canvas返回后重新进入页面面，不能正常绘制，而且设置的宽高都不生效             var canvasList = document.getElementById('canvasDiv');             canvas = document.createElement('Canvas');             canvasList.appendChild(canvas)             canvas.height = document.documentElement.clientHeight/2;             canvas.width = document.documentElement.clientWidth-4;             canvasTop = canvas.offsetTop + 2;             board = canvas.getContext('2d');             board.lineWidth = 2; //设置画笔粗细             board.strokeStyle = "#000";             board.lineJoin = "round"; //设置画笔轨迹基于圆点拼接