JS执行耗时任务Loading效果

//让执行耗时任务时UI可以更新
	function loading_renderAll() { 
		//显示加载中
		startLoading();
		doSomethingTimeConsumingThen(function () {
		  	// 关闭加载中
		  	alert("关闭加载中");
			stopLoading();
		});
	}
	function doSomethingTimeConsumingThen (onDone) {
	  setTimeout(onDone, 1);

	}
loading_renderAll()