Professional Ajax(programming)

Professional Ajax(programming)


Professional Ajax(programming)

Ajax  (an acronym for Asynchronous JavaScript and XML) is a group of interrelated web development techniques used on the client-side to create asynchronous web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. Data can be retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not required (JSON is often used instead), and the requests do not need to be asynchronous.


Ajax is not a single technology, but a group of technologies. HTML and CSS can be used in combination to mark up and style information. The DOM is accessed with JavaScript to dynamically display, and allow the user to interact with, the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.


History 

 

In the 1990s, most web sites were based on complete HTML pages. Each user action required that the page be reloaded from the server (or a new page loaded). This process was inefficient, as reflected by the user experience: all page content disappeared then reappeared. Each time a page was reloaded due to a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and used excessive bandwidth.


In 1996, the iframe tag was introduced by Internet Explorer to load content asynchronously.


In 1998, Microsoft Outlook Web Access team implemented the first component XMLHTTP by client script.

Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail (2004) and Google Maps (2005).
The term Ajax was coined on 18 February 2005 by Jesse James Garrett in an article entitled "Ajax: A New Approach to Web Applications", based on techniques used on Google pages.


On 5 April 2006, the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official web standard.

 

Drawbacks 

 

In pre-HTML5 browsers, pages dynamically created using successive Ajax requests did not automatically register themselves with the browser's history engine, so clicking the browser's "back" button may not have returned the browser to an earlier state of the Ajax-enabled page, but may have instead returned to the last full page visited before it. Such behavior — navigating between pages instead of navigating between page states — may be desirable, but if fine-grained tracking of page state is required then a pre-HTML5 workaround was to use invisible iframes to trigger changes in the browser's history. A workaround implemented by Ajax techniques is to change the URL fragment identifier (the part of a URL after the '#') when an Ajax-enabled page is accessed and monitor it for changes. HTML5 provides an extensive API standard for working with the browser's history engine.

Because most web crawlers do not execute JavaScript code, publicly indexable web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, thereby allowing search engines to index it.
 

Similarly, some web applications which use Ajax are built in a way that cannot be read by screen-reading technologies, such as JAWS. The WAI-ARIA standards provide a way to provide hints in such a case.


The same origin policy prevents some Ajax techniques from being used across domains,although the W3C has a draft of the XMLHttpRequest object that would enable this functionality. Methods exist to sidestep this security feature by using a special Cross Domain Communications channel embedded as an iframe within a page, or by the use of JSONP.


The asynchronous callback-style of programming required can lead to complex code that is hard to maintain, to debug[ and to test.




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