[转载] jQuery源码分析-07数据缓存-Cache

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7. 数据缓存 Cache

jQuery提供了jQuery.data()和jQuery.fn.data(),实现对缓存的操作。jQuery.fn.data()内部通过jQuery.data()实现。jQuery.data()有三种用法:

/**

 * jQuery.data( elem, key, value ) 在指定元素上存储/添加任意的数据,处理了循环引用和内存泄漏问题

 * jQuery.data( elem, key ) 返回指定元素上name指定的值

 * jQuery.data( elem ) 返回全部数据

 */

7.1        实现思路

l  写入(引用jQuery.data()接口的三个参数:elem,key,value)

在匹配的DOM元素(elem)上附加一个唯一ID,在$.cache中添加同样的ID属性,该ID属性的值是一个对象,其中存储了key/value映射。

其中有几个关键点:

n  所有数据都存储在全局变量$.cache中

n  唯一ID是一个整型值,初始为0,调用data接口时自动加一,生成新的唯一ID

n  唯一ID附加在以$.expando命名的属性上,$.expando是动态生成的,类似于一个时间戳,以尽可能的避免与用户变量冲突

l  读取

从匹配的DOM元素(elem)上取到唯一ID,在$.cache中找到唯一ID对应的对象,再从对应的对象中找到key对应的值

7.2        验证

下面用代码验证一下(在火狐中用firebug调试):

// 查看存储唯一ID的属性名

$.expando // 返回 "jQuery161013869011191548697"

// 在body节点上写入数据

$('body').data( 'a', 1 )

$('body').data( 'b', 2 )

// 读取body节点的唯一ID

$('body')[0][ $.expando ] // 返回5

$('body')[0][ "jQuery161013869011191548697" ] // 返回5

// 读取数据

$.cache[5] // 返回 Object { a=1, b=2},验证成功!

7.3        源码分析

7.3.1  jQuery.data

// 数据缓存

 

var rbrace = /^(?:\{.*\}|\[.*\])$/, // 花括号或方括号

    rmultiDash = /([a-z])([A-Z])/g; // 驼峰写法,大小写之间会被插入破折号

 

jQuery.extend({

    cache: {},

 

    // Please use with caution

    uuid: 0, // 唯一id种子,DOM元素第一次调用data接口存储数据时,会用uuid++的方式,生成一个新的唯一id

 

    // Unique for each copy of jQuery on the page

    // Non-digits removed to match rinlinejQuery

    // 页面上jQuery副本的唯一标识

    // 非数字符号被移除以匹配rinlinejQuery

    expando: "jQuery" + ( jQuery.fn.jquery + Math.random() ).replace( /\D/g, "" ),

 

    // The following elements throw uncatchable exceptions if you

    // attempt to add expando properties to them.

    //

    // YunG:

    // 如果尝试在embed、object、applet上附加属性值,将会抛出未捕获的异常

    //

    // embed:

    // embed标签用于播放一个多媒体对象,包括Flash、音频、视频等

    // http://221.199.150.103/jsj/html/page/book/xhtml/m_embed.htm

    //

    // object:

    // object元素用于向页面添加多媒体对象,包括Flash、音频、视频等。它规定了对象的数据和参数,以及可用来显示和操作数据的代码。

    // <object>与</object>之间的文本是替换文本,如果用户的浏览器不支持此标签会显示这些文本。

    // object元素中一般会包含<param>标签,<param>标签可用来定义播放参数。

    // http://221.199.150.103/jsj/html/page/book/xhtml/m_object.htm?F=14,L=1

    //

    // <embed>和<object>标签的区别:两者都是用来播放多媒体文件的对象,object元素用于IE浏览器,embed元素用于非IE浏览器,为了保证兼容性,通常我们同时使用两个元素,浏览器会自动忽略它不支持的标签。同时使用两个元素时,应该把<embed>标签放在<object>标签的内部。

    //

    // applet:

    // 不赞成使用

    noData: {

       "embed": true,

       // Ban all objects except for Flash (which handle expandos)

       "object": "clsid:D27CDB6E-AE6D-11cf-96B8-444553540000",

       "applet": true

    },

    // 判断一个元素是否有与之关联的数据(通过jQuery.data设置),用在事件处理中

    hasData: function( elem ) {

       // 如果是DOM元素,则从jQuery.cache中读取,关联jQuery.cache和DOM元素的id存储在属性jQuery.expando中

       // 如果是非DOM元素,则直接从elem上取,数据存储在 jQuery.expando 属性中

       // elem的属性jQuery.expando,要么值是id,要么值是要存储的数据

       // elem被替换为所存储的数据

       elem = elem.nodeType ? jQuery.cache[ elem[jQuery.expando] ] : elem[ jQuery.expando ];

