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4.5 显示进度条
无一例外地，几乎每个应用都会时不时地调用一个长时间运行的事务。如果你关心系统的可用性，就要确保用户能很容易地看到系统的状态。如果是一个胖客户应用，对于长时间运行事务的问题，解决办法很简单：只需显示一个进度条，以便用户知道目前所处状况。不过，在Ajax之前，要在Web应用中做到这一点很不容易。本节将使你了解如何使用Ajax为Web应用建立进度条。
在代码清单4-9所示的例子中，再次在pollCallback()方法中使用了setTimeout()，从而每隔2秒调用一次服务器。在processResult()方法中，只是从服务器查找已完成比例（百分数）的第一位数字，从而得出要将进度条中的哪些进度块着色（灰色）。
代码清单4-9 progressBar.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD
HTML 4.01 Transitional//EN">
<html>
<head>
<title>Ajax
Progress Bar</title>
<script
type="text/javascript">
var xmlHttp;
var key;
var bar_color = 'gray';
var span_id = "block";
var clear = " "
function createXMLHttpRequest() {
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}
function go() {
createXMLHttpRequest();
checkDiv();
var url = "ProgressBarServlet?task=create";
var button = document.getElementById("go");
button.disabled = true;
xmlHttp.open("GET", url, true);
xmlHttp.onreadystatechange = goCallback;
xmlHttp.send(null);
}
function goCallback() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
setTimeout("pollServer()", 2000);
}
}
}
function pollServer() {
createXMLHttpRequest();
var url = "ProgressBarServlet?task=poll&key="
+ key;
xmlHttp.open("GET", url, true);
xmlHttp.onreadystatechange = pollCallback;
xmlHttp.send(null);
}
function pollCallback() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
var percent_complete =
xmlHttp.responseXML
.getElementsByTagName("percent")[0].firstChild.data;
var index = processResult(percent_complete);
for (var i = 1; i <= index; i++) {
var elem = document.getElementById("block" + i);
elem.innerHTML = clear;
elem.style.backgroundColor = bar_color;
var next_cell = i + 1;
if (next_cell > index && next_cell <= 9) {
document.getElementById("block" + next_cell)
.innerHTML =
percent_complete + "%";
}
}
if (index < 9) {
setTimeout("pollServer()", 2000);
} else {
document.getElementById("complete").innerHTML
= "Complete!";
document.getElementById("go").disabled = false;
}
}
}
}
function processResult(percent_complete) {
var ind;
if (percent_complete.length == 1) {
ind = 1;
} else if (percent_complete.length == 2) {
ind = percent_complete.substring(0, 1);
} else {
ind = 9;
}
return ind;
}
function checkDiv() {
var progress_bar = document.getElementById("progressBar");
if (progress_bar.style.visibility == "visible")
{
clearBar();
document.getElementById("complete").innerHTML
= "";
} else {
progress_bar.style.visibility = "visible"
}
}
function clearBar() {
for (var i = 1; i < 10; i++) {
var elem = document.getElementById("block" + i);
elem.innerHTML = clear;
elem.style.backgroundColor = "white";
}
}
</script>
</head>
<body>
<h1>Ajax
Progress Bar Example</h1>
Launch long-running
process:
<input type="button" value="Launch"
id="go" onclick="go();"/>
<p>
<table
align="center">
<tbody>
<tr><td>
<div id="progressBar"
style="padding:2px;border:solid black 2px;visibility:hidden">
<span id="block1"> </span>
<span id="block2"> </span>
<span id="block3"> </span>
<span id="block4"> </span>
<span id="block5"> </span>
<span id="block6"> </span>
<span id="block7"> </span>
<span id="block8"> </span>
<span id="block9"> </span>
</div>
</td></tr>
<tr><td align="center" id="complete"></td></tr>
</tbody>
</table>
</body>
</html>
这个例子的服务器代码“模拟”了一个长时间运行的事务（见代码清单4-10）。在真实环境中，这期间可能还要同时创建多个新实例并注册，之前还需要客户发出请求。为简单起见，我们忽略了这点，也没有编写线程代码。
代码清单4-10 ProgressBarServlet.java
package ajaxbook.chap4;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ProgressBarServlet
extends HttpServlet {
private int
counter = 1;
/** Handles
the HTTP <code>GET</code> method.
* @param
request servlet request
* @param
response servlet response
*/
protected void doGet(HttpServletRequest
request, HttpServletResponse response)
throws ServletException,
IOException {
String task = request.getParameter("task");
String res = "";
if (task.equals("create")) {
res = "<key>1</key>";
counter = 1;
}
else {
String percent = "";
switch (counter) {
case 1: percent = "10"; break;
case 2: percent = "23"; break;
case 3: percent = "35"; break;
case 4: percent = "51"; break;
case 5: percent = "64"; break;
case 6: percent = "73"; break;
case 7: percent = "89"; break;
case 8: percent = "100"; break;
}
counter++;
res = "<percent>" + percent + "</percent>";
}
PrintWriter out = response.getWriter();
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
out.println("<response>");
out.println(res);
out.println("</response>");
out.close();
}
}
图4-10显示了实际工作中的进度条，图4-11显示了完成时的情况。
图4-10 进度条示例
图4-11 进度条完成
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