正如我們早前在書中提過的, 沒面模式為一個龐大的（可能更復(fù)雜的）代碼結(jié)構(gòu)提供了一個更簡單的抽象接口。

門面在jQuery庫中能夠經(jīng)常見到，它們?yōu)殚_發(fā)者處理DOM節(jié)點，動畫或者令人特別感興趣的跨域Ajax提供了簡單的實現(xiàn)入口。

下面的代碼是jQuery $.ajax()方法的門面:

$.get( url, data, callback, dataType );
$.post( url, data, callback, dataType );
$.getJSON( url, data, callback );
$.getScript( url, callback ); 

這些方法背后真正執(zhí)行的代碼是這樣的：

// $.get()
$.ajax({
  url: url,
  data: data,
  dataType: dataType
}).done( callback );

// $.post
$.ajax({
  type: "POST",
  url: url,
  data: data,
  dataType: dataType
}).done( callback );

// $.getJSON()
$.ajax({
  url: url,
  dataType: "json",
  data: data,
}).done( callback );

// $.getScript()
$.ajax({
  url: url,
  dataType: "script",
}).done( callback );

更有趣的是，上面代碼中的門面實際上是它們自身具有的能力，它們隱藏了代碼背后很多復(fù)雜的操作。

這是因為jQuery.ajax()在jQuery核心代碼中的實現(xiàn)是一段不平凡的代碼，至少是這樣的。至少它規(guī)范了XHR（XMLHttpRequest）之間的差異而且讓我們能夠簡單的執(zhí)行常見的HTTP動作（比如：get、post等），以及處理延遲等等。

由于顯示與上面所講的門面相關(guān)的代碼將會占據(jù)整個章節(jié)，這里僅僅給出了jQuery核心代碼中規(guī)劃化XHR的代碼:

// Functions to create xhrs
function createStandardXHR() {
  try {
    return new window.XMLHttpRequest();
  } catch( e ) {}
}

function createActiveXHR() {
  try {
    return new window.ActiveXObject( "Microsoft.XMLHTTP" );
  } catch( e ) {}
}

// Create the request object
jQuery.ajaxSettings.xhr = window.ActiveXObject ?
  /* Microsoft failed to properly
   * implement the XMLHttpRequest in IE7 (can't request local files),
   * so we use the ActiveXObject when it is available
   * Additionally XMLHttpRequest can be disabled in IE7/IE8 so
   * we need a fallback.
   */
  function() {
    return !this.isLocal && createStandardXHR() || createActiveXHR();
  } :
  // For all other browsers, use the standard XMLHttpRequest object
  createStandardXHR;
  ...

下面的代碼也處于實際的jQuery XHR（jqXHR）實現(xiàn)的上層，它是我們實際上經(jīng)常打交道的方便的門面:

// Request the remote document
   jQuery.ajax({
     url: url,
     type: type,
     dataType: "html",
     data: params,
     // Complete callback (responseText is used internally)
     complete: function( jqXHR, status, responseText ) {
       // Store the response as specified by the jqXHR object
       responseText = jqXHR.responseText;
       // If successful, inject the HTML into all the matched elements
       if ( jqXHR.isResolved() ) {
         // Get the actual response in case
         // a dataFilter is present in ajaxSettings
         jqXHR.done(function( r ) {
           responseText = r;
         });
         // See if a selector was specified
         self.html( selector ?
           // Create a dummy div to hold the results
           jQuery("

<div>

   ")
             // inject the contents of the document in, removing the scripts
             // to avoid any 'Permission Denied' errors in IE
             .append(responseText.replace(rscript, ""))

             // Locate the specified elements
             .find(selector) :

           // If not, just inject the full result
           responseText );
       }

       if ( callback ) {
         self.each( callback, [ responseText, status, jqXHR ] );
       }
     }
   });

   return this;
 }

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