jQuery中数据缓存$.data的用法及源码完全解析

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-
function dataAttr(elem,key,data){}
// 检查数据缓存对象是否为空
function isEmptyDataObject(obj){}
jQuery.extend({
   // 清空数据缓存对象

cleanData: function(elems){}
});

三、$.data(elem, name, data), $.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

<!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>jQuery.data demo</title>
 <style>
 div {
  color: blue;
 }
 span {
  color: red;
 }
 </style>
 <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>

<div>
 The values stored were
 <span></span>
 and
 <span></span>
</div>

<script>
var div = $( "div" )[ 0 ];
jQuery.data( div, "test", {
 first: 16,
 last: "pizza!"
});
$( "span:first" ).text( jQuery.data( div, "test" ).first );
$( "span:last" ).text( jQuery.data( div, "test" ).last );
</script>

</body>
</html>

$.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

<!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>jQuery.data demo</title>
 <style>
 div {
  margin: 5px;
  background: yellow;
 }
 button {
  margin: 5px;
  font-size: 14px;
 }
 p {
  margin: 5px;
  color: blue;
 }
 span {
  color: red;
 }
 </style>
 <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>

<div>A div</div>
<button>Get "blah" from the div</button>
<button>Set "blah" to "hello"</button>
<button>Set "blah" to 86</button>
<button>Remove "blah" from the div</button>
<p>The "blah" value of this div is <span>?</span></p>

<script>
$( "button" ).click( function() {
 var value,
  div = $( "div" )[ 0 ];
 switch ( $( "button" ).index( this ) ) {
 case 0 :
  value = jQuery.data( div, "blah" );
  break;
 case 1 :
  jQuery.data( div, "blah", "hello" );
  value = "Stored!";
  break;
 case 2 :
  jQuery.data( div, "blah", 86 );
  value = "Stored!";
  break;
 case 3 :
  jQuery.removeData( div, "blah" );
  value = "Removed!";
  break;
 }
 $( "span" ).text( "" + value );
});
</script>

</body>
</html>

$.data(elem, name, data), $.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 with no cache
   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 for each element since their data
   // ends up in the global cache
   if ( 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
  // cache in order to avoid key collisions between internal data and user-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events
  if ( 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;
  }

  // Check for both 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 use only.
 _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" data-last-value="43" data-hidden="true" data-options='{"name":"John"}'></div>

  $( "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
  } else if ( typeof key === "object" ) {
   return this.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 first
   if ( 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 {
   return this.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 ) {
  return this.each(function() {
   jQuery.removeData( this, key );
  });
 }
});

// 6. 函数dataAttr(elem, key, data)解析HTML5属性data-
function dataAttr( 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：

<!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>jQuery.removeData demo</title>
 <style>
 div {
  margin: 2px;
  color: blue;
 }
 span {
  color: red;
 }
 </style>
 <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>

<div>value1 before creation: <span></span></div>
<div>value1 after creation: <span></span></div>
<div>value1 after removal: <span></span></div>
<div>value2 after removal: <span></span></div>

<script>
var div = $( "div" )[ 0 ];
$( "span:eq(0)" ).text( "" + $( "div" ).data( "test1" ) ); //undefined
jQuery.data( div, "test1", "VALUE-1" );
jQuery.data( div, "test2", "VALUE-2" );
$( "span:eq(1)" ).text( "" + jQuery.data( div, "test1" ) ); // VALUE-1
jQuery.removeData( div, "test1" );
$( "span:eq(2)" ).text( "" + jQuery.data( div, "test1" ) ); // undefined
$( "span:eq(3)" ).text( "" + jQuery.data( div, "test2" ) ); // value2
</script>

</body>
</html>

<!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>removeData demo</title>
 <style>
 div {
  margin: 2px;
  color: blue;
 }
 span {
  color: red;
 }
 </style>
 <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>

<div>value1 before creation: <span></span></div>
<div>value1 after creation: <span></span></div>
<div>value1 after removal: <span></span></div>
<div>value2 after removal: <span></span></div>

<script>
$( "span:eq(0)" ).text( "" + $( "div" ).data( "test1" ) ); // undefined
$( "div" ).data( "test1", "VALUE-1" );
$( "div" ).data( "test2", "VALUE-2" );
$( "span:eq(1)" ).text( "" + $( "div").data( "test1" ) ); // VALUE-1
$( "div" ).removeData( "test1" );
$( "span:eq(2)" ).text( "" + $( "div" ).data( "test1" ) ); // undefined
$( "span:eq(3)" ).text( "" + $( "div" ).data( "test2" ) ); // VALUE-2
</script>

</body>
</html>

$.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 continuing
  if ( !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 keys
    if ( !jQuery.isArray( name ) ) {

     // try the string as a key before any manipulation
     if ( 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 destroyed
    if ( !( 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].data
  if ( !pvt ) {
   delete cache[ id ].data;

   // Don't destroy the parent cache unless the internal data object
   // had been the only thing left in it
   if ( !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 cases
   if ( jQuery.support.deleteExpando ) {
    delete elem[ internalKey ];
   } else if ( elem.removeAttribute ) {
    elem.removeAttribute( internalKey );
   } else {
    elem[ internalKey ] = null;
   }
  }
 }
});

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

// checks a cache object for emptiness
function isEmptyDataObject( obj ) {
 for ( var name in obj ) {

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

 return true;
}

六、$.hasData(elem)
使用方法：

<!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>jQuery.hasData demo</title>
 <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>

<p>Results: </p>

<script>
var $p = jQuery( "p" ), p = $p[ 0 ];
$p.append( jQuery.hasData( p ) + " " ); // false

$.data( p, "testing", 123 );
$p.append( jQuery.hasData( p ) + " " ); // true

$.removeData( p, "testing" );
$p.append( jQuery.hasData( p ) + " " ); // false

$p.on( "click", function() {} );
$p.append( jQuery.hasData( p ) + " " ); // true

$p.off( "click" );
$p.append( jQuery.hasData( p ) + " " ); // false
</script>

</body>
</html>
$.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
 }
});