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By Root 4175 0

uses a simple function-based coding style, taking advantage of JavaScript’s variable argument lists and loose typing. For example, it wraps the common document.getElementById() method, which accepts only strings as input, with a function that accepts either strings or DOM elements, resolving the element ID if a string is passed in but returning a DOM element unmodified if that is passed in as argument. Hence, xGetElementById() can be called to ensure that an argument has been resolved from ID to DOM node, without having to test whether it’s already been resolved. Being able to substitute a DOM element for its text ID is particularly useful when creating dynamically generated code, such as when passing a string to the setTimeout() method or to a callback handler. A similarly concise style is used in the methods for manipulating DOM element styling, with the same function acting as both getter and setter. For example, the statement xWidth(myElement)

will return the width of the DOM element myElement, where myElement is either a DOM element or the ID of a DOM element. By adding an extra argument, like so xWidth(myElement,420)

we set the width of the element. Hence, to set the width of one element equal to another, we can write

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Third-party libraries and frameworks

105

xWidth(secondElement,xWidth(firstElement))

x does not contain any code for creating network requests, but it is nonetheless a useful library for constructing the user interfaces for Ajax applications, written in a clear, understandable style.

Sarissa

Sarissa is a more targeted library than x, and is concerned chiefly with XML

manipulation in JavaScript. It supports Internet Explorer’s MSXML ActiveX components (version 3 and up), Mozilla, Opera, Konqueror, and Safari for basic functionality, although some of the more advanced features such as XPath and XSLT

are supported by a smaller range of browsers.

The most important piece of functionality for Ajax developers is cross-browser support for the XMLHttpRequest object. Rather than creating a Façade object of its own, Sarissa uses the Adapter pattern to create a JavaScript-based XMLHttpRequest object on browsers that don’t offer a native object by that name (chiefly Internet Explorer). Internally, this object will make use of the ActiveX

objects that we described in chapter 2, but as far as the developer is concerned, the following code will work on any browser once Sarissa has been imported: var xhr = new XMLHttpRequest();

xhr.open("GET", "myData.xml");

xhr.onreadystatechange = function(){

if(xhr.readyState == 4){

alert(xhr.responseXML);

}

}

xhr.send(null);

Compare this code with listing 2.11 and note that the API calls are identical to those of the native XMLHttpRequest object provided by Mozilla and Safari browsers. As noted already, Sarissa also provides a number of generic support mechanisms for working with XML documents, such as the ability to serialize arbitrary JavaScript objects to XML. These mechanisms can be useful in processing the XML documents returned from an Ajax request to the server, if your project uses XML as the markup for response data. (We discuss this issue, and the alternatives, in chapter 5.)

Prototype

Prototype is a general-purpose helper library for JavaScript programming, with an emphasis on extending the JavaScript language itself to support a more object-oriented programming style. Prototype has a distinctive style of JavaScript Licensed to jonathan zheng

106

CHAPTER 3

Introducing order to Ajax

coding, based on these added language features. Although the Prototype code itself can be difficult to read, being far removed from the Java/C# style, using Prototype, and libraries built on top of it, is straightforward. Prototype can be thought of a library for library developers. Ajax application writers are more likely to use libraries built on top of Prototype than to use Prototype itself. We’ll look at some of these libraries in the following sections. In the meantime, a brief discussion of Prototype