• jQuery之.on()方法

    还不是完全清楚如何使用.on()进行jQuery事件绑定的同学先看这里http://api.jquery.com/on/

    jQuery绑定事件的方法有几种，推荐使用.on()方法绑定，原因有两点：

    1.on()方法可以绑定动态添加到页面元素的事件

    比如动态添加到页面的DOM元素，用.on()方法绑定的事件不需要关心注册该事件的元素何时被添加进来，也不需要重复绑定。有的同学可能习惯于用.bind()、.live()或.delegate()，查看源码就会发现，它们实际上调用的都是.on()方法，并且.live()方法在jQuery1.9版本已经被移除。

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    bind: function( types, data, fn ) {
        return this.on( types, null, data, fn );
    },
     
    live: function( types, data, fn ) {
        jQuery( this.context ).on( types, this.selector, data, fn );
        return this;
    },
     
    delegate: function( selector, types, data, fn ) {
        return this.on( types, selector, data, fn );
    }

    移除.on()绑定的事件用.off()方法。

     

    2.on()方法绑定事件可以提升效率

    很多文章都提到了利用事件冒泡和代理来提升事件绑定的效率，大多都没列出具体的差别，所以为了求证，我做一个小测试。

    假设页面添加了5000个li，用chrome开发者工具Profiles测试页面载入时间。

    普通绑定（姑且这么称呼它）

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    $('li').click(function(){
        console.log(this)
    });

    绑定过程的执行时间

    2013-08-13_190358

    普通绑定相当于在5000li上面分别注册click事件，内存占用约4.2M，绑定时间约为72ms。

    .on()绑定

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    $(document).on('click', 'li', function(){
        console.log(this)
    })

    绑定过程的执行时间

    2013-08-13_191010

    .on()绑定利用事件代理，只在document上注册了一个click事件，内存占用约2.2M，绑定时间约为1ms。

    .on()源码分析

    .on()方法分析包含其调用的两个主要方法： 
    .add()进行事件注册

    .dispatch()进行事件代理

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    /* jQuery 1.10.2 */
    on: function( types, selector, data, fn, /*INTERNAL*/ one ) {
        var type, origFn;
     
        // Types can be a map of types/handlers
        if ( typeof types === "object" ) {
            // ( types-Object, selector, data )
            if ( typeof selector !== "string" ) {
                // ( types-Object, data )
                data = data || selector;
                selector = undefined;
            }
            // 遍历types对象，针对每一个属性绑定on()方法
            // 将types[type]作为fn传入
            for ( type in types ) {
                this.on( type, selector, data, types[ type ], one );
            }
            return this;
        }
     
        // 参数修正
        // jQuery这种参数修正的方法很好
        // 可以兼容多种参数形式
        // 可见在灵活调用的背后做了很多处理
        if ( data == null && fn == null ) {
            // ( types, fn )
            fn = selector;
            data = selector = undefined;
        } else if ( fn == null ) {
            if ( typeof selector === "string" ) {
                // ( types, selector, fn )
                fn = data;
                data = undefined;
            } else {
                // ( types, data, fn )
                fn = data;
                data = selector;
                selector = undefined;
            }
        }
        if ( fn === false ) {
            // fn传入false时，阻止该事件的默认行为
            // function returnFalse() {return false;}
            fn = returnFalse;
        } else if ( !fn ) {
            return this;
        }
     
        // one()调用on()
        if ( one === 1 ) {
            origFn = fn;
            fn = function( event ) {
                // Can use an empty set, since event contains the info
                // 用一个空jQuery对象，这样可以使用.off方法，
                // 并且event带有remove事件需要的信息
                jQuery().off( event );
                return origFn.apply( this, arguments );
            };
            // Use same guid so caller can remove using origFn
            // 事件删除依赖于guid
            fn.guid = origFn.guid || ( origFn.guid = jQuery.guid++ );
        }
     
        // 这里调用jQuery的each方法遍历调用on()方法的jQuery对象
        // 如$('li').on(...)则遍历每一个li传入add()
        // 推荐使用$(document).on()或者集合元素的父元素
        return this.each( function() {
            jQuery.event.add( this, types, fn, data, selector );
        });
    },
     
    // 事件注册
    add: function( elem, types, handler, data, selector ) {
        var tmp, events, t, handleObjIn,
            special, eventHandle, handleObj,
            handlers, type, namespaces, origType,
            elemData = jQuery._data( elem );
     
        // Don't attach events to noData or
        // text/comment nodes (but allow plain objects)
        // 不符合绑定条件的节点
        if ( !elemData ) {
            return;
        }
     
        // Caller can pass in an object of custom data in lieu of the handler
        // 传入的handler为事件对象
        if ( handler.handler ) {
            handleObjIn = handler;
            handler = handleObjIn.handler;
            selector = handleObjIn.selector;
        }
     
        // Make sure that the handler has a unique ID,
        // used to find/remove it later
        // 为handler分配一个ID，用于之后的查找或删除
        if ( !handler.guid ) {
            handler.guid = jQuery.guid++;
        }
     
