完全了解AJAX (四)

作者:zym007 日期:2006-12-23 16:59:01

字体大小: 小中大

响应状况

采用先前的代码，我们改变open method中的url参数到一个不存在的文件中：

xmlhttp.open("GET", "filethatdoesnotexist.html", true);

再次运行该代码，我们看到和先前的代码执行了相同的结果-为什么会这样？

如果这是一个正常http请求，我们预计404错误会出现-在AJAX中也具有相同的状况属性。该属性仅可读，包含HTTP请求状况代码，我们这里的文件对应的代码是404。应用于一般http请求的相同状况代码仍适用AJAX。（注意：为了正常运行，页面应该读取来自一个HTTP服务器的请求，如果从文件系统中读取，该状况代码将始终是未定义的。）

因此，我们需要增加一个if陈述，用来测试请求是否成功，如果是一个200的响应代码，将被成功返回。如果不成功，我们必须选择恰当的方式来解决错误。

function myReturnMethod() {

if (xmlhttp.readyState==4){

if (xmlhttp.status == 200) {

alert("Request successfully completed");

}

else if(xmlhttp.status == 404) {

alert("Requested file not found");

}

else {

alert("Error has occurred with status code:

"+xmlhttp.status);

}

}

}

ResponseText与responseXML

尽管我们已经可以成功地完成一个AJAX请求，但还需要对返回的数据做出处理。通过应用responseText 或responseXML属性，我们有两种处理的方式。最简单的方法是直接获取从服务器返回的数据，使用responseText属性中返回的纯文本数据。这样得到的数据可以是任何我们想要的形式；简单的文本响应，符号划界值或一整本字符串名册。在我们的返回方法中，我们将在警告中输出responseText，这将输出我们请求的整个文件内容。

在这个阶段，如果你还没有向我们发出请求的文件- another_file.html中输入任何内容，你应该做了。

function myReturnMethod() {

if (xmlhttp.readyState==4){

if (xmlhttp.status == 200) {

alert(xmlhttp.responseText);

}

else if(xmlhttp.status == 404) {

alert("Requested file not found");

}

else {

alert("Error has occurred with status code:

"+xmlhttp.status);

}

}

}

另一个可供选择的找回返回数据的方法是使用responseXML属性，其将以XML文档对象的形式返回数据，利用Javascript的DOM功能可以对其研究。为了看到这一动作的结果，我们需要返回一个恰当的XML格式文档；我们就创建一个新文件来请求(xmlresult.html), 并写入以下代码：

<?xml version="1.0" encoding="UTF-8"?>

<root>

XML Formatted Result

</root>

然后我们需要将打开命令改为：

xmlhttp.open("GET", "xmlresult.html", true);

在我们的返回方法中所作的最大改变是，我们需要在那里替换警告(xmlhttp.responseText)，代码如下：

xmldoc = xmlhttp.responseXML;

rootnode = xmldoc.getElementsByTagName('root').item(0);

alert(rootnode.firstChild.data);

瞧，我们正在利用DOM来返回数据。

通过结合我们所了解到的现有的javascript功能，该数据可被用于应付一个页面的内容。例如：

document.getElementById('resultdiv').innerHTML = xmlhttp.responseText;

何时使用AJAX

既然已经知道如何创建一个请求以及返回结果，那么还有一个问题是何时该使用AJAX呢？

对于一项新技术自然会有两方面的极端意见，一些网络开发者认为，如果没有AJAX，生活将变得更简单；另一些人则巴不得见到由AJAX构建的整个网站。通常这种情况下，真理往往介于这两种观点之间。当决定在你的网站上使用AJAX时，你也应当考虑其中带来的后果。首先，所有应用了AJAX的页面无法加入书签。随着AJAX的使用，也为你的网站引入了一种“状态”，而除了初始页面外，用户无法返回到任何其他页面。

原因是在用户的浏览器上url没有改变-如果其发生改变，AJAX就没有存在的必要了。

其次，如果想要支持每一位访问网站的用户，就需要付出双倍的工作。对于你创建使用AJAX网站的每个片段，你都必须迎合没有javascript功能用户的需要。如果为了javascript和非javascript用户具有相同的页面级别，需要做大量额外的工作，必须使用noscript标签。也就是说，AJAX通过减少浏览和交互时所需要的页面重载次数，从而提高了网站的可用性。结合javascript效果库，我们可以创建一些非常引人注目的页面交互功能，这在两年前是不可能实现的。因此，我的观点是在你的网站的非主要位置使用AJAX。例如，在某个网站，如果将AJAX结构应用到文章中将是极其错误的想法，因为你再也无法将未读完的文章加入书签。另一方面，一种文章评论或对话系统则非常适合应用AJAX。

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文章来自: Builder

标签: Ajax

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