动态调整canvas大小时必须重新获取上下文（ canvas.getContext(‘2d’) ）及重新设置ctx的属性，否则会导致后续的绘制操作无效

动态调整canvas:

var canvas = document.getElementsByTagName("canvas")[0];var ctx = canvas.getContext('2d');window.onresize = function () {console.log("窗口大小改变")resizeCanvas();
}function resizeCanvas() {canvas.width = window.innerWidth;canvas.height = window.innerHeight;//每次修改canvas大小后需要重新获取canvas上下文，否则会导致绘画无效ctx = canvas.getContext('2d');
}