Re-factor the EventBus to allow servicing of "external" event listeners *after* the viewer components have updated

Since the goal has always been, essentially since the `EventBus` abstraction was added, to remove all dispatching of DOM events[1] from the viewer components this patch tries to address one thing that came up when updating the examples:
The DOM events are always dispatched last, and it's thus guaranteed that all internal event listeners have been invoked first.
However, there's no such guarantees with the general `EventBus` functionality and the order in which event listeners are invoked is *not* specified. With the promotion of the `EventBus` in the examples, over DOM events, it seems like a good idea to at least *try* to keep this ordering invariant[2] intact.

Obviously this won't prevent anyone from manually calling the new *internal* viewer component methods on the `EventBus`, but hopefully that won't be too common since any existing third-party code would obviously use the `on`/`off` methods and that all of the examples shows the *correct* usage (which should be similarily documented on the "Third party viewer usage" Wiki-page).

---
[1] Looking at the various Firefox-tests, I'm not sure that it'll be possible to (easily) re-write all of them to not rely on DOM events (since getting access to `PDFViewerApplication` might be generally difficult/messy depending on scopes).
In any case, even if technically feasible, it would most likely add *a lot* of complication that may not be desireable in the various Firefox-tests. All-in-all, I'd be fine with keeping the DOM events only for the `MOZCENTRAL` target and gated on `Cu.isInAutomation` (or similar) rather than a preference.

[2] I wouldn't expect any *real* bugs in a custom implementation, simply based on event ordering, but it nonetheless seem like a good idea if any "external" events are still handled last.

web/app.js

@@ -1597,45 +1597,45 @@ const PDFViewerApplication = {
     _boundEvents.beforePrint = this.beforePrint.bind(this);
     _boundEvents.afterPrint = this.afterPrint.bind(this);
 
-    eventBus.on("resize", webViewerResize);
-    eventBus.on("hashchange", webViewerHashchange);
-    eventBus.on("beforeprint", _boundEvents.beforePrint);
-    eventBus.on("afterprint", _boundEvents.afterPrint);
-    eventBus.on("pagerendered", webViewerPageRendered);
-    eventBus.on("updateviewarea", webViewerUpdateViewarea);
-    eventBus.on("pagechanging", webViewerPageChanging);
-    eventBus.on("scalechanging", webViewerScaleChanging);
-    eventBus.on("rotationchanging", webViewerRotationChanging);
-    eventBus.on("sidebarviewchanged", webViewerSidebarViewChanged);
-    eventBus.on("pagemode", webViewerPageMode);
-    eventBus.on("namedaction", webViewerNamedAction);
-    eventBus.on("presentationmodechanged", webViewerPresentationModeChanged);
-    eventBus.on("presentationmode", webViewerPresentationMode);
-    eventBus.on("openfile", webViewerOpenFile);
-    eventBus.on("print", webViewerPrint);
-    eventBus.on("download", webViewerDownload);
-    eventBus.on("firstpage", webViewerFirstPage);
-    eventBus.on("lastpage", webViewerLastPage);
-    eventBus.on("nextpage", webViewerNextPage);
-    eventBus.on("previouspage", webViewerPreviousPage);
-    eventBus.on("zoomin", webViewerZoomIn);
-    eventBus.on("zoomout", webViewerZoomOut);
-    eventBus.on("zoomreset", webViewerZoomReset);
-    eventBus.on("pagenumberchanged", webViewerPageNumberChanged);
-    eventBus.on("scalechanged", webViewerScaleChanged);
-    eventBus.on("rotatecw", webViewerRotateCw);
-    eventBus.on("rotateccw", webViewerRotateCcw);
-    eventBus.on("switchscrollmode", webViewerSwitchScrollMode);
-    eventBus.on("scrollmodechanged", webViewerScrollModeChanged);
-    eventBus.on("switchspreadmode", webViewerSwitchSpreadMode);
-    eventBus.on("spreadmodechanged", webViewerSpreadModeChanged);
-    eventBus.on("documentproperties", webViewerDocumentProperties);
-    eventBus.on("find", webViewerFind);
-    eventBus.on("findfromurlhash", webViewerFindFromUrlHash);
-    eventBus.on("updatefindmatchescount", webViewerUpdateFindMatchesCount);
-    eventBus.on("updatefