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:

function( status, statusText, responses, headers ) {}

• status is the HTTP status code of the response, like 200 for a typical success, or 404 for when the resource is not found.
• statusText is the statusText of the response.
• responses (Optional 可选) is a map of dataType/value that contains the response in all the formats the transport could provide (for instance, a native XMLHttpRequest object would set reponses to { xml: XMLData, text: textData } for a response that is an XML document)
• headers (Optional) is a string containing all the response headers if the transport has access to them (akin to what XMLHttpRequest.getAllResponseHeaders() would provide).

Just like prefilters, a transport's factory function can be attached to specific dataType:

$.ajaxTransport( "script", function( options, originalOptions, jqXHR ) {
    /* Will only be called for script requests */
});

The following example shows how a minimal image transport could be implemented:

$.ajaxTransport( "image", function( s ) {

  if ( s.type === "GET" && s.async ) {

    var image;

    return {

      send: function( _ , callback ) {

        image = new Image();

        function done( status ) {
          if ( image ) {
            var statusText = ( status == 200 ) ? "success" : "error",
            tmp = image;
            image = image.onreadystatechange = image.onerror = image.onload = null;
            callback( status, statusText, { image: tmp } );
          }
        }

        image.onreadystatechange = image.onload = function() {
          done( 200 );
        };
        image.onerror = function() {
          done( 404 );
        };

        image.src = s.url;
      },

      abort: function() {
        if ( image ) {
          image = image.onreadystatechange = image.onerror = image.onload = null;
        }
      }
    };
  }
});

Handling Custom Data Types
在jquery源码里, 已定义了几种处理类型
The jQuery Ajax implementation comes with a set of standard dataTypes, such as text, json, xml, and html.

Use the converters option in $.ajaxSetup() to augment or modify the data type conversion strategies used by $.ajax().

The unminified jQuery source itself includes a list of default converters, which effectively illustrates how they can be used:

// List of data converters
// 1) key format is "source_type destination_type"
//    (a single space in-between)
// 2) the catchall symbol "*" can be used for source_type
converters: {

  // Convert anything to text
  "* text": window.String,

  // Text to html (true = no transformation)
  "text html": true,

  // Evaluate text as a json expression
  "text json": jQuery.parseJSON,

  // Parse text as xml
  "text xml": jQuery.parseXML
}

When you specify a converters option globally in $.ajaxSetup() or per call in $.ajax(), the object will map onto the default converters, overwriting those you specify and leaving the others intact.

For example, the jQuery source uses $.ajaxSetup() to add a converter for "text script":

jQuery.ajaxSetup({
  accepts: {
    script: "text/javascript, application/javascript"
  },
  contents: {
    script: /javascript/
  },
  converters: {
    "text script": jQuery.globalEval
  }
});