無一例外地，幾乎每個應用都會時不時地調用一個長時間啟動並執行事務。如果你關心系統的可用性，就要確保使用者能很容易地看到系統的狀態。如果是一個胖客戶應用，對於長時間運行事務的問題，解決辦法很簡單：只需顯示一個進度條，以便使用者知道目前所處狀況。不過，在Ajax之前，要在Web應用中做到這一點很不容易。本節將使你瞭解如何使用Ajax為Web應用建立進度條。
在代碼清單4-9所示的例子中，再次在pollCallback()方法中使用了setTimeout()，從而每隔2秒調用一次伺服器。在processResult()方法中，只是從伺服器尋找已完成比例（百分數）的第一位元字，從而得出要將進度條中的哪些進度塊著色（灰色）。
代碼清單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>
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