## 使用canvas畫布利用js通過鼠標實現矩形的繪制（任意方向的繪制圖形）(如果要繪制其他圖形，做一點小改動就行了)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style type="text/css">
#cav{
position: absolute;
border:1px solid black;
}
</style>
</head>
<body>
<canvas id='cav' width="500" height="400" style='background-color: #F0F8FF'></canvas>
<script>
function MyPaint(id,color='red'){
var canvas = document.getElementById(id);
this.canvas = canvas;
this.color = color;
this.ctx = this.canvas.getContext('2d');
this.p1= {};
this.p2 = {};
}
MyPaint.prototype.paintRect = function(){
var myPaint = this;
this.canvas.addEventListener('mousedown',function(e){
console.info('mousedown',this);
this.p1.x = e.offsetX;
this.p1.y = e.offsetY;
this.canvas.addEventListener("mousemove",myDrect);
this.canvas.addEventListener("mouseup",function(){
console.info("mouseup");
console.info(myDrect);
this.removeEventListener("mousemove",myDrect);
})
}.bind(this));
function myDrect(e){
myPaint.p2.x = e.offsetX;
myPaint.p2.y = e.offsetY;
myPaint.ctx.width = myPaint.width;
myPaint.ctx.height = myPaint.height;
myPaint.ctx.fillStyle = '#FF0000';
myPaint.ctx.strokeStyle = '#FF0000';
var width = Math.abs(myPaint.p1.x-myPaint.p2.x);
var height = Math.abs(myPaint.p1.y-myPaint.p2.y);
myPaint.ctx.clearRect(0,0,myPaint.canvas.width,myPaint.canvas.height);
myPaint.ctx.beginPath();
if(myPaint.p2.x>=myPaint.p1.x){
if(myPaint.p2.y>=myPaint.p1.y){
myPaint.ctx.rect(myPaint.p1.x,myPaint.p1.y,width,height);
}else{
myPaint.ctx.rect(myPaint.p1.x,myPaint.p1.y,width,-height);
}
}else{
if(myPaint.p2.y>=myPaint.p1.y){
myPaint.ctx.rect(myPaint.p1.x,myPaint.p1.y,-width,height);
}else{
myPaint.ctx.rect(myPaint.p1.x,myPaint.p1.y,-width,-height);
}
}
myPaint.ctx.stroke();
myPaint.ctx.save();
}
}
var mp = new MyPaint('cav');
mp.paintRect();
</script>
</body>
</html>