jquery中函数ready用法分析

While JavaScript provides the load event for executing code when a page is rendered, this event does not get triggered until all assets such as images have been completely received. In most cases, the script can be run as soon as the DOM hierarchy has been fully constructed. The handler passed to .ready() is guaranteed to be executed after the DOM is ready, so this is usually the best place to attach all other event handlers and run other jQuery code. When using scripts that rely on the value of CSS style properties, it's important to reference external stylesheets or embed style elements before referencing the scripts.

In cases where code relies on loaded assets (for example, if the dimensions of an image are required), the code should be placed in a handler for the load event instead.

虽然JavaScript提供了load事件，当页面渲染完成之后会执行这个函数，在所以元素加载完成之前，这个函数不会被调用，例如图像。但是在大多数情况下，只要DOM结构加载完，脚本就可以尽快运行。传递给.ready()的事件句柄在DOM准备好后立即执行，因此通常情况下，最好把绑定事件句柄和其他jQuery代码都到这里来。但是当脚本依赖于CSS样式属性时，一定要在脚本之前引入外部样式或内嵌样式的元素。   
  
如果代码依赖于需加载完的元素（例如，想获取一个图片的尺寸大小），应该用.load()事件代替，并把代码放到load事件句柄中。

$(handler) or $(document).ready(handler) →  ready() → bindReady() → 执行readyList.add( fn ) fn

 jQuery.fn = jQuery.prototype = {   
            constructor: jQuery,   
            init: function( selector, context, rootjQuery ) {   
                // HANDLE: $(function)   
                // Shortcut for document ready   
                // 如果函数，则认为是DOM ready句柄   
                if ( jQuery.isFunction( selector ) ) {   
                    return rootjQuery.ready( selector );   
                }   
            },   
           
            ready: function( fn ) {   
                // Attach the listeners   
                jQuery.bindReady(); // 绑定DOM ready监听器，跨浏览器，兼容标准浏览器和IE浏览器   
           
                // Add the callback   
                     readyList.add( fn );// 将ready句柄添加到ready异步句柄队列   
           
                return this;   
            }   
        };

    bindReady: function() { // jQuery.bindReady   
                if ( readyList ) {   
                    return;   
                }   
  
                readyList =jQuery.Callbacks( "once memory" )// 初始化ready异步事件句柄队列   
  
                // Catch cases where $(document).ready() is called after the   
                // browser event has already occurred.   
                // 如果DOM已经完毕，立即调用jQuery.ready   
                if ( document.readyState === "complete" ) {   
                    // Handle it asynchronously to allow scripts the opportunity to delay ready   
                    // 重要的是异步   
                    return setTimeout( jQuery.ready, 1 );   
                }   
            //下面是一些防御性的编程 故此省略        ......
}