<html>
<head>
<meta charset="utf-8">
<title>HTML5 canvas 做画板画图 可以做电子白板</title>
<style type="text/css">
<!--
#container { position: relative;}
#imageView { border: 1px solid #000; }
--></style>
</head>
<body>
<div id="container">
<canvas id="imageView" width="800" height="500">
</canvas>
</div>
<script type="text/javascript">
var canvas;
var context;
var tool;
/**
* called on window load.
*/
if(window.addEventListener){
window.addEventListener('load',
init(),
false);
}
/**
* init.
*/
function init(){
/**
* find the canvas element.
*/
canvas = document.getElementById('imageView');
if(!canvas){
return;
}
if(!canvas.getContext){
return;
}
/**
* get the 2D canvas context.
*/
context = canvas.getContext('2d');
if(!context){
return;
}
/**
* pencil tool instance.
*/
tool = new tool_pencil();
/**
* attach the mousedown, mousemove and mouseup event listeners.
*/
canvas.addEventListener('mousedown', ev_canvas, false);
canvas.addEventListener('mousemove', ev_canvas, false);
canvas.addEventListener('mouseup', ev_canvas, false);
}
/**
* This painting tool
* works like a drawing pencil
* which tracks the mouse movements.
*
* @returns {tool_pencil}
*/
function tool_pencil(){
var tool = this;
this.started = false;
/**
* This is called when you start holding down the mouse button.
* This starts the pencil drawing.
*/
this.mousedown = function (ev){
/**
* when you click on the canvas and drag your mouse
* the cursor will changes to a text-selection cursor
* the "ev.preventDefault()" can prevent this.
*/
ev.preventDefault();
context.beginPath();
context.moveTo(ev._x,ev._y);
tool.started = true;
};
/**
* This is called every time you move the mouse.
* Obviously, it only draws if the tool.started state is set to true
*/
this.mousemove = function (ev){
if(tool.started){
context.lineTo(ev._x,ev._y);
context.stroke();
}
};
/**
* This is called when you release the mouse button.
*/
this.mouseup = function (ev) {
if(tool.started){
tool.mousemove(ev);
tool.started = false;
}
};
}
/**
* general-purpose event handler.
* determines the mouse position relative to the canvas element.
*
* @param ev
*/
function ev_canvas(ev){
var x,y;
if(ev.offsetX || ev.offsetY == 0){
ev._x = ev.offsetX;
ev._y = ev.offsetY;
}
/**
* call the event handler of the tool.
*/
var func = tool[ev.type];
if (func) {
func(ev);
}
}
</script>
</body>
</html>
原文地址：http://blog.csdn.net/graceup/article/details/43491137
