JQuery的构造器$()

jQuery的$符号非常神奇，它可以接受一个字符，也可以接受一个文档对象或window对象，亦可以传个函数进行变为domReady加载器。显然，能做到这一步，其实现是相当的复杂，这个实现就是它的init方法，jQuery的真实构造器。它功能也随着版本的升级而升级，越来越长。

2009-01-13发布的1.3版:

init: function( selector, context ) {
    // Make sure that a selection was provided
    selector = selector || document;
 
    // 处理节点参数,直接添加属性到新实例上
    if ( selector.nodeType ) {
        this[0] = selector;
        this.length = 1;
        this.context = selector;
        return this;
    }
    // 处理字符串参数
    if ( typeof selector === "string" ) {
        // 判定是否为HTML片断还是ID
        var match = quickExpr.exec( selector );
      
        if ( match && (match[1] || !context) ) {
 
            // 如果是HTML片断,转换一个由节点构造的数组
            if ( match[1] )
                selector = jQuery.clean( [ match[1] ], context );
 
            // 如果是ID,则查找此元素,如果找到放进空数组中
            else {
                var elem = document.getElementById( match[3] );
 
                // Make sure an element was located
                if ( elem ){
                    // 处理 IE and Opera 混淆ID与NAME的bug
                    if ( elem.id != match[3] )
                        return jQuery().find( selector );
                    var ret = jQuery( elem );
                    ret.context = document;
                    ret.selector = selector;
                    return ret;
                }
                selector = [];
            }
        } else
        //使用Sizzle处理其他CSS表达式,生成实例并返回
            return jQuery( context ).find( selector );
        // 处理函数参数，直接domReady
    } else if ( jQuery.isFunction( selector ) )
        return jQuery( document ).ready( selector );
 
    //处理jQuery对象参数，简单地将其两个属性赋给新实例
    if ( selector.selector && selector.context ) {
        this.selector = selector.selector;
        this.context = selector.context;
    }
    //将上面得到节点数组，用setArray方法把它们变成实例的元素
    return this.setArray(jQuery.makeArray(selector));
},

2009-02-19发布的1.32版:

init: function( selector, context ) {
    // Make sure that a selection was provided
    selector = selector || document;
    // 处理节点参数,直接添加属性到新实例上
    if ( selector.nodeType ) {
        this[0] = selector;
        this.length = 1;
        this.context = selector;
        return this;
    }
     //处理字符串参数
    if ( typeof selector === "string" ) {
        //判定是否为HTML片断还是ID
        var match = quickExpr.exec( selector );
        if ( match && (match[1] || !context) ) {
           // 如果是HTML片断,转换一个由节点构造的数组
            if ( match[1] )
                selector = jQuery.clean( [ match[1] ], context );
            else {
                var elem = document.getElementById( match[3] );
 
                // 如果是ID,则查找此元素,如果找到放进空数组中
                if ( elem && elem.id != match[3] )
                    return jQuery().find( selector );
 
                //这里对1.3版做了些优化,更简洁
                var ret = jQuery( elem || [] );
                ret.context = document;
                ret.selector = selector;
                return ret;
            }
        } else
            //使用Sizzle处理其他CSS表达式,生成实例并返回
            return jQuery( context ).find( selector );
 
         // 处理函数参数，进行domReady操作
    } else if ( jQuery.isFunction( selector ) )
        return jQuery( document ).ready( selector );
 
     //处理jQuery对象参数，简单地将其两个属性赋给新实例
    if ( selector.selector && selector.context ) {
        this.selector = selector.selector;
        this.context = selector.context;
    }
//这里对1.3版做了些扩展,允许传珍上元素集合(HTMLCollection)与节点集合(NodeList),
//元素数组可能是我们用字符串转换过来的,也可以是用户直接传进来的
    return this.setArray(jQuery.isArray( selector ) ? selector : jQuery.makeArray(selector));
},

2010-01-13发布的1.4版:

init: function( selector, context ) {
    var match, elem, ret, doc;
 
    //处理空白字符串,null,undefined参数(新增),返回一个非常纯净的实例
    if ( !selector ) {
        return this;
    }
 
