$("p").on("click",function(){
  alert("The paragraph was clicked.");
});

$("#box1").off("click");

{
  type: type,
  origType: origType,
  data: data,
  handler: handler,
  guid: guid,
  selector: selector,
  needsContext: needsContext,
  namespace: namespace
}

"div#box":{ //元素
  "Jquery623873":{ //元素的缓存
    "events":{ 
      "click":[
       {  //元素click事件的事件数据
             type: type,
             origType: origType,
             data: data,
             handler: handler,
             guid: guid,
             selector: selector,
             needsContext: needsContext,
             namespace: namespace
               }
      ],
      "mousemove":[
       {
             type: type,
             origType: origType,
             data: data,
             handler: handler,
             guid: guid,
             selector: selector,
             needsContext: needsContext,
             namespace: namespace
               }
      ]
    }
  }
}

jQuery.fn.extend( {

  on: function( types, selector, data, fn ) {
    return on( this, types, selector, data, fn );
  },
  one: function( types, selector, data, fn ) {
    return on( this, types, selector, data, fn, 1 );
  },
  off: function( types, selector, fn ) {

    //此处省略处理参数的代码

    return this.each( function() {
      jQuery.event.remove( this, types, fn, selector );
    } );
  }
} );

function on( elem, types, selector, data, fn, one ) {
  var origFn, type;

  //此处省略处理参数的代码

  //是否是通过one绑定，是的话使用一个函数代理当前事件回调函数，代理函数只执行一次
  //这里使用到了代理模式
  if ( one === 1 ) {   
    origFn = fn;
    fn = function( event ) {

      // Can use an empty set, since event contains the info
      jQuery().off( event );
      return origFn.apply( this, arguments );
    };

    // Use same guid so caller can remove using origFn
    fn.guid = origFn.guid || ( origFn.guid = jQuery.guid++ );
  }

  /************************************************
  *** jquery将所有选择到的元素到放到一个数组里，然后
  *** 对每个元素到使用event对象的add方法绑定事件
  *************************************************/
  return elem.each( function() {
    jQuery.event.add( this, types, fn, data, selector );
  } );
}

jQuery.event = {

  add: function( elem, types, handler, data, selector ) {

    var handleObjIn, eventHandle, tmp,
      events, t, handleObj,
      special, handlers, type, namespaces, origType,
      elemData = dataPriv.get( elem );  //这句将检查elem是否被缓存，如果没有将会创建一个缓存添加到elem元素上。形式诸如：elem["jQuery310057655476080253721"] = {}


    // Don't attach events to noData or text/comment nodes (but allow plain objects)
    if ( !elemData ) {
      return;
    }


    //用户可以传入一个自定义数据对象来代替事件回调函数，将事件回调函数放在这个数据对象的handler属性里
    if ( handler.handler ) {
      handleObjIn = handler;
      handler = handleObjIn.handler;
      selector = handleObjIn.selector;
    }

    //每个事件回调函数都会生成一个唯一的id，以后find/remove的时候会用到

    if ( !handler.guid ) {
      handler.guid = jQuery.guid++;
    }

    // 如果元素第一次绑定事件，则初始化元素的事件数据结构和主回调函数（main）
    //说明：每个元素有一个主回调函数，作为绑定多个事件到该元素时的回调的入口
    if ( !( events = elemData.events ) ) {
      events = elemData.events = {};
    }
    //这里就是初始化主回调函数的代码
    if ( !( eventHandle = elemData.handle ) ) {
      eventHandle = elemData.handle = function( e ) {

        // Discard the second event of a jQuery.event.trigger() and
        // when an event is called after a page has unloaded
        return typeof jQuery !== "undefined" && jQuery.event.triggered !== e.type "" ).match( rnotwhite ) || [ "" ];
    t = types.length;
    while ( t-- ) {
      tmp = rtypenamespace.exec( types[ t ] ) || []; 
      type = origType = tmp[ 1 ];
      namespaces = ( tmp[ 2 ] || "" ).split( "." ).sort();

      // There *must* be a type, no attaching namespace-only handlers
      if ( !type ) {
        continue;
      }

