引子

为什么Jquery能实现不传回调函数也能解绑事件？如下：

$("p").on("click",function(){alert("The paragraph was clicked.");
});$("#box1").off("click");

事件绑定解绑机制

调用on函数的时候，将生成一份事件数据，结构如下：

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

并将该数据加入到元素的缓存中。jquery中每个元素都可以有一个缓存（只有有需要的时候才生成），其实就是该元素的一个属性。jquery为每个元素的每种事件都建立一个队列，用来保存事件处理函数，所以可以对一个元素添加多个事件处理函数。缓存的结构如下：

"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}]}}
}

当要解绑事件的时候，如果没指定fn参数，jquery就会从该元素的缓存里拿到要解绑的事件的处理函数队列，从里面拿出fn参数,然后调用removeEventListener进行解绑。

源代码

代码注释可能不太清楚，可以复制出来看

jquery原型中的on,one,off方法：

事件绑定从这里开始

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 );} );}
} );

独立出来供one和on调用的on函数：

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 infojQuery().off( event );return origFn.apply( this, arguments );};// Use same guid so caller can remove using origFnfn.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 );} );
}

处理参数的代码也可以看一下，实现on("click",function(){})这样调用 on:function(types, selector, data, fn)也不会出错。其实就是内部判断，如果data, fn参数为空的时候，把selector赋给fn

event对象是事件绑定的一个关键对象：

这里处理把事件绑定到元素和把事件信息添加到元素缓存的工作：

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 unloadedreturn typeof jQuery !== "undefined" && jQuery.event.triggered !== e.type ?jQuery.event.dispatch.apply( elem, arguments ) : undefined;};}// 处理事件绑定，考虑到可能会通过空格分隔传入多个事件，这里要进行多事件处理types = ( types || "" ).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 handlersif ( !type ) {continue;}// If event changes its type, use the special event handlers for the changed typespecial = jQuery.event.special[ type ] || {};// If selector defined, determine special event api type, otherwise given typetype = ( selector ? special.delegateType : special.bindType ) || type;// Update special based on newly reset typespecial = jQuery.event.special[ type ] || {};// 事件回调函数的数据对象handleObj = jQuery.extend( {type: type,origType: origType,data: data,handler: handler,guid: handler.guid,selector: selector,needsContext: selector && jQuery.expr.match.needsContext.test( selector ),namespace: namespaces.join( "." )}, 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 falseif ( !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;}}
};

千万注意，对象和数组传的是引用！比如将事件数据保存到缓存的代码：

handlers = events[ type ] = [];if ( selector ) {handlers.splice( handlers.delegateCount++, 0, handleObj );
} else {handlers.push( handleObj );
}

handlers的改变，events[ type ]会同时改变。

dataPriv就是管理缓存的对象：

其工作就是给元素创建一个属性，这个属性是一个对象，然后把与这个元素相关的信息放到这个对象里面，缓存起来。这样需要使用到这个对象的信息时，只要知道这个对象就可以拿到：

function Data() {//从这里可以看出expando 只在创建Data类的实例的时候初始化一次，即每个元素都以expando作为缓存的名称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 assignmentif ( 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 keysif ( 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-whitespacekey = key in cache ?[ key ] :( key.match( rnotwhite ) || [] );}i = key.length;while ( i-- ) {delete cache[ key[ i ] ];}}// Remove the expando if there's no more dataif ( 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 );}
};