How to use selector and context in jQuery object?

 $("div ul").selector//值为“div ul”    
 $("div.test").selector//值为“div.test”    
 $("#test ul li:first").selector//值为“test ul li:first”

function( selector, context, rootjQuery ) {var match, elem, ret, doc;// Handle $(""), $(null), or $(undefined)
        //如果selector为空格，！selector为false
if (!selector) {//此时this为空jQuery对象
return this;
}// Handle $(DOMElement)
        //nodeType节点类型，利用是否有nodeType属性来判断是否是DOM元素
if ( selector.nodeType ) {//将第一个元素和属性context指向selector
this.context = this[0] = selector;this.length = 1;return this;
}// The body element only exists once, optimize finding it
        //因为body只出现一次，利用!context进行优化
if ( selector === "body" && !context && document.body ) {//context指向document对象
this.context = document;this[0] = document.body;this.selector = selector;this.length = 1;return this;
}// Handle HTML strings
if ( typeof selector === "string" ) {    // Are we dealing with HTML string or an ID?
//以<开头以>结尾，且长度大于等于3，这里假设是HTML片段，跳过queckExpr正则检查
if ( selector.charAt(0) === "<" && selector.charAt( selector.length - 1 ) === ">" && selector.length >= 3 ) {// Assume that strings that start and end with <> are HTML and skip the regex check
match = [ null, selector, null ];
} else {
match = quickExpr.exec( selector );
}// Verify a match, and that no context was specified for #id
if ( match && (match[1] || !context) ) {// HANDLE: $(html) -> $(array)
if ( match[1] ) {
context = context instanceof jQuery ? context[0] : context;
doc = ( context ? context.ownerDocument || context : document );// If a single string is passed in and it&#39;s a single tag
// just do a createElement and skip the rest
ret = rsingleTag.exec( selector );        //如果是单独标签
if (ret) {//如果context是普通对象
    if (jQuery.isPlainObject(context)) {    //之所以放在数组中，是方便后面的jQuery.merge()方法调用
    selector = [document.createElement(ret[1])];    //调用attr方法，传入参数context
jQuery.fn.attr.call( selector, context, true );
} else {
selector = [ doc.createElement( ret[1] ) ];
}        //复杂HTML的处理方法
} else {
ret = jQuery.buildFragment( [ match[1] ], [ doc ] );
selector = ( ret.cacheable ? jQuery.clone(ret.fragment) : ret.fragment ).childNodes;
}return jQuery.merge( this, selector );// HANDLE: $("#id")
} else {
elem = document.getElementById( match[2] );// Check parentNode to catch when Blackberry 4.6 returns
// nodes that are no longer in the document #6963
if ( elem && elem.parentNode ) {// Handle the case where IE and Opera return items
    // by name instead of ID
    //即使是documen.getElementById这样核心的方法也要考虑到浏览器兼容问题，可能找到的是name而不是id
if ( elem.id !== match[2] ) {return rootjQuery.find( selector );
}// Otherwise, we inject the element directly into the jQuery object
this.length = 1;this[0] = elem;
}this.context = document;this.selector = selector;return this;
}// HANDLE: $(expr, $(...))
//没有指定上下文，执行rootjQuery.find()，制定了上下文且上下文是jQuery对象，执行context.find()
} else if ( !context || context.jquery ) {return ( context || rootjQuery ).find( selector );// HANDLE: $(expr, context)
// (which is just equivalent to: $(context).find(expr)
//如果指定了上下文，且上下文不是jQuery对象
} else {    //先创建一个包含context的jQuery对象，然后调用find方法
return this.constructor( context ).find( selector );
}// HANDLE: $(function)
// Shortcut for document ready
} else if ( jQuery.isFunction( selector ) ) {return rootjQuery.ready( selector );
}        //selector是jquery对象
if ( selector.selector !== undefined ) {this.selector = selector.selector;this.context = selector.context;
}        //合并到当前jQuery对象
return jQuery.makeArray( selector, this );
}