jQuery数据缓存$.data 的使用以及源码解析

一、实现原理：

对于DOM元素，通过分配一个唯一的关联id把DOM元素和该DOM元素的数据缓存对象关联起来，关联id被附加到以jQuery.expando的值命名的属性上，数据存储在全局缓存对象jQuery.cache中。在读取、设置、移除数据时，将通过关联id从全局缓存对象jQuery.cache中找到关联的数据缓存对象，然后在数据缓存对象上执行读取、设置、移除操作。
对于Javascript对象，数据则直接存储在该Javascript对象的属性jQuery.expando上。在读取、设置、移除数据时，实际上是对Javascript对象的数据缓存对象执行读取、设置、移除操作。
为了避免jQuery内部使用的数据和用户自定义的数据发生冲突，数据缓存模块把内部数据存储在数据缓存对象上，把自定义数据存储在数据缓存对象的属性data上。

二、总体结构：

// 数据缓存 Data
jQuery.extend({
     // 全局缓存对象
     cache: {},
     // 唯一 id种子
     uuid:0,
     // 页面中每个jQuery副本的唯一标识
     expando: "jQuery" + ( jQuery.fn.jquery + Math.random() ).replace( /\D/g, "" ),
     // 是否有关联的数据
     hasData: function(){},
     // 设置、读取自定数据或内部数据
     data: function(elem, name, data, pvt) {},
     // 移除自定义数据或内部数据
     removeData: function(elem, name, pvt) {},
     // 设置、读取内部数据
     _data: function(elem, name, data) {},
     // 是否可以设置数据
     acceptData: function(elem){}
});
jQuery.fn.extend({
     // 设置、读取自定义数据，解析HTML5属性data-
     data: function(key,value){},
     // 移除自定义数据
     removeData: function(key){}
});
// 解析HTML5属性 data-functiondataAttr(elem,key,data){}
// 检查数据缓存对象是否为空functionisEmptyDataObject(obj){}
jQuery.extend({
     // 清空数据缓存对象


cleanData: function(elems){}
});

三、.data(elem, name)

$.data(elem, name, data)的使用方法：
如果传入参数name, data, 则设置任意类型的数据

  
  
  
  




  The values stored were
  
  and
  

$.data(elem, name)的使用方法：

如果传入key, 未传入参数data, 则读取并返回指定名称的数据

  
  
  
  



A div
Get "blah" from the div
Set "blah" to "hello"
Set "blah" to 86
Remove "blah" from the div
The "blah" value of this div is ?

.data(elem, name) 源码解析：

jQuery.extend({
  // 1. 定义jQuery.data(elem, name, data, pvt)
  data: function( elem, name, data, pvt /* Internal Use Only */ ) {
    // 2. 检查是否可以设置数据
    if ( !jQuery.acceptData( elem ) ) {
      return; // 如果参数elem不支持设置数据，则立即返回
    }

    // 3 定义局部变量
    var privateCache, thisCache, ret,
      internalKey = jQuery.expando,
      getByName = typeof name === "string",

      // We have to handle DOM nodes and JS objects differently because IE6-7
      // can't GC object references properly across the DOM-JS boundary
      isNode = elem.nodeType, // elem是否是DOM元素

      // Only DOM nodes need the global jQuery cache; JS object data is
      // attached directly to the object so GC can occur automatically
      cache = isNode ? jQuery.cache : elem, // 如果是DOM元素，为了避免javascript和DOM元素之间循环引用导致的浏览器(IE6/7)垃圾回收机制不起作用，要把数据存储在全局缓存对象jQuery.cache中；对于javascript对象，来及回收机制能够自动发生，不会有内存泄露的问题，因此数据可以查收存储在javascript对象上

      // 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 withnocache
      id = isNode ? elem[ internalKey ] : elem[ internalKey ] && internalKey,
      isEvents = name === "events";

    // Avoid doing any more work than we need to when trying to get data on an
    // object that has no data at all
    // 4. 如果是读取数据，但没有数据，则返回
    if ( (!id || !cache[id] || (!isEvents && !pvt && !cache[id].data)) && getByName && data === undefined ) {
      return;
      // getByName && data === undefined 如果name是字符串，data是undefined, 说明是在读取数据
      // !id || !cache[id] || (!isEvents && !pvt && !cache[id].data 如果关联id不存在，说明没有数据；如果cache[id]不存在，也说明没有数据；如果是读取自动以数据，但cache[id].data不存在，说明没有自定义数据
    }

    // 5. 如果关联id不存在，则分配一个
    if ( !id ) {
      // Only DOM nodes need a new unique ID foreach element since their data
      // ends up in the globalcacheif ( isNode ) {
        elem[ internalKey ] = id = ++jQuery.uuid; // 对于DOM元素，jQuery.uuid会自动加1，并附加到DOM元素上
      } else {
        id = internalKey; // 对于javascript对象，关联id就是jQuery.expando
      }
    }

    // 6. 如果数据缓存对象不存在，则初始化为空对象{}
    if ( !cache[ id ] ) {
      cache[ id ] = {};

