这是创建一个在两端绘制箭头的线对象的方法
有趣的部分是计算箭头的角度，如下所示：
var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
startRadians+=((this.x2>=this.x1)?-90:90)*Math.PI/180;
var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
endRadians+=((this.x2>=this.x1)?90:-90)*Math.PI/180;
剩下的只是画一条线和 2 个三角形作为箭头来计算旋转
Line.prototype.drawArrowhead=function(ctx,x,y,radians){
ctx.save();
ctx.beginPath();
ctx.translate(x,y);
ctx.rotate(radians);
ctx.moveTo(0,0);
ctx.lineTo(5,20);
ctx.lineTo(-5,20);
ctx.closePath();
ctx.restore();
ctx.fill();
}
这是代码和小提琴：http://jsfiddle.net/m1erickson/Sg7EZ/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var context=canvas.getContext("2d");
function Line(x1,y1,x2,y2){
this.x1=x1;
this.y1=y1;
this.x2=x2;
this.y2=y2;
}
Line.prototype.drawWithArrowheads=function(ctx){
// arbitrary styling
ctx.strokeStyle="blue";
ctx.fillStyle="blue";
ctx.lineWidth=1;
// draw the line
ctx.beginPath();
ctx.moveTo(this.x1,this.y1);
ctx.lineTo(this.x2,this.y2);
ctx.stroke();
// draw the starting arrowhead
var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
startRadians+=((this.x2>this.x1)?-90:90)*Math.PI/180;
this.drawArrowhead(ctx,this.x1,this.y1,startRadians);
// draw the ending arrowhead
var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
endRadians+=((this.x2>this.x1)?90:-90)*Math.PI/180;
this.drawArrowhead(ctx,this.x2,this.y2,endRadians);
}
Line.prototype.drawArrowhead=function(ctx,x,y,radians){
ctx.save();
ctx.beginPath();
ctx.translate(x,y);
ctx.rotate(radians);
ctx.moveTo(0,0);
ctx.lineTo(5,20);
ctx.lineTo(-5,20);
ctx.closePath();
ctx.restore();
ctx.fill();
}
// create a new line object
var line=new Line(50,50,150,150);
// draw the line
line.drawWithArrowheads(context);
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
