jquery 1.5 ajax的改进
源文见: http://api.jquery.com/extending-ajax/

相比之前的版本, 重写了ajax模块, 引入更多的扩展点. 三个概念:
Prefilters
A prefilter is a callback function that is called before each request is sent, and prior to any $.ajax() option handling.
Prefilters are registered using $.ajaxPrefilter(), and a typical registration looks like this:

$.ajaxPrefilter( function( options, originalOptions, jqXHR ) {
  // Modify options, control originalOptions, store jqXHR, etc
});

• options are the request options
• originalOptions are the options as provided to the ajax method, unmodified and, thus, without defaults from ajaxSettings
• jqXHR is the jqXHR object of the request

It is also possible to attach a prefilter to requests with a specific dataType. For example, the following applies the given prefilter to JSON and script requests only:

$.ajaxPrefilter( "json script", function( options, originalOptions, jqXHR ) {
  // Modify options, control originalOptions, store jqXHR, etc
});

Converters  (注册自己的数据转换处理)
A converter is a callback function that is called when a response of a certain dataType is received while another dataType is expected.

Converters are stored into ajaxSettings and can be added globally as follows:

$.ajaxSetup({
  converters: {
    "text mydatatype": function( textValue ) {
      if ( valid( textValue ) ) {
        // Some parsing logic here
        return mydatatypeValue;
      } else {
        // This will notify a parsererror for current request
        throw exceptionObject;
      }
    }
  }
});

Converters are useful to introduce custom dataTypes. They can also be used to transform data into desired formats. Note: all custom dataTypes must be lowercase.

With the example above, it is now possible to request data of type "mydatatype" as follows:

$.ajax( url, {
  dataType: "mydatatype"
});

或者也可以直接写在ajax单独的一次处理里

$.ajax( url, {
  dataType: "xml text mydatatype",
  converters: {
    "xml text": function( xmlValue ) {
      // Extract relevant text from the xml document
      return textValue;
    }
  }
});

Transports (自定义整个的传输过程, 这是迫不得已才用)

A transport is an object that provides two methods, send and abort, that are used internally by $.ajax() to issue requests. A transport is the most advanced way to enhance $.ajax() and should be used only as a last resort when prefilters and converters are insufficient.

Since each request requires its own transport object instance, tranports cannot be registered directly. Therefore, you should provide a function that returns a transport instead.

Transports factories are registered using $.ajaxTransport(). A typical registration looks like this:

$.ajaxTransport( function( options, originalOptions, jqXHR ) {
  if( /* transportCanHandleRequest */ ) {
    return {
      send: function( headers, completeCallback ) {
        /* send code */
      },
      abort: function() {
        /* abort code */
      }
    };
  }
});

• options are the request options
• originalOptions are the options as provided to the ajax method, unmodified and, thus, without defaults from ajaxSettings
• jqXHR is the jqXHR object of the request
• headers is a map of request headers (key/value) that the transport can transmit if it supports it
• completeCallback is the callback used to notify ajax of the completion of the request

completeCallback has the following signature:
<