jQuery源碼解析之jQuery.event.dispatch()

1、起源
jQuery.event.add()方法最終是用addEventListener綁定事件的：

elem.addEventListener( type, eventHandle )

而eventHandle方法正是等於jQuery.event.dispatch()：

if ( !( eventHandle = elemData.handle ) ) {
        eventHandle = elemData.handle = function( e ) {
          return typeof jQuery !== "undefined" && jQuery.event.triggered !== e.type ?
            jQuery.event.dispatch.apply( elem, arguments ) : undefined;
        };
   }

2、$.event.dispatch()
做用：
觸發綁定的事件的處理程序

源碼：

//源碼5472行
    //nativeEvent即原生MouseEvent
    //觸發事件的處理程序
    dispatch: function( nativeEvent ) {
      //修正event對象
      // Make a writable jQuery.Event from the native event object
      var event = jQuery.event.fix( nativeEvent );
      console.log(event,'event5479')
      
      var i, j, ret, matched, handleObj, handlerQueue,
        args = new Array( arguments.length ),
        //獲取click事件的處理程序集合，結構以下：
        //[
        // {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 1},
        // {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 2},
        // delegateCount:0,
        //]
        //從數據緩存中獲取事件處理集合
        handlers = ( dataPriv.get( this, "events" ) || {} )[ event.type ] || [],
        //click:{
        // trigger:{},
        // _default:{}
        //}
        special = jQuery.event.special[ event.type ] || {};
      // Use the fix-ed jQuery.Event rather than the (read-only) native event
      args[ 0 ] = event;

      for ( i = 1; i < arguments.length; i++ ) {
        args[ i ] = arguments[ i ];
      }
      //this即目標元素
      //delegateTarget:委託目標
      event.delegateTarget = this;
      //這段代碼壓根不會執行，由於全局搜索沒找到preDispatch
      // 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
      //結構以下
      //[{
      // elem:xx,
      // handlers:[
      //  {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 1},
      //  {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 2},
      //  ]
      //}]
      //獲取handler隊列
      handlerQueue = jQuery.event.handlers.call( this, event, handlers );
      // Run delegates first; they may want to stop propagation beneath us
      i = 0;
      //沒有執行stopPropagation()的話
      console.log(handlerQueue,'handlerQueue5525')
      //先判斷有沒有冒泡
      //再判斷有沒有阻止剩下的handler執行
      while ( ( matched = handlerQueue[ i++ ] ) && !event.isPropagationStopped() ) {
        console.log(matched,'matched5542')
        event.currentTarget = matched.elem;

        j = 0;
        //handleObj即單個事件處理程序
        //沒有執行stopImmediatePropagation()的話

        //依次執行每個handler
        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.rnamespace || event.rnamespace.test( handleObj.namespace ) ) {
            //經過循環將爲event添加handleObj和handleObj.data
            event.handleObj = handleObj;
            event.data = handleObj.data;
            //關鍵代碼，執行事件處理程序handler
            ret = ( ( jQuery.event.special[ handleObj.origType ] || {} ).handle ||
              handleObj.handler ).apply( matched.elem, args );
            if ( ret !== undefined ) {
              //event.result賦值ret
              if ( ( event.result = ret ) === false ) {
                //阻止默認行爲
                event.preventDefault();
                //阻止冒泡
                event.stopPropagation();
              }
            }
          }
        }
      }

      // Call the postDispatch hook for the mapped type
      if ( special.postDispatch ) {
        special.postDispatch.call( this, event );
      }
      console.log(handlers,'event5587')
      //undefined
      return event.result;
    },

解析：

（1）jQuery.event.fix()
做用：
將原生事件對象MouseEvent修正（fix）成jQuery的event對象

源碼：

//源碼5700行
    fix: function( originalEvent ) {
      //若是存在屬性id則原樣返回（由於已處理成jQueryEvent）
      return originalEvent[ jQuery.expando ] ?
        originalEvent :
        new jQuery.Event( originalEvent );
    },

解析：
能夠看到fix的本質是新建一個event對象，再看jQuery.Event()方法

（2）jQuery.Event()
源碼：

//click，false
  //修正event對象
  //源碼5777行
  //src即MouseEvent
  jQuery.Event = function( src, props ) {

