html5在使用drawImage绘图的时候，出现刷新就不显示的问题，原来要想是图片正常显示需要为图片添加一个onload事件，直接看代码
<!DOCTYPE HTML>
<html>
<head>
<meta content="text/html" charset="utf8">
</head>
<body>
<canvas id="myCanvas4" width="200" height="100" style="border:1px solid #c3c3c3;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c=document.getElementById("myCanvas4");
var cxt=c.getContext("2d");
var img=new Image();
img.src="./images/50ab2f61403bf44f.png";
img.onload=function(){
cxt.drawImage(img,0,0);
};
cxt.drawImage(img,0,0);
</script>
</body>
</html>