angualr 加载速度慢，为页面初始化完成前添加loading

1、在项目根目录下index.html添加loading模板：

 <div class="loading-box" id="loading">
      <div class="mark"></div>
      <img width="20" height="20" src="assets/images/loading.gif">
  </div>

2.添加js，初始化完成前显示loading：

 <script type="text/javascript">
    const initialize = function () {
        let loading = document.getElementById("loading");
        loading.style.display = 'block';
        document.onreadystatechange = function () {
            if (document.readyState == "complete") {
                loading.style.display = 'none';
            }
        }
    }
    initialize();
</script>