      // Avoids exposing jQuery metadata on plain JS objects when the object
      // is serialized using JSON.stringify
      if ( !isNode ) {
        cache[ id ].toJSON = jQuery.noop; // 对于javascript对象，设置方法toJSON为空函数，以避免在执行JSON.stringify()时暴露缓存数据。如果一个对象定义了方法toJSON(),JSON.stringify()在序列化该对象时会调用这个方法来生成该对象的JSON元素
      }
    }

    // An object can be passed to jQuery.data instead of a key/value pair; this gets
    // shallow copied over onto the existing cache
    // 7. 如果参数name是对象或函数，则批量设置数据
    if ( typeof name === "object" || typeof name === "function" ) {
      if ( pvt ) {
        cache[ id ] = jQuery.extend( cache[ id ], name ); // 对于内部数据，把参数name中的属性合并到cache[id]中
      } else {
        cache[ id ].data = jQuery.extend( cache[ id ].data, name ); // 对于自定义数据，把参数name中的属性合并到cache[id].data中
      }
    }

    // 8. 如果参数data不是undefined, 则设置单个数据
    privateCache = thisCache = cache[ id ];

    // jQuery data() is stored in a separate object inside the object's internal data
    // cacheinorderto avoid key collisions between internal dataanduser-defined
    // data.
    if ( !pvt ) {
      if ( !thisCache.data ) {
        thisCache.data = {};
      }

      thisCache = thisCache.data;
    }

    if ( data !== undefined ) {
      thisCache[ jQuery.camelCase( name ) ] = data;
    }

    // Users should not attempt to inspect the internal events object using jQuery.data,
    // it is undocumented and subject to change. But does anyone listen? No.
    // 9. 特殊处理eventsif ( isEvents && !thisCache[ name ] ) { // 如果参数name是字符串"events",并且未设置过自定义数据"events",则返回事件婚车对象，在其中存储了事件监听函数。
      return privateCache.events;
    }

    // Checkforboth converted-to-camel and non-converted data property names
    // If a data property was specified
    //10. 如果参数name是字符串，则读取单个数据
    if ( getByName ) {

      // First Try to find as-is property data
      ret = thisCache[ name ]; // 先尝试读取参数name对应的数据

      // Test for null|undefined property data
      if ( ret == null ) { // 如果未取到，则把参数name转换为驼峰式再次尝试读取对应的数据

        // Try to find the camelCased property
        ret = thisCache[ jQuery.camelCase( name ) ];
      }
    } else { // 11. 如果未传入参数name,data,则返回数据缓存对象
      ret = thisCache;
    }

    return ret;
  },

  // For internal useonly.
  _data: function( elem, name, data ) {
    return jQuery.data( elem, name, data, true );
  },
});

四、.data(key, value), .data(key)

使用方法：

 $( "body" ).data( "foo", 52 ); // 传入key, value
    $( "body" ).data( "bar", { myType: "test", count: 40 } ); // 传入key, value
    $( "body" ).data( { baz: [ 1, 2, 3 ] } ); // 传入key, value
    $( "body" ).data( "foo" ); // 52 // 传入key
    $( "body" ).data(); // 未传入参数

HTML5 data attriubutes:

 


    $( "div" ).data( "role" ) === "page";
    $( "div" ).data( "lastValue" ) === 43;
    $( "div" ).data( "hidden" ) === true;
    $( "div" ).data( "options" ).name === "John";

.data(key, value), .data(key) 源码解析

jQuery.fn.extend({
  // 1. 定义.data(key, value)
  data: function( key, value ) {
    var parts, attr, name,
      data = null;

    // 2. 未传入参数的情况if ( typeof key === "undefined" ) {
      if ( this.length ) { // 如果参数key是undefined, 即参数格式是.data(), 则调用方法jQuery.data(elem, name, data, pvt)获取第一个匹配元素关联的自定义数据缓存对象，并返回。
        data = jQuery.data( this[0] );

        if ( this[0].nodeType === 1 && !jQuery._data( this[0], "parsedAttrs" ) ) {
          attr = this[0].attributes;
          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 ] );
            }
          }
          jQuery._data( this[0], "parsedAttrs", true );
        }
      }

      return data;
    // 3. 参数key 是对象的情况,即参数格式是.data(key),则遍历匹配元素集合，为每个匹配元素调用方法jQuery.data(elem, name, data,pvt)批量设置数据
    } elseif ( typeof key === "object" ) {
      returnthis.each(function() {
        jQuery.data( this, key );
      });
    }
    // 4. 只传入参数key的情况  如果只传入参数key, 即参数格式是.data(key),则返回第一个匹配元素的指定名称数据
    parts = key.split(".");
    parts[1] = parts[1] ? "." + parts[1] : "";

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

      // Try to fetch any internally stored data firstif ( data === undefined && this.length ) {
        data = jQuery.data( this[0], key );
        data = dataAttr( this[0], key, data );
      }

      return data === undefined && parts[1] ?
        this.data( parts[0] ) :
        data;

