借助 Promise判断多张图片是否加载完成

var urls = ["https://sina-static.oss-cn-beijing.aliyuncs.com/images/slide/300250_1.jpeg", "https://sina-static.oss-cn-beijing.aliyuncs.com/images/slide/300250_2.jpeg"];
let promiseAll = [],
   imgs = [],
   total = urls.length;
for (let i = 0; i < total; i++) {
   promiseAll[i] = new Promise((resolve, reject) => {
       imgs[i] = new Image();
       imgs[i].src = urls[i];
       imgs[i].onload = function() {
           resolve(imgs[i]);
       };
   })
}
Promise.all(promiseAll).then(img => {
   metaRefer(data, n);
})
//动态创建meta refresh 标签
function metaRefer(data, n) {
	let meta = document.createElement('meta');
	meta.content = "刷新时间";
	meta.httpEquiv = "Refresh";
	document.getElementsByTagName('head')[0].appendChild(meta);
	n++;
	window.parent.postMessage(n, '*');
}