        // Init the element's event structure and main handler,
        // if this is the first
        // 初始化events结构
        if ( !(events = elemData.events) ) {
            events = elemData.events = {};
        }
        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 !== core_strundefined &&
                (!e || jQuery.event.triggered !== e.type) ?
                    jQuery.event.dispatch.apply( eventHandle.elem, arguments ) :
                    undefined;
            };
            // Add elem as a property of the handle fn
            // to prevent a memory leak with IE non-native events
            // 添加elem为eventHandle的属性，防止IE非本地事件的内存泄露？
            // 搜索整个源码，只有110行用到了eventHandle.elem
            eventHandle.elem = elem;
        }
     
        // Handle multiple events separated by a space
        // 处理多个以空格分隔的事件类型
        types = ( types || "" ).match( core_rnotwhite ) || [""];
        t = types.length;
        while ( t-- ) {
            tmp = rtypenamespace.exec( types[t] ) || [];
            type = origType = tmp[1];
            // 存储所有命名空间
            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
            // 对于改变了事件类型的特殊事件
            special = jQuery.event.special[ type ] || {};
     
            // If selector defined, determine special event api type,
            // otherwise given type
            type = ( selector ? special.delegateType : special.bindType ) || type;
     
            // Update special based on newly reset type
            special = jQuery.event.special[ type ] || {};
     
            // handleObj is passed to all event handlers
            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 );
     
            // Init the event handler queue if we're the first
            // 初始化handler队列，只初始化一次
            if ( !(handlers = events[ type ]) ) {
                handlers = events[ type ] = [];
                handlers.delegateCount = 0;
     
                // Only use addEventListener/attachEvent
                // if the special events handler returns false
                if ( !special.setup ||
                special.setup.call( elem, data, namespaces, eventHandle ) === false ) {
                    // Bind the global event handler to the element
                    // 二级DOM事件/IE事件模型
                    // eventHandle会调用jQuery.event.dispatch进行事件代理
                    if ( elem.addEventListener ) {
                        elem.addEventListener( type, eventHandle, false );
     
                    } else if ( elem.attachEvent ) {
                        elem.attachEvent( "on" + type, eventHandle );
                    }
                }
            }
     
            if ( special.add ) {
                special.add.call( elem, handleObj );
     
                if ( !handleObj.handler.guid ) {
                    handleObj.handler.guid = handler.guid;
                }
            }
     
            // Add to the element's handler list, delegates in front
            if ( selector ) {
                handlers.splice( handlers.delegateCount++, 0, handleObj );
            } else {
                handlers.push( handleObj );
            }
     
            // Keep track of which events have ever been used,
            // for event optimization
            // 跟踪每个事件是否被使用过，为了事件优化
            jQuery.event.global[ type ] = true;
        }
     
        // Nullify elem to prevent memory leaks in IE
        // 将变量置空，防止循环引用导致IE内存泄露
        elem = null;
    },
     
    // 事件代理
    dispatch: function( event ) {
     
        // Make a writable jQuery.Event from the native event object
        // jQuery定义的event对象，兼容标准事件模型与IE事件模型
        event = jQuery.event.fix( event );
     
        var i, ret, handleObj, matched, j,
            handlerQueue = [],
            args = core_slice.call( arguments ),
            handlers = ( jQuery._data( this, "events" ) || {} )[ event.type ] || [],
            special = jQuery.event.special[ event.type ] || {};
     
        // Use the fix-ed jQuery.Event rather than the (read-only) native event
        // 使用jQuery.Event代替浏览器的event
        args[0] = event;
        // 事件的代理节点，比如document
        event.delegateTarget = this;
     
        // Call the preDispatch hook for the mapped type,
        // and let it bail if desired
        if ( special.preDispatch &&
            special.preDispatch.call( this, event ) === false ) {
            return;
        }
     
        // Determine handlers
        // 遍历事件发生节点至代理节点之间的所有节点
        // 匹配每一个发生节点=?绑定节点
        handlerQueue = jQuery.event.handlers.call( this, event, handlers );
     
        // Run delegates first; they may want to stop propagation beneath us
        i = 0;
        // 遍历匹配的节点，并且没有被阻止冒泡
        while ( (matched = handlerQueue[ i++ ]) && !event.isPropagationStopped() ) {
            event.currentTarget = matched.elem;
     
            j = 0;
            while ( (handleObj = matched.handlers[ j++ ]) &&
             !event.isImmediatePropagationStopped() ) {
     
                // Triggered event must either 1) have no namespace, or
                // 2) have namespace(s) a subset or equal to those
                // in the bound event (both can have no namespace).
                if ( !event.namespace_re ||
                 event.namespace_re.test( handleObj.namespace ) ) {
     
                    event.handleObj = handleObj;
                    event.data = handleObj.data;
     
                    // 传入绑定事件的具体节点，调用事件发生函数
                    ret = ( (jQuery.event.special[ handleObj.origType ] || {}).handle ||
                     handleObj.handler )
                            .apply( matched.elem, args );
     
                    if ( ret !== undefined ) {
                        if ( (event.result = ret) === false ) {
                            event.preventDefault();
                            event.stopPropagation();
                        }
                    }
                }
            }
        }
     
        // Call the postDispatch hook for the mapped type
        if ( special.postDispatch ) {
            special.postDispatch.call( this, event );
        }
     
        return event.result;
    }
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  • 原文地址：https://www.cnblogs.com/Alex80/p/4516397.html
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