    // Allow instantiation without the 'new' keyword
    if ( !( this instanceof jQuery.Event ) ) {
      return new jQuery.Event( src, props );
    }

    // Event object
    //src.type=click
    if ( src && src.type ) {
      //MouseEvent
      this.originalEvent = src;
      //click
      this.type = src.type;

      // Events bubbling up the document may have been marked as prevented
      // by a handler lower down the tree; reflect the correct value.
      this.isDefaultPrevented = src.defaultPrevented ||
      src.defaultPrevented === undefined &&

      // Support: Android <=2.3 only
      src.returnValue === false ?
        returnTrue :
        returnFalse;

      // Create target properties
      // Support: Safari <=6 - 7 only
      // Target should not be a text node (#504, #13143)
      this.target = ( src.target && src.target.nodeType === 3 ) ?
        src.target.parentNode :
        src.target;

      this.currentTarget = src.currentTarget;
      this.relatedTarget = src.relatedTarget;

      // Event type
    } else {
      //click
      this.type = src;
    }

    // Put explicitly provided properties onto the event object
    //false
    if ( props ) {
      jQuery.extend( this, props );
    }

    // Create a timestamp if incoming event doesn't have one
    this.timeStamp = src && src.timeStamp || Date.now();

    // Mark it as fixed
    //修正的標誌
    this[ jQuery.expando ] = true;
  };

解析：
簡單來講，就是把原生event事件上的經常使用屬性賦值到了jQuery的event上

$("#A").on("click" ,function (event) {
    //這個就是jQuery.Event()構建出的event
    console.log(event,"A被點擊了")
  })

jQuery的event結構以下：

//click的event就是jQuery.Event
jQuery.Event{
  handleObj{
    data:undefined,
    guid: 2,
    handler:function(){console.log("A被點擊了")}，
    namespace: "clickA",
    origType: "click",
    selector: "#B",
    type: "click.clickA",
  },
  originalEvent:{
    //就是MouseEvent
  },
  target:div#B,
  type: "click",
  delegateTarget: div#A,
  //fix 的標誌
  jQuery331087940272164138: true,
  currentTarget: div#A,
  isDefaultPrevented:xxx,
  timeStamp:Date.now(),
  isDefaultPrevented:function(){return false}
}

注意下originalEvent 和jQuery.extend( this, props )
前者就是原生MouseEvent，只是將原生event做爲jQuery.event的originalEvent屬性了；
後者是擴展屬性，若是開發者想額外加入自定義屬性的話。

（3）dataPriv.get( this, "events" )
注意：
jQuery的數據緩存裏的events和上面說的event是不一樣的

數據緩存的events是用來結構以下：

{
  click:[
    {
      type: "click", 
      origType: "click", 
      data: undefined, 
      handler: function(){console.log("B委託A綁定click事件")}, 
      guid: 1,
      namespace: "",
      needsContext: undefined,
      selector: #B,
    },
    {
      type: "click", 
      origType: "click", 
      data: undefined, 
      handler: function(){console.log("A綁定click事件")}, 
      guid: 2,
      namespace: "",
      needsContext: undefined,
      selector: undefined,
    },
    //事件委託的數量
    delegateCount:1,
  ],
  focus:[
    {
      type: "focus", 
      origType: "focus", 
      data: undefined, 
      handler: function(){console.log("A綁定focus事件")}, 
      guid: 3,
      namespace: "",
      needsContext: undefined,
      selector: undefined,
    },
     delegateCount:0,
  ],
}

（4） jQuery.event.handlers
做用：
獲取handler隊列

源碼：

jQuery.event = {
    //源碼5547行
    //組裝事件處理隊列  
    //event是fix過的MouseEvent, handlers  
    handlers: function( event, handlers ) {
      var i, handleObj, sel, matchedHandlers, matchedSelectors,
        handlerQueue = [],
        //0
        delegateCount = handlers.delegateCount,
        //目標元素
        cur = event.target;
      //handlers，第一個handler是委託事件，第二個handler是自身事件
      // Find delegate handlers
      if ( delegateCount &&

