jquery构造器的实现

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。