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();};};