      // If event changes its type, use the special event handlers for the changed type
      special = jQuery.event.special[ type ] || {};

      // If selector defined, determine special event api type, otherwise given type
      type = ( selector "." )
      }, handleObjIn );

      // 加入第一次绑定该类事件，会初始化一个数组作为事件回调函数队列，每个元素的每一种事件有一个队列
      if ( !( handlers = events[ type ] ) ) {
        handlers = events[ type ] = [];
        handlers.delegateCount = 0;

        // Only use addEventListener if the special events handler returns false
        if ( !special.setup ||
          special.setup.call( elem, data, namespaces, eventHandle ) === false ) {

          if ( elem.addEventListener ) {
            elem.addEventListener( type, eventHandle );
          }
        }
      }

      if ( special.add ) {
        special.add.call( elem, handleObj );

        if ( !handleObj.handler.guid ) {
          handleObj.handler.guid = handler.guid;
        }
      }

      // 加入到事件回调函数队列
      if ( selector ) {
        handlers.splice( handlers.delegateCount++, 0, handleObj );
      } else {
        handlers.push( handleObj );
      }

      // Keep track of which events have ever been used, for event optimization
      // 用来追踪哪些事件从未被使用，用以优化
      jQuery.event.global[ type ] = true;
    }

  }
};

handlers = events[ type ] = [];

if ( selector ) {
  handlers.splice( handlers.delegateCount++, 0, handleObj );
} else {
  handlers.push( handleObj );
}

function Data() {
  this.expando = jQuery.expando + Data.uid++;
}

Data.uid = 1;

//删除部分没用到代码
Data.prototype = {

  cache: function( owner ) {

    // 取出缓存，可见缓存就是目标对象的一个属性
    var value = owner[ this.expando ];

    // 如果对象还没有缓存，则创建一个
    if ( !value ) {
      value = {};

      // We can accept data for non-element nodes in modern browsers,
      // but we should not, see #8335.
      // Always return an empty object.
      if ( acceptData( owner ) ) {

        // If it is a node unlikely to be stringify-ed or looped over
        // use plain assignment
        if ( owner.nodeType ) {
          owner[ this.expando ] = value;

        // Otherwise secure it in a non-enumerable property
        // configurable must be true to allow the property to be
        // deleted when data is removed
        } else {
          Object.defineProperty( owner, this.expando, {
            value: value,
            configurable: true
          } );
        }
      }
    }

    return value;
  },
  get: function( owner, key ) {
    return key === undefined ?
      this.cache( owner ) :

      // Always use camelCase key (gh-2257) 驼峰命名
      owner[ this.expando ] && owner[ this.expando ][ jQuery.camelCase( key ) ];
  },
  remove: function( owner, key ) {
    var i,
      cache = owner[ this.expando ];

    if ( cache === undefined ) {
      return;
    }

    if ( key !== undefined ) {

      // Support array or space separated string of keys
      if ( jQuery.isArray( key ) ) {

        // If key is an array of keys...
        // We always set camelCase keys, so remove that.
        key = key.map( jQuery.camelCase );
      } else {
        key = jQuery.camelCase( key );

        // If a key with the spaces exists, use it.
        // Otherwise, create an array by matching non-whitespace
        key = key in cache ?
          [ key ] :
          ( key.match( rnotwhite ) || [] );
      }

      i = key.length;

      while ( i-- ) {
        delete cache[ key[ i ] ];
      }
    }

    // Remove the expando if there's no more data
    if ( key === undefined || jQuery.isEmptyObject( cache ) ) {

      // Support: Chrome <=35 - 45
      // Webkit & Blink performance suffers when deleting properties
      // from DOM nodes, so set to undefined instead
      // https://bugs.chromium.org/p/chromium/issues/detail?id=378607 (bug restricted)
      if ( owner.nodeType ) {
        owner[ this.expando ] = undefined;
      } else {
        delete owner[ this.expando ];
      }
    }
  },
  hasData: function( owner ) {
    var cache = owner[ this.expando ];
    return cache !== undefined && !jQuery.isEmptyObject( cache );
  }
};