写字板事例：
写字板分析：1.点击鼠标開始写字（onmosedown）2.按下鼠标写字（onmousemove）3.松开鼠标，停下写字（撤销onmousemove事件）：
代码:
<strong><!doctype html>
</strong><html>
<head>
<meta charset="utf-8">
<title>Canvas</title>
<style type="text/css">
body {
margin: 0px;
padding: 0px;
}
#canvas {
margin: 0px;
padding: 0px;
border: 1px solid #000000;
}
</style>
</head>
<body>
<canvas id="canvas" width="400" height="400">当前浏览器不支持canvas</canvas>
<script type="text/javascript" src="utils.js"></script>
<script type="text/javascript" src="arrow.js"></script>
<script type="text/javascript">
window.onload=function(){
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
/*var mouse=utils.captureMousePosition(canvas); */
//加入mousedown监听事件。在用户鼠标按下后開始画线。并注冊mousemove事件
canvas.addEventListener("mousedown",function(event){
context.beginPath();
context.moveTo(event.clientX, event.clientY);
//alert("-------------")
canvas.addEventListener("mousemove",onMouseMove,false);
},false);
//在鼠标移动时划线
function onMouseMove(event){
context.lineTo(event.clientX,event.clientY);
context.stroke();
}
//在鼠标按键松开后，注销鼠标移动事件
canvas.addEventListener("mouseup",function(event){
canvas.removeEventListener("mousemove",onMouseMove,false);
},false);
}
</script>
</body>
</html>