    // 处理节点参数,直接添加属性到新实例上
    if ( selector.nodeType ) {
        this.context = this[0] = selector;//写法上优化
        this.length = 1;
        return this;
    }
 
    //处理字符串参数
    if ( typeof selector === "string" ) {
        //  判定是否为HTML片断还是ID
        match = quickExpr.exec( selector );
        if ( match && (match[1] || !context) ) {
 
            //如果是HTML片断
            if ( match[1] ) {
                //取得文档对象
                doc = (context ? context.ownerDocument || context : document);
 
                // 如果是单个标签,直接使用 document.createElement创建此节点并放入数组中
                ret = rsingleTag.exec( selector );
 
                if ( ret ) {
                    //如果后面跟着一个纯净的JS对象,则为此节点添加相应的属性或样式
                    if ( jQuery.isPlainObject( context ) ) {
                        selector = [ document.createElement( ret[1] ) ];
                        jQuery.fn.attr.call( selector, context, true );
                    } else {
                        selector = [ doc.createElement( ret[1] ) ];
                    }
 
                } else {
                    //改由buildFragment来生成节点集合(NodeList)
                    ret = buildFragment( [ match[1] ], [ doc ] );
                    selector = (ret.cacheable ? ret.fragment.cloneNode(true) : ret.fragment).childNodes;
                }
            } else {
                // 如果是ID,则查找此元素,如果找到放进空数组中
                elem = document.getElementById( match[2] );
 
                if ( elem ) {
                    // 处理 IE and Opera 混淆ID与NAME的bug
                    if ( elem.id !== match[2] ) {
                        return rootjQuery.find( selector );
                    }
                    //这里也做了一些优化,原来是很傻地再生成一个jQuery实例
                    this.length = 1;
                    this[0] = elem;
                }
                this.context = document;
                this.selector = selector;
                return this;
            }
 
            // 如果字符是很简单的标签选择器,那基本没有必要走Sizzle路线,直接getElementsByTagName,很好的优化
        } else if ( !context && /^\w+$/.test( selector ) ) {
            this.selector = selector;
            this.context = document;
            selector = document.getElementsByTagName( selector );
 
            // 如果第二个参数不存在或者是jQuery对象,那么用它或rootjQuery调用find查找目标节点(走Sizzle路线)
        } else if ( !context || context.jquery ) {
            return (context || rootjQuery).find( selector );
 
            // HANDLE: $(expr, context)
            // (which is just equivalent to: $(context).find(expr)
        } else {
            //如果第二个参数已指定为某元素节点,转为jQuery对象,走Sizzle路线
            return jQuery( context ).find( selector );
        }
 
        // 处理函数参数，直接domReady
 
    } else if ( jQuery.isFunction( selector ) ) {
        return rootjQuery.ready( selector );
    }
    //处理jQuery对象参数，简单地将其两个属性赋给新实例
    if (selector.selector !== undefined) {
        this.selector = selector.selector;
        this.context = selector.context;
    }
//这里又做了些许修改,缘于makeArray可以接受第二个参数(可以是数组或类数组,这时相当合并操作)
    return jQuery.isArray( selector ) ?
        this.setArray( selector ) ://内部用push方法,迅速将一个普通对象变成类数组对象
        jQuery.makeArray( selector, this );
},

接着是广受欢迎的2010-02-13发布的1.42版
init: function( selector, context ) {
    var match, elem, ret, doc;
 
    // 处理空白字符串,null,undefined参数
    if ( !selector ) {
        return this;
    }
    // 处理节点参数
    if ( selector.nodeType ) {
        this.context = this[0] = selector;
        this.length = 1;
        return this;
    }  
    // 处理body参数(新增)
    if ( selector === "body" && !context ) {
        this.context = document;
        this[0] = document.body;
        this.selector = "body";
        this.length = 1;
        return this;
    }
 
