<本文譯自http://dojotoolkit.org/reference-guide/dojo/DeferredList.html#dojo-deferredlist>
dojo.Deferred提供了一種“一問一答”的事件處理機制，dojo.DeferredList則在其之上提供了“一個答案回答多個問題”的機制。<譯註：“一個答案回答多個問題”更符合publisher-subscriber模型。dojo.DeferredList處理的是一個事件必須以多個事件的完成爲前提的情況，即Windows API WaitForMultipleObjects應用的場合。>
一個常見的任務是，當一串由不同服務提供的資源都可用時，通知某個阻塞的任務開始執行。例如，一個跨多個書店的搜索操作。
<譯註：此處省略關於爲什麼需要DeferredList的若干說明。大意是，以一個跨多個書店的搜索操作爲例，用戶需要看到的時所有書店的搜索結果的綜合結果。一種不好的做法是，依次發出對每個書店的搜索請求，蒐集結果，在所有搜索請求完成之後再綜合結果。這樣整個操作串行化了，耗時爲每個操作耗時的總和；如果使用DeferredList，則耗時爲單個操作的最長時間。其實使用DeferredList還有另一個好處，就是代碼形式上的簡潔>
代碼示例：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html dir="ltr">
<head>
<style type="text/css">
body, html { font-family:helvetica,arial,sans-serif; font-size:90%; }
</style>
<script src="http://ajax.googleapis.com/ajax/libs/dojo/1.6/dojo/dojo.xd.js"
djConfig="parseOnLoad: true">
</script>
<script type="text/javascript">
dojo.require("dojo.DeferredList");
dojo.require("dijit.form.Button");
dojo.addOnLoad(function() {
// stub search functions to simulate network delay
function searchAmazon() {
var d = new dojo.Deferred();
setTimeout(function() {
d.callback("We found books at amazon");
},
500);
return d;
}
function searchBol() {
var d = new dojo.Deferred();
setTimeout(function() {
d.callback("We found books at bol");
},
700);
return d;
}
function searchGoogle() {
var d = new dojo.Deferred();
setTimeout(function() {
d.callback("We found books at google");
},
200);
return d;
}
dojo.connect(dijit.byId("search"), "onClick", function() {
var d1 = searchAmazon(),
d2 = searchBol(),
d3 = searchGoogle();
dojo.byId("statusSearch").innerHTML = "Searching....";
// create a deferred list to aggregate the state
var dl = new dojo.DeferredList([d1, d2, d3]);
// a DeferredList has much the same API as a Deferred
dl.addCallback(function(res) {
// "res" is an array of results
dojo.byId("statusSearch").innerHTML = "Result: " + res[0][1] + ", " + res[1][1] + ", " + res[2][1];
console.log(res);
});
});
});
</script>
<link rel="stylesheet" type="text/css" href="http://ajax.googleapis.com/ajax/libs/dojo/1.6/dijit/themes/claro/claro.css"
/>
</head>
<body class=" claro ">
<button dojoType="dijit.form.Button" id="search">
Search
</button>
<div style="margin: 10px;">
Status:
<span id="statusSearch">
</span>
</div>
<!-- NOTE: the following script tag is not intended for usage in real
world!! it is part of the CodeGlass and you should just remove it when
you use the code -->
<script type="text/javascript">
dojo.addOnLoad(function() {
if (document.pub) {
document.pub();
}
});
</script>
</body>
</html><譯註：代碼searchXXX使用setTimeout對異步操作進行了模擬，真實的代碼更象是下面這樣：
function searchAmazon(query){
return dojo.xhr("GET", {
url: "/books/amazon",
content: { q: query }
});
}