 

       return !!elem && !isEmptyDataObject( elem );

    },

    /**

     * jQuery.data( elem, key, value ) 在指定元素上存储/添加任意的数据,处理了循环引用和内存泄漏问题

     * jQuery.data( elem, key ) 返回指定元素上name指定的值

     * jQuery.data( elem ) 返回全部数据

     */

    /**

     * pvt 私有的,是否是内部使用的独立对象,pvt为true时用于事件处理

     */

    data: function( elem, name, data, pvt /* Internal Use Only */ ) {

       // 是否可以附加数据,不可以则直接返回

       if ( !jQuery.acceptData( elem ) ) {

           return;

       }

 

       var internalKey = jQuery.expando, // 内部key?

           getByName = typeof name === "string", // name必须是字符串?

           thisCache,

 

           // We have to handle DOM nodes and JS objects differently because IE6-7

           // can't GC object references properly across the DOM-JS boundary

           // 必须区分处理DOM元素和JS对象,因为IE6-7不能垃圾回收对象跨DOM对象和JS对象进行的引用属性

           isNode = elem.nodeType,

 

           // Only DOM nodes need the global jQuery cache; JS object data is

           // attached directly to the object so GC can occur automatically

           // 如果是DOM元素,则使用全局的jQuery.cache(为什么?DOM元素不能存储非字符串?无法垃圾回收?)

           // 如果是JS对象,则直接附加到对象上

           cache = isNode ? jQuery.cache : elem,

 

           // Only defining an ID for JS objects if its cache already exists allows

           // the code to shortcut on the same path as a DOM node with no cache

           // 如果JS对象的cache已经存在,则需要为JS对象定义一个ID

           // 如果是DOM元素,则直接取elem[ jQuery.expando ],返回id(有可能是undefined)

           // 如果是JS对象,且JS对象的属性jQuery.expando存在,返回jQuery.expando(有可能是 undefined)

           id = isNode ? elem[ jQuery.expando ] : elem[ jQuery.expando ] && jQuery.expando;

 

       // Avoid doing any more work than we need to when trying to get data on an

       // object that has no data at all

       // 避免做更多的不必要工作,当尝试在一个没有任何数据的对象上获取数据时

          

       // name是字符串,data未定义,说明是在取数据

       // 但是对象没有任何数据,直接返回

       // ?id不存在,说明没有数据;或者,id存在,但是属性internalKey不存在,也说明没有数据

       // ?internalKey到底是干什么用的?

       if ( (!id || (pvt && id && !cache[ id ][ internalKey ]))

              && getByName && data === undefined ) {

           return;

       }

       // id不存在的话就生成一个

       if ( !id ) {

           // Only DOM nodes need a new unique ID for each element since their data

           // ends up in the global cache

           // 只有DOM元素需要一个唯一ID,因为DOM元素的数据是存储在全局cache中的。

           // 用uuid种子递增分配唯一ID

           if ( isNode ) {

              elem[ jQuery.expando ] = id = ++jQuery.uuid;

           } else {

              // JS对象则直接使用jQuery.expando,既然是直接附加到对象上,又何必要id呢?

              // 避免与其他属性冲突!

              id = jQuery.expando;

           }

       }

 

       // 数据存储在一个映射对象中({})

       if ( !cache[ id ] ) {

           cache[ id ] = {}; // 初始化存储对象

          

           // 这是什么东东?既然是一个“恶作剧”,那就就忽略它,不深究。

           // TODO: This is a hack for 1.5 ONLY. Avoids exposing jQuery

           // metadata on plain JS objects when the object is serialized using

           // JSON.stringify

           if ( !isNode ) {

              cache[ id ].toJSON = jQuery.noop;

           }

       }

 

       // An object can be passed to jQuery.data instead of a key/value pair; this gets

       // shallow copied over onto the existing cache

       // data接口接收对象和函数,浅拷贝

       if ( typeof name === "object" || typeof name === "function" ) {

           // 私有数据,存储在cache[ id ][ internalKey ]中

           // 什么类型的数据算私有数据呢?事件处理函数,还有么?

           if ( pvt ) {

              cache[ id ][ internalKey ] = jQuery.extend(cache[ id ][ internalKey ], name);

           } else {

              cache[ id ] = jQuery.extend(cache[ id ], name);