    // 处理字符串参数,分七种情形：
    //①单个标签,带对象属性包           --->   jQuery.merge
    //②单个标签,不带对象属性包         --->   attr + jQuery.merge
    //③复杂的HTML片断                 --->   buildFragment + jQuery.merge
    //④ID选择器,与找到的元素的ID不同   --->   getElementById + Sizzle + pushStack
    //⑤ID选择器,与找到的元素的ID相同   --->   getElementById + 简单属性添加
    //⑥标签选择器                     --->   getElementsByTagName + jQuery.merge
    //⑦其他CSS表达式                  --->   Sizzle + pushStack
    if ( typeof selector === "string" ) {
        match = quickExpr.exec( selector );
        if ( match && (match[1] || !context) ) {
            if ( match[1] ) {
                doc = (context ? context.ownerDocument || context : document);
                ret = rsingleTag.exec( selector );
                if ( ret ) {
                    if ( jQuery.isPlainObject( context ) ) {
                        selector = [ document.createElement( ret[1] ) ];
                        jQuery.fn.attr.call( selector, context, true );
 
                    } else {
                        selector = [ doc.createElement( ret[1] ) ];
                    }
                } else {
                    ret = buildFragment( [ match[1] ], [ doc ] );
                    selector = (ret.cacheable ? ret.fragment.cloneNode(true) : ret.fragment).childNodes;
                }
                     
                return jQuery.merge( this, selector );
            } else {
                elem = document.getElementById( match[2] );
 
                if ( elem ) {
                    if ( elem.id !== match[2] ) {
                        return rootjQuery.find( selector );
                    }
                    this.length = 1;
                    this[0] = elem;
                }
 
                this.context = document;
                this.selector = selector;
                return this;
            }
        } else if ( !context && /^\w+$/.test( selector ) ) {
            this.selector = selector;
            this.context = document;
            selector = document.getElementsByTagName( selector );
            return jQuery.merge( this, selector );
             
        } else if ( !context || context.jquery ) {
            return (context || rootjQuery).find( selector );
        } else {
            return jQuery( context ).find( selector );
        }
        // 处理函数参数，直接domReady
    } else if ( jQuery.isFunction( selector ) ) {
        return rootjQuery.ready( selector );
    }
    //处理jQuery对象参数
    if (selector.selector !== undefined) {
        this.selector = selector.selector;
        this.context = selector.context;
    }
    //无论是数组还是类数组（如NodeList），统统使用jQuery.makeArray来为实例添加新的元素
    return jQuery.makeArray( selector, this );
},

另附上makeArray方法与merge方法，merge方法好神奇啊，

makeArray: function( array, results ) {
    var ret = results || [];
 
    if ( array != null ) {
        // The window, strings (and functions) also have 'length'
        // The extra typeof function check is to prevent crashes
        // in Safari 2 (See: #3039)
        if ( array.length == null || typeof array === "string" || jQuery.isFunction(array) || (typeof array !== "function" && array.setInterval) ) {
            push.call( ret, array );
        } else {
            jQuery.merge( ret, array );
        }
    }
 
    return ret;
},
merge: function( first, second ) {
    var i = first.length, j = 0;
 
    if ( typeof second.length === "number" ) {
        for ( var l = second.length; j < l; j++ ) {
            first[ i++ ] = second[ j ];
        }
     
    } else {
        while ( second[j] !== undefined ) {
            first[ i++ ] = second[ j++ ];
        }
    }
 
    first.length = i;
 
    return first;
},

2011-01-23发布的1.5版，其init方法与1.42的变化不大：只有两处做了改动：

//1.42
-  ret = buildFragment( [ match[1] ], [ doc ] );
-  selector = (ret.cacheable ? ret.fragment.cloneNode(true) : ret.fragment).childNodes;
//1.5
+ ret = jQuery.buildFragment( [ match[1] ], [ doc ] );
+ selector = (ret.cacheable ? jQuery.clone(ret.fragment) : ret.fragment).childNodes;
 
//1.42
- return jQuery( context ).find( selector );
//1.5
+ return this.constructor( context ).find( selector );//目的就是为了不再生成新实例

2011-05-02发布的jquery1.6，变化不大，只是对HTML片断进行了更严密的判定：

// Are we dealing with HTML string or an ID?
   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 );
   }

总体来说,jQuery的构造器已经做得非常之完美,基本上达到"改无可改"的地步了。但是要保证其高效运作，我们还需要一点选择器的知识与了解buildFragment方法的运作，因为这两个实在太常用了，但也是最耗性能的。