如何在页面加载时显示运行进度条

我想在页面加载时显示运行进度条像这儿，在我的页面中。 我在示例中使用了一个简单的加载图像，但我想在运行进度栏中将其转换。这是我的代码：

$(window).load(function() {
    alert("hi");
    $('#loading').hide();
});

#loading {
    width: 100%;
    height: 100%;
    top: 0px;
    left: 0px;
    position: fixed;
    display: block;
    opacity: 0.7;
    background-color: #fff;
    z-index: 99;
    text-align: center;
}

#loading-image {
    position: absolute;
    top: 100px;
    left: 240px;
    z-index: 100;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.0/jquery.min.js"></script>
<div id="loading">
    <img id="loading-image" src="http://cdn.nirmaltv.com/images/generatorphp-thumb.gif" alt="Loading..." />
</div>
<div id="txt">
    <h2>Let AJAX change this text</h2>
</div>
<button>Change Content</button>

这里有一个JSFiddle

我已从下面复制了相关代码这一页。希望这可以帮助你。

$.ajax({
  xhr: function() {
    var xhr = new window.XMLHttpRequest();
    //Upload progress
    xhr.upload.addEventListener("progress", function(evt) {
      if (evt.lengthComputable) {
        var percentComplete = evt.loaded / evt.total;
        //Do something with upload progress
        console.log(percentComplete);
      }
    }, false);
    //Download progress
    xhr.addEventListener("progress", function(evt) {
      if (evt.lengthComputable) {
        var percentComplete = evt.loaded / evt.total;
        //Do something with download progress
        console.log(percentComplete);
      }
    }, false);
    return xhr;
  },
  type: 'POST',
  url: "/",
  data: {},
  success: function(data) {
    //Do something success-ish
  }
});