           }

       }

       // 存储对象,存放了所有数据的映射对象

       thisCache = cache[ id ];

 

       // Internal jQuery data is stored in a separate object inside the object's data

       // cache in order to avoid key collisions between internal data and user-defined

       // data

       // jQuery内部数据存在一个独立的对象(thisCache[ internalKey ])上,为了避免内部数据和用户定义数据冲突

       //

       // 如果是私有数据

       if ( pvt ) {

           // 存放私有数据的对象不存在,则创建一个{}

           if ( !thisCache[ internalKey ] ) {

              thisCache[ internalKey ] = {};

           }

           // 使用私有数据对象替换thisCache

           thisCache = thisCache[ internalKey ];

       }

 

       // 如果data不是undefined,表示传入了data参数,则存储data到name属性上

       // 这里为什么要统一为驼峰写法呢?

       // 这里的问题是:如果传入的是object/function,不做转换,只有传入的name是字符串才会转换。

       if ( data !== undefined ) {

           thisCache[ jQuery.camelCase( name ) ] = data;

       }

 

       // 又是一个hack,忽略它,不研究

       // TODO: This is a hack for 1.5 ONLY. It will be removed in 1.6. Users should

       // not attempt to inspect the internal events object using jQuery.data, as this

       // internal data object is undocumented and subject to change.

       if ( name === "events" && !thisCache[name] ) {

           return thisCache[ internalKey ] && thisCache[ internalKey ].events;

       }

       // 如果name是字符串,则返回data

       // 如果不是,则返回整个存储对象

       return getByName ? thisCache[ jQuery.camelCase( name ) ] : thisCache;

    },

    // 在指定元素上移除存放的数据

    removeData: function( elem, name, pvt /* Internal Use Only */ ) {

       if ( !jQuery.acceptData( elem ) ) {

           return;

       }

 

       var internalKey = jQuery.expando, isNode = elem.nodeType,

 

           // See jQuery.data for more information

           cache = isNode ? jQuery.cache : elem,

 

           // See jQuery.data for more information

           id = isNode ? elem[ jQuery.expando ] : jQuery.expando;

 

       // If there is already no cache entry for this object, there is no

       // purpose in continuing

       if ( !cache[ id ] ) {

           return;

       }

 

       if ( name ) {

           var thisCache = pvt ? cache[ id ][ internalKey ] : cache[ id ];

 

           if ( thisCache ) {

              delete thisCache[ name ];

 

              // If there is no data left in the cache, we want to continue

              // and let the cache object itself get destroyed

              if ( !isEmptyDataObject(thisCache) ) {

                  return;

              }

           }

       }

 

       // See jQuery.data for more information

       if ( pvt ) {

           delete cache[ id ][ internalKey ];

 

           // Don't destroy the parent cache unless the internal data object

           // had been the only thing left in it

           if ( !isEmptyDataObject(cache[ id ]) ) {

              return;

           }

       }

 

       var internalCache = cache[ id ][ internalKey ];

 

       // Browsers that fail expando deletion also refuse to delete expandos on

       // the window, but it will allow it on all other JS objects; other browsers

       // don't care

       // 如果不支持在DOM元素上delete,设置为null

       if ( jQuery.support.deleteExpando || cache != window ) {

           delete cache[ id ];

       } else {

           cache[ id ] = null;

       }

 

       // We destroyed the entire user cache at once because it's faster than

       // iterating through each key, but we need to continue to persist internal

       // data if it existed

       // // 如果还有数据,就清空一次再设置,增加性能

       if ( internalCache ) {

           cache[ id ] = {};

           // TODO: This is a hack for 1.5 ONLY. Avoids exposing jQuery

           // metadata on plain JS objects when the object is serialized using

           // JSON.stringify

           if ( !isNode ) {

              cache[ id ].toJSON = jQuery.noop;

           }

 

           cache[ id ][ internalKey ] = internalCache;

 

       // Otherwise, we need to eliminate the expando on the node to avoid

       // false lookups in the cache for entries that no longer exist

        // 已经没有任何数据了,就全部删除

       } else if ( isNode ) {

           // IE does not allow us to delete expando properties from nodes,

           // nor does it have a removeAttribute function on Document nodes;

           // we must handle all of these cases

           // 如果支持delete,就删除。

           // IE使用removeAttribute,所以尝试一次。再失败就只能设置为null了。

           if ( jQuery.support.deleteExpando ) {

              delete elem[ jQuery.expando ];

           } else if ( elem.removeAttribute ) {

              elem.removeAttribute( jQuery.expando );

           } else {

              elem[ jQuery.expando ] = null;

           }

       }

    },

 

    // For internal use only.