        // Support: IE <=9
        // Black-hole SVG <use> instance trees (trac-13180)
        cur.nodeType &&

        // Support: Firefox <=42
        // Suppress spec-violating clicks indicating a non-primary pointer button (trac-3861)
        // https://www.w3.org/TR/DOM-Level-3-Events/#event-type-click
        // Support: IE 11 only
        // ...but not arrow key "clicks" of radio inputs, which can have `button` -1 (gh-2343)
        !( event.type === "click" && event.button >= 1 ) ) {
        //循環，event.target冒泡到cur.parentNode，
        //直至綁定的目標元素#A，退出循環
        for ( ; cur !== this; cur = cur.parentNode || this ) {
          console.log(cur,'cur5618')
          // Don't check non-elements (#13208)
          // Don't process clicks on disabled elements (#6911, #8165, #11382, #11764)
          if ( cur.nodeType === 1 && !( event.type === "click" && cur.disabled === true ) ) {

            matchedHandlers = [];
            matchedSelectors = {};
            //在每一層，依次將委託的事件push進matchedHandlers
            //順序由下到上
            for ( i = 0; i < delegateCount; i++ ) {
              handleObj = handlers[ i ];
              //sel就是#C
              // Don't conflict with Object.prototype properties (#13203)
              sel = handleObj.selector + " ";

              if ( matchedSelectors[ sel ] === undefined ) {
                matchedSelectors[ sel ] = handleObj.needsContext ?
                  jQuery( sel, this ).index( cur ) > -1 :

                  //注意：jQuery.find()和jQuery().find()是不同的
                  jQuery.find( sel, this, null, [ cur ] ).length;
              }
              if ( matchedSelectors[ sel ] ) {
                matchedHandlers.push( handleObj );
              }
            }
            //而後將該層委託事件的數組放進handlers中
            //handlerQueue是全部層委託事件的集合
            if ( matchedHandlers.length ) {
              handlerQueue.push( { elem: cur, handlers: matchedHandlers } );
            }
          }
        }
      }

      // Add the remaining (directly-bound) handlers
      //最終冒泡到this元素
      cur = this;
      //1<2
      //將除委託事件的事件（如自身綁定的事件）放入handlerQueue中
      if ( delegateCount < handlers.length ) {
        handlerQueue.push( { elem: cur, handlers: handlers.slice( delegateCount ) } );
      }
      //[{
      // elem:xx,
      // handlers:[
      //  {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 1},
      //  {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 2},
      //  ]
      //}]
      return handlerQueue;
    },

}

解析：
注意下這個雙層循環，目的是把每一層的委託事件的集合push進matchedHandlers，而後再將matchedHandlers放進handlerQueue隊列

在處理完每層的委託事件後，將剩下的自身綁定事件再push進handlerQueue隊列中

也就是說，handlerQueue的結構以下：

[
//委託事件
  {
   elem:xx,
   handlers:[
      {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 1},
      {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 2},
    ]
  },
//自身綁定事件
  {
   elem:xxx,
   handlers:[
      {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 3},
      {type: "click", origType: "click", data: undefined, handler: ƒ, guid: 4},
    ]
  },
]

（5）回過頭再往下看dispatch 源碼，是兩個while循環，舉個例子來講明下：

<div id="A" style="background-color: deeppink">
  這是A

  <div id="B" style="background-color: bisque">
    這是B
  </div>

</div>

  $("#A").on("click" ,function (event) {
    console.log(event,"A被點擊了")
  })

  $("#A").on("click" ,"#B",function (event) {
    console.log(event,"點擊了B，即B委託A的click事件被點擊了")
  })

那麼會
先循環並執行委託事件，
即handler=function (event) {console.log(event,"點擊了B，即B委託A的click事件被點擊了")}，
再循環並執行目標元素自身綁定事件，
即handler=function (event) {console.log(event,"A被點擊了")}
前提是冒泡不被阻止

最後，執行click事件的事件處理程序的關鍵代碼以下：

handleObj.handler.apply( matched.elem, args )

---

（完）