jquery ajax 源码分析

1. jQuery1.5开始，AJAX模块新增了三个方法。 jQuery. ajaxPrefilter， jQuery. ajaxTransport，ajaxConvert。

2. ajax内部的处理可分为基于XMLHttpRequest的ajax请求和基于script的jsonp跨域请求。

AJAX模块的整体架构如下：

jQuery.extend({

    // Counter for holding the number of active queries
    active: 0,

    // Last-Modified header cache for next request
    lastModified: {},
    etag: {},

    ajaxSettings: {
        url: ajaxLocation,
        type: "GET",
            ...
    },

    ajaxSetup: function( target, settings ) { ...},

    ajaxPrefilter: addToPrefiltersOrTransports( prefilters ),
    ajaxTransport: addToPrefiltersOrTransports( transports ),

    // Main method
    ajax: function( url, options ) {
                ...
        return jqXHR;
    },

    getJSON: function( url, data, callback ) {
        return jQuery.get( url, data, callback, "json" );
    },

    getScript: function( url, callback ) {
        return jQuery.get( url, undefined, callback, "script" );
    }
});
jQuery.each( [ "get", "post" ], function( i, method ) {
    jQuery[ method ] = function( url, data, callback, type ) {
        return jQuery.ajax({
            ...
        });
    };
});

ajaxPrefilter：在每个请求被发送和$.ajax()处理前，设置自定义Ajax选项或修改现有选项。
ajaxTransport：创建一个对象，用于处理Ajax数据的实际传输。它提供了两种方法，send 和 abort。

• dataType为"script"的ajaxPrefilter例子：

// Handle cache's special case and crossDomain
jQuery.ajaxPrefilter( "script", function( s ) {
    if ( s.cache === undefined ) {
        s.cache = false;
    }
    if ( s.crossDomain ) {
        s.type = "GET";
    }
});

• dataType为"script"的ajaxTransport例子：

// Bind script tag hack transport
jQuery.ajaxTransport( "script", function( s ) {
    // This transport only deals with cross domain requests
    if ( s.crossDomain ) {
        var script, callback;
        return {
            send: function( _, complete ) {
                script = jQuery("<script>").prop({
                    async: true,
                    charset: s.scriptCharset,
                    src: s.url
                }).on(
                    "load error",
                    callback = function( evt ) {
                        script.remove();
                        callback = null;
                        if ( evt ) {
                            complete( evt.type === "error" ? 404 : 200, evt.type );
                        }
                    }
                );
                document.head.appendChild( script[ 0 ] );
            },
            abort: function() {
                if ( callback ) {
                    callback();
                }
            }
        };
    }
});

** 最终初始化完成的prefilters：**

prefilters

** 最终初始化完成的transports：**

transports

ajax代码具体实现：

// Main method
    ajax: function( url, options ) {

        // If url is an object, simulate pre-1.5 signature
        if ( typeof url === "object" ) {
            options = url;
            url = undefined;
        }

        // Force options to be an object
        options = options || {};

        var transport,
            ...
            s = jQuery.ajaxSetup( {}, options ),
            deferred = jQuery.Deferred(),
                        ...
            
            // Fake xhr
            jqXHR = {
                readyState: 0,
                // Builds headers hashtable if needed
                getResponseHeader: function( key ) {
                    ...
                },
                // Caches the header
                setRequestHeader: function( name, value ) {
                    ...
                },
                                ...
                // Cancel the request
                abort: function( statusText ) {
                    var finalText = statusText || strAbort;
                    if ( transport ) {
                        transport.abort( finalText );
                    }
                    done( 0, finalText );
                    return this;
                }
            };

        // Attach deferreds
        deferred.promise( jqXHR ).complete = completeDeferred.add;
        jqXHR.success = jqXHR.done;
        jqXHR.error = jqXHR.fail;

        
        s.url = ( ( url || s.url || ajaxLocation ) + "" ).replace( rhash, "" )
            .replace( rprotocol, ajaxLocParts[ 1 ] + "//" );

        // Alias method option to type as per ticket #12004
        s.type = options.method || options.type || s.method || s.type;

        // Extract dataTypes list
        s.dataTypes = jQuery.trim( s.dataType || "*" ).toLowerCase().match( rnotwhite ) || [ "" ];

        // Convert data if not already a string
        if ( s.data && s.processData && typeof s.data !== "string" ) {
            s.data = jQuery.param( s.data, s.traditional );
        }

        // Apply prefilters
        inspectPrefiltersOrTransports( prefilters, s, options, jqXHR );

        // If request was aborted inside a prefilter, stop there
        if ( state === 2 ) {
            return jqXHR;
        }

        // We can fire global events as of now if asked to
        // Don't fire events if jQuery.event is undefined in an AMD-usage scenario (#15118)
        fireGlobals = jQuery.event && s.global;

        // Watch for a new set of requests
        if ( fireGlobals && jQuery.active++ === 0 ) {
            jQuery.event.trigger("ajaxStart");
        }

        // Uppercase the type
        s.type = s.type.toUpperCase();
                ...

        // Install callbacks on deferreds
        for ( i in { success: 1, error: 1, complete: 1 } ) {
            jqXHR[ i ]( s[ i ] );
        }

        // Get transport
        transport = inspectPrefiltersOrTransports( transports, s, options, jqXHR );

        // If no transport, we auto-abort
        if ( !transport ) {
            done( -1, "No Transport" );
        } else {
            jqXHR.readyState = 1;

            // Send global event
            if ( fireGlobals ) {
                globalEventContext.trigger( "ajaxSend", [ jqXHR, s ] );
            }
            // Timeout
            if ( s.async && s.timeout > 0 ) {
                timeoutTimer = setTimeout(function() {
                    jqXHR.abort("timeout");
                }, s.timeout );
            }

            try {
                state = 1;
                transport.send( requestHeaders, done );
            } catch ( e ) {
                // Propagate exception as error if not done
                if ( state < 2 ) {
                    done( -1, e );
                // Simply rethrow otherwise
                } else {
                    throw e;
                }
            }
        }

        // Callback for when everything is done
        function done( status, nativeStatusText, responses, headers ) {
            
        }

        return jqXHR;
    },