canvas 绘制跟随鼠标移动的线条

坦克大作战游戏中常见，点击某点，坦克的枪口会自动对准被点击的点！

window.onload = function() {
var canvas = document.getElementById("canvas");
var obj = canvas.getContext('2d');

obj.moveTo( 300, 500);
obj.lineTo( 300, 480);
obj.lineWidth = 6;
obj.strokeStyle = "#ff0000";
obj.stroke();

canvas.onmousedown = function( e ){
var mx = e.layerX;
var my = e.layerY;

var sx = 300;
var sy = 500;

var angle = Math.atan2( ( my - sy), ( mx - sx ) )

obj.clearRect(0, 0, 600, 600);
obj.beginPath();
obj.moveTo( sx, sy );
obj.lineTo( sx + 20 * Math.cos( angle ), sy + 20 * Math.sin(angle));
obj.stroke();
};
};