本来想的挺复杂实际操作了一下15分钟完成了，挺简单的。
预览地址：http://dtdxrk.github.io/game/blackboard/index.html
分享一下思路：
1.创建画布
2.添加按钮
3.设置事件
没啥好说的就这样吧。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>blackboard</title>
<style>
*{margin: 0;padding: 0;}
#wapper{margin:50px auto 0;display: block;}
#wapper:hover{cursor: pointer;}
.btn-list{ 810px;margin: 5px auto;}
.btn{padding:5px 10px;margin-right: 10px;}
</style>
</head>
<body>
<canvas id="wapper" width="800" height="400"></canvas>
<!-- btn -->
<div class="btn-list">chalkColor : <button class="btn" id="btn-white">white</button><button class="btn" id="btn-pink">pink</button><button class="btn" id="btn-green">green</button></div>
<div class="btn-list">lineWidth : <button class="btn" id="btn-5">5</button><button class="btn" id="btn-10">10</button><button class="btn" id="btn-15">15</button></div>
<div class="btn-list">clearAll : <button class="btn" id="btn-clearall">clearAll</button></div>
<!-- btn end-->
<script>
var blackboard = {
color:'#fff',
lineWidth:5,
isDrawing:false,
x:0,
y:0,
init:function(){
this.wapper = document.getElementById('wapper');
this._2d = this.wapper.getContext('2d');
this.clear_blackboard();
this.init_font();
this.bind_btnEvent();
},
clear_blackboard:function(){
/*绘制背景颜色*/
this._2d.beginPath();
this._2d.fillStyle = '#000';/*设置颜色*/
this._2d.fillRect(0,0,800,600);/*fillRect(起点x，起点y，矩形长，矩形宽)*/
},
init_font:function(){
this._2d.beginPath();
this._2d.font='100px Arial';
this._2d.fillStyle = '#fff';
this._2d.textAlign = 'center';
this._2d.fillText('blackboard',400,200);/*strokeText(text,x,y,max width)*/
},
bind_btnEvent:function(){
var that = this;
that.wapper.onmouseout = function(e){
that.isDrawing = false;
}
that.wapper.onmousemove = function(e){
if(that.isDrawing){
that.drawing(e.offsetX, e.offsetY);
that.x = e.offsetX;
that.y = e.offsetY;
}
}
that.wapper.onmouseup = function(e){
that.isDrawing = false;
return false;
}
that.wapper.onmousedown = function(e){
that.x = e.offsetX;
that.y = e.offsetY;
that.isDrawing = true;
return false;
}
/*btn-clearall*/
document.getElementById('btn-clearall').onclick = function(){
that.clear_blackboard();
}
document.getElementById('btn-white').onclick = function(){
that.color = 'white';
}
document.getElementById('btn-pink').onclick = function(){
that.color = 'pink';
}
document.getElementById('btn-green').onclick = function(){
that.color = 'green';
}
document.getElementById('btn-5').onclick = function(){
that.lineWidth = 5;
}
document.getElementById('btn-10').onclick = function(){
that.lineWidth = 10;
}
document.getElementById('btn-15').onclick = function(){
that.lineWidth = 15;
}
},
drawing:function(x,y){
this._2d.beginPath();/*创建新的路径*/
this._2d.lineWidth = this.lineWidth;/*设置线条宽度*/
this._2d.strokeStyle = this.color;/*设置线条颜色*/
this._2d.lineCap = 'round';/*向线条的每个末端添加圆形线帽。*/
this._2d.moveTo(this.x, this.y);/*将画笔光标移动到画布坐标10，10*/
this._2d.lineTo(x,y);/*画一条线*/
this._2d.stroke();/*绘制定义的路径*/
}
}
blackboard.init();
</script>
</body>
</html>