javascript

Chapter 15: XML in JavaScript
When an XML file is loaded asynchronously, you need to assign an onreadystatechange event handler
to XML DOM document. There are four different ready states:
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1 — The DOM is loading data.
2 — The DOM has completed loading the data.
3 — The DOM may be used although some sections may not be available.
4 — The DOM is completely loaded and ready to be used.
Practically speaking, the only ready state of interest is 4, which indicates that the XML file has been
completely downloaded and parsed into a DOM. You can retrieve the ready state of the XML document
via the readyState property. Loading an XML file asynchronously typically uses the following pattern:
var xmldom = createDocument();
xmldom.async = true;
xmldom.onreadystatechange = function(){
if (xmldom.readyState == 4){
if (xmldom.parseError != 0){
alert(“An error occurred:\nError Code: “
+ xmldom.parseError.errorCode + “\n”
+ “Line: “ + xmldom.parseError.line + “\n”
+ “Line Pos: “ + xmldom.parseError.linepos + “\n”
+ “Reason: “ + xmldom.parseError.reason);
} else {
alert(xmldom.documentElement.tagName); //”root”
alert(xmldom.documentElement.firstChild.tagName); //”child”
var anotherChild = xmldom.createElement(“child”);
xmldom.documentElement.appendChild(anotherChild);
var children = xmldom.getElementsByTagName(“child”);
alert(children.length); //2
};
}
}
alert(xmldom.xml);
xmldom.load(“example.xml”);
Note that the assignment of the onreadystatechange event handler must happen before the call to
load() to ensure that it gets called in time. Also note that inside of the event handler, you must use the
name of the XML document variable, xmldom , instead of the this object. ActiveX controls disallow the
use of this as a security precaution. Once the ready state of the document reaches 4, you can safely
check to see if there ’ s a parsing error and begin your XML processing.
Even though it ’ s possible to load XML files via the XML DOM document object,
it ’ s generally accepted to use an XMLHttpRequest object for this instead. The
XMLHttpRequest object, and Ajax in general, are discussed in Chapter 17 .
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