed */
if (y + dy < HEIGHT){
y += dy;
}
break;
case 37: /* Left arrow was pressed */
if (x - dx > 0){
x -= dx;
}
break;
case 39: /* Right arrow was pressed */
if (x + dx < WIDTH){
x += dx;
}
break;
}
}
function draw() {
clear();
ctx.fillStyle = "white";
ctx.strokeStyle = "black";
rect(0,0,WIDTH,HEIGHT);
ctx.fillStyle = "purple";
circle(x, y, 10);
}
init();
window.addEventListener('keydown',doKeyDown,true);
</script>
</section>
</body>
</html>