    // 内部使用

    _data: function( elem, name, data ) {

       return jQuery.data( elem, name, data, true );

    },

 

    // A method for determining if a DOM node can handle the data expando

    // YunG:

    // 判断一个DOM元素是否可以附加数据

    acceptData: function( elem ) {

       if ( elem.nodeName ) {

           // embed object applet

           var match = jQuery.noData[ elem.nodeName.toLowerCase() ];

 

           if ( match ) {

              // return match === true

              // return elem.getAttribute("classid") !== match

              return !(match === true || elem.getAttribute("classid") !== match);

           }

       }

 

       return true;

    }

});

7.3.2  jQuery.fn.data

jQuery.fn.extend({

    // 在匹配的元素上存储任意数据,解决了循环引用和内存泄漏(TODO)?

    // 支持HTML5 data-Attributes

   

    // data( key, value ) data( object )

    // 返回jQuery对象中指定key的值,或者返回含有全部数据的结合

   

    // data( key ) data( )

    // HTML5? http://api.jquery.com/data/

    data: function( key, value ) {

       var data = null;

       // 守护方法,用于处理特殊的key

       // key是undefined,则认为是取当前jQuery对象中第一个元素的全部数据

       if ( typeof key === "undefined" ) {

           if ( this.length ) {

              data = jQuery.data( this[0] );

              // 如果是Element

              if ( this[0].nodeType === 1 ) {

                  var attr = this[0].attributes, name;

                  for ( var i = 0, l = attr.length; i < l; i++ ) {

                     name = attr[i].name;

 

                     if ( name.indexOf( "data-" ) === 0 ) {

                         name = jQuery.camelCase( name.substring(5) );

 

                         dataAttr( this[0], name, data[ name ] );

                     }

                  }

              }

           }

 

           return data;

       // key是对象,则对当前jQuery对象迭代调用$.fn.each,

       // 在每一个匹配的元素上存储数据key

       } else if ( typeof key === "object" ) {

           return this.each(function() {

              jQuery.data( this, key );

           });

       }

      

       // 到这列,key是字符串

       var parts = key.split(".");

       parts[1] = parts[1] ? "." + parts[1] : "";

       // 如果value为undefined,则任务是取当前jQuery对象中第一个元素指定名称的数据

       if ( value === undefined ) {

           data = this.triggerHandler("getData" + parts[1] + "!", [parts[0]]);

 

           // Try to fetch any internally stored data first

           // 优先取内部数据

           if ( data === undefined && this.length ) {

              data = jQuery.data( this[0], key );

              // 读取HTML5的data属性

              data = dataAttr( this[0], key, data );

           }

 

           return data === undefined && parts[1] ?

              this.data( parts[0] ) :

              data;

       // key是字符串,value不是undefined,则存储

       } else {

           return this.each(function() {

              var $this = jQuery( this ),

                  args = [ parts[0], value ];

              // 触发事件

              $this.triggerHandler( "setData" + parts[1] + "!", args );

              jQuery.data( this, key, value );

              $this.triggerHandler( "changeData" + parts[1] + "!", args );

           });

       }

    },

 

    removeData: function( key ) {

       return this.each(function() {

           jQuery.removeData( this, key );

       });

    }

});

7.3.3  支持HTML5的data- 属性

// HTML 5 data- Attributes http://ejohn.org/blog/html-5-data-attributes/

// 如果在指定元素elem没有找到key对应的数据data,就尝试读取HTML5的data属性

function dataAttr( elem, key, data ) {

    // If nothing was found internally, try to fetch any

    // data from the HTML5 data-* attribute

    if ( data === undefined && elem.nodeType === 1 ) {

       var name = "data-" + key.replace( rmultiDash, "$1-$2" ).toLowerCase();

 

       data = elem.getAttribute( name );

 

       if ( typeof data === "string" ) {

           try {

              data = data === "true" ? true :

                  data === "false" ? false :

                     data === "null" ? null :

                         !jQuery.isNaN( data ) ? parseFloat( data ) :

                            rbrace.test( data ) ? jQuery.parseJSON( data ) :

                                data;

           } catch( e ) {}

 

           // Make sure we set the data so it isn't changed later

           jQuery.data( elem, key, data );

 

       } else {

           data = undefined;

       }

    }

 

    return data;

}

posted @ 2014-10-13 11:53  岁月沉香  阅读(104)  评论(0编辑  收藏  举报