    // 5. 传入参数key和value的情况 即参数格式是.data(key, value),则为每个匹配元素设置任意类型的数据，并触发自定义事件setData, changeData
    } else {
      returnthis.each(function() {
        var self = jQuery( this ),
          args = [ parts[0], value ];

        self.triggerHandler( "setData" + parts[1] + "!", args );
        jQuery.data( this, key, value );
        self.triggerHandler( "changeData" + parts[1] + "!", args );
      });
    }
  },

  removeData: function( key ) {
    returnthis.each(function() {
      jQuery.removeData( this, key );
    });
  }
});

// 6. 函数dataAttr(elem, key, data)解析HTML5属性data-functiondataAttr( elem, key, data ) {
  // If nothing was found internally, try to fetch any// data from the HTML5 data-* attribute// 只有参数data为undefined时，才会解析HTML5属性data-if ( data === undefined && elem.nodeType === 1 ) {

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

    data = elem.getAttribute( name );

    if ( typeof data === "string" ) {
      try {
        data = data === "true" ? true :
        data === "false" ? false :
        data === "null" ? null :
        jQuery.isNumeric( 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;
}

五、$.removeData(elem, name)，.removeData(key)

使用方法：

  
  
  
  



value1 before creation: 
value1 after creation: 
value1 after removal: 
value2 after removal: 










  
  
  
  



value1 before creation: 
value1 after creation: 
value1 after removal: 
value2 after removal: 

$.removeData(elem, name)，.removeData(key) 源码解析：

$.extend({
    // jQuery.removeData(elem,name,pvt)用于移除通过jQuery.data()设置的数据
  removeData: function( elem, name, pvt /* Internal Use Only */ ) {
    if ( !jQuery.acceptData( elem ) ) {
      return;
    }

    var thisCache, i, l,

      // Reference to internal data cache key
      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[ internalKey ] : internalKey;

    // If there is already no cache entry for this object, there is no// purpose in continuingif ( !cache[ id ] ) {
      return;
    }

        // 如果传入参数name, 则移除一个或多个数据if ( name ) {

      thisCache = pvt ? cache[ id ] : cache[ id ].data;

      if ( thisCache ) { // 只有数据缓存对象thisCache存在时，才有必要移除数据// Support array or space separated string names for data keysif ( !jQuery.isArray( name ) ) {

          // try the string as a key before any manipulationif ( name in thisCache ) {
            name = [ name ];
          } else {

            // split the camel cased version by spaces unless a key with the spaces exists
            name = jQuery.camelCase( name );
            if ( name in thisCache ) {
              name = [ name ];
            } else {
              name = name.split( " " );
            }
          }
        }

        // 遍历参数name中的数据名，用运算符delete逐个从数据缓存对象thisCache中移除for ( i = 0, l = name.length; i < l; i++ ) {
          delete thisCache[ name[i] ];
        }

        // If there is no data left in the cache, we want to continue// and let the cache object itself get destroyedif ( !( pvt ? isEmptyDataObject : jQuery.isEmptyObject )( thisCache ) ) {
          return;
        }
      }
    }

    // See jQuery.data for more information// 删除自定义数据缓存对象cache[id].dataif ( !pvt ) {
      delete cache[ id ].data;

      // Don't destroy the parent cache unless the internal data object// had been the only thing left in itif ( !isEmptyDataObject(cache[ id ]) ) {
        return;
      }
    }

    // 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// Ensure that `cache` is not a window object #10080// 删除数据缓存对象cache[id]if ( jQuery.support.deleteExpando || !cache.setInterval ) {
      delete cache[ id ];
    } else {
      cache[ id ] = null;
    }

    // We destroyed the cache and need to eliminate the expando on the node to avoid// false lookups in the cache for entries that no longer exist// 删除DOM元素上扩展的jQuery.expando属性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 casesif ( jQuery.support.deleteExpando ) {
        delete elem[ internalKey ];
      } elseif ( elem.removeAttribute ) {
        elem.removeAttribute( internalKey );
      } else {
        elem[ internalKey ] = null;
      }
    }
  }
});

jQuery.fn.extend({
    removeData: function( key ) {
      returnthis.each(function() {
        jQuery.removeData( this, key );
      });
    }
});

// checks a cache object for emptinessfunctionisEmptyDataObject( obj ) {
  for ( var name in obj ) {

    // if the public data object is empty, the private is still emptyif ( name === "data" && jQuery.isEmptyObject( obj[name] ) ) {
      continue;
    }
    if ( name !== "toJSON" ) {
      return false;
    }
  }

  return true;
}

六、$.hasData(elem)

使用方法：

  
  
  



Results: 

$.hasData(elem) 源码解析：

$.extend({
    hasData: function( elem ) {
      elem = elem.nodeType ? jQuery.cache[ elem[jQuery.expando] ] : elem[ jQuery.expando ];
      return !!elem && !isEmptyDataObject( elem );
      // 如果关联的数据缓存对象存在，并且含有数据，则返回true, 否则返回false。 这里用两个逻辑非运算符! 把变量elem转换为布尔值
  }
});