为什么会一直显示正在加载ajax图像？

我是ajax新手，已经学了一些w3 schools在线课程。现在我已经可以在网站上加载内容了，但是当加载发生时，只会向用户显示一个加载图像。我在网上搜了很久也发现了一些相同的问题，但他们使用的是其他类型的Ajax，这是我所不知道的。

我的代码：

<script>
 function loadXMLDoc(a)
  {
     var xmlhttp;
     var temp = "myname.php?id=" + a;


     if (window.XMLHttpRequest)
        {// code for IE7+, Firefox, Chrome, Opera, Safari
             xmlhttp=new XMLHttpRequest();
        }
     else
        {// code for IE6, IE5
           xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
        }
           xmlhttp.onreadystatechange=function()
          {
          if (xmlhttp.readyState==4 && xmlhttp.status==200)
            {
                 document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
            }
          }
   xmlhttp.open("GET",temp,true);
   xmlhttp.send();
 }
</script>



<h2>AJAX</h2>
  <button type="button" onclick="loadXMLDoc('1')">Request 1</button>
  <button type="button" onclick="loadXMLDoc('2')">Request 2</button>
  <button type="button" onclick="loadXMLDoc('3')">Request 3</button>
<div id="myDiv"></div>

如果有人可以根据我的编码给我一个例子或链接，将不胜感激。

Create an <img> element that is hidden to begin with, then when the ajax is called, show it, and when completed, hide it.

<script>
 function loadXMLDoc(a)
  {
     var xmlhttp;
     var temp = "myname.php?id=" + a;

     // show the loading image
     document.getElementById("loading").style.display = "";

     if (window.XMLHttpRequest)
        {// code for IE7+, Firefox, Chrome, Opera, Safari
             xmlhttp=new XMLHttpRequest();
        }
     else
        {// code for IE6, IE5
           xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
        }
           xmlhttp.onreadystatechange=function()
          {
          if (xmlhttp.readyState==4 && xmlhttp.status==200)
            {
                 // hide the loading image
                 document.getElementById("loading").style.display = "none";

                 document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
            }
          }
   xmlhttp.open("GET",temp,true);
   xmlhttp.send();
 }
</script>



<h2>AJAX</h2>

<img src="loading.gif" alt="Loading..." id="loading" style="display:none;" />

  <button type="button" onclick="loadXMLDoc('1')">Request 1</button>
  <button type="button" onclick="loadXMLDoc('2')">Request 2</button>
  <button type="button" onclick="loadXMLDoc('3')">Request 3</button>
<div id="myDiv"></div>

For future reference, the Mozilla Development Network docs are a good resource for plain Javascript ajax.

1) add a loading animation image (img tag) in the HTML document and place it where you want to display, but you hide it with CSS by setting display: none

2) before your xmlhttp.send(), show the image by setting the image display: ''

3) in the xmlhttp.onreadystatechange callback function, hide the image again to indicate the loading is finished.