return;
}
// get 2D context of canvas and draw rectangel
tempContext = canvas.getContext(“2d”);
tempContext.fillStyle=”blue”;
x = canvas.width/2;
y = canvas.height/2;
tempContext.fillRect(x, y, 80, 40);
// key event – use DOM element as object
canvas.addEventListener(‘keydown’, doKeyDown, true);
canvas.focus();
// key event – use window as object
window.addEventListener(‘keydown’, doKeyDown, true);
// mouse event
canvas.addEventListener(“mousedown”, doMouseDown, false);
canvas.addEventListener(‘mousemove’, doMouseMove, false);
canvas.addEventListener(‘mouseup’, doMouseUp, false);
}
function getPointOnCanvas(canvas, x, y) {
var bbox = canvas.getBoundingClientRect();
return { x: x – bbox.left * (canvas.width / bbox.width),
y: y – bbox.top * (canvas.height / bbox.height)
};
}
function doKeyDown(e) {
var keyID = e.keyCode ? e.keyCode :e.which;
if(keyID === 38 || keyID === 87) { // up arrow and W
clearCanvas();
y = y – 10;
tempContext.fillRect(x, y, 80, 40);
e.preventDefault();
}
if(keyID === 39 || keyID === 68) { // right arrow and D
clearCanvas();
x = x + 10;
tempContext.fillRect(x, y, 80, 40);
e.preventDefault();
}
if(keyID === 40 || keyID === 83) { // down arrow and S
clearCanvas();
y = y + 10;
tempContext.fillRect(x, y, 80, 40);
e.preventDefault();
}
if(keyID === 37 || keyID === 65) { // left arrow and A
clearCanvas();
x = x – 10;
tempContext.fillRect(x, y, 80, 40);
e.preventDefault();
}
}
function clearCanvas() {
tempContext.clearRect(0, 0, 500, 500)
}
function doMouseDown(event) {
var x = event.pageX;
var y = event.pageY;
var canvas = event.target;
var loc = getPointOnCanvas(canvas, x, y);
console.log(“mouse down at point( x:” + loc.x + “, y:” + loc.y + “)”);
tempContext.beginPath();
tempContext.moveTo(loc.x, loc.y);
started = true;
}
function doMouseMove(event) {
var x = event.pageX;
var y = event.pageY;
var canvas = event.target;
var loc = getPointOnCanvas(canvas, x, y);
if (started) {
tempContext.lineTo(loc.x, loc.y);
tempContext.stroke();
}
}
function doMouseUp(event) {
console.log(“mouse up now”);
if (started) {
doMouseMove(event);
started = false;
}
}
HTML部分：
<body>
<h1>HTML Canvas Event Demo – By Gloomy Fish</h1>
<pre>Press W, A, S, D keys to move</pre>
<div id=”my_painter”>
<canvas id=”event_canvas” tabindex=”0″></canvas>
</div>
