jquery ajax

jQuery.ajax( options ) Returns: XMLHttpRequest
example:

$.ajax({
   type: "POST",
   url: "some.php",
   data: "name=John&location=Boston",
   success: function(msg){
     alert( "Data Saved: " + msg );
   }
});

以上options写了许多属性， 一看也能明白它们的含义。不过显得有些麻烦了，于是就有了简化版本：
jQuery.post( url, [data], [callback] ) Returns: XMLHttpRequest
以上的code就可以写成：

  $post("some.php","name=John&location=Boston",function(msg){
               alert( "Data Saved: " + msg );
        });

除了post，当然还有get：
jQuery.get( url, [data], [callback] ) Returns: XMLHttpRequest

比较特殊的，还有：
jQuery.getJSON( url, [data], [callback] )   Returns: XMLHttpRequest
以get方式，从服务端获取json对象。get和post其实返回对象也可以是json， html，xml，json都行，因此这个方法感觉有些多余了， 实际用处应该不大。

还有一个特殊方式
jQuery.getScript( url, [callback] )    Returns: XMLHttpRequest
url指定了一个javascript脚本， 当脚本加载完毕则执行callback。这个形式不像前面都是异步请求返回业务需要的数据， 这里是动态加载javascript， 比较有意思。
sample:

 $(document).ready(function(){
    $.getScript("http://dev.jquery.com/view/trunk/plugins/color/jquery.color.js", function(){
  $("#go").click(function(){
    $(".block").animate( { backgroundColor: 'pink' }, 1000)
      .animate( { backgroundColor: 'blue' }, 1000);
  });

Ajax Events:
ajax事件分了两种：local和global， local的是对ajax调用本身而言的，而global可以将事件注册到dom组件上。
local event:

$.ajax({
   beforeSend: function(){
     // Handle the beforeSend event
   },
   complete: function(){
     // Handle the complete event
   }
   // ...
 });

beforeSend和complete都是ajax调用中内部的事件，因此不可以绑定到其他组件。
global event:

 $("#loading").bind("ajaxSend", function(){
   $(this).show();
 }).bind("ajaxComplete", function(){
   $(this).hide();
 });

ajaxSend是global的，因此既可以在ajax内部使用，也能绑定在dom元素之上。
global事件是可以禁止的：

 $.ajax({
   url: "test.html",
   global: false,
   // ...
 });

事件类型还有一个规律， 凡是带了ajax开头的都是global的，否则就是local的。
而且几乎总有一个global事件跟local事件对应。

#  beforeSend (Local Event)
This event, which is triggered before an Ajax request is started, allows you to modify the XMLHttpRequest object (setting additional headers, if need be.)
# ajaxSend (Global Event)
This global event is also triggered before the request is run.
# success (Local Event)
This event is only called if the request was successful (no errors from the server, no errors with the data).
# ajaxSuccess (Global Event)
This event is also only called if the request was successful.
# error (Local Event)
This event is only called if an error occurred with the request (you can never have both an error and a success callback with a request).
# ajaxError (Global Event)
This global event behaves the same as the local error event.
# complete (Local Event)
This event is called regardless of if the request was successful, or not. You will always receive a complete callback, even for synchronous requests.
# ajaxComplete (Global Event)

事件响应function基本都是如此：
function (event, XMLHttpRequest, ajaxOptions) {
  this; // dom element listening
}
对于error的，最后还多一个thrownError参数

当需要调整某个ajax请求的参数的时候，ajaxSetup能够派上用场:

$.ajaxSetup({
  url: "/xmlhttp/",
  global: false,
  type: "POST"
});