js下判断 iframe 是否加载完成的完美方法

var iframe = document.createElement("iframe");
iframe.src = "http://sc.jb51.net";
if (iframe.attachEvent){
iframe.attachEvent("onload", function(){
alert("Local iframe is now loaded.");
});
} else {
iframe.onload = function(){
alert("Local iframe is now loaded.");
};
}
document.body.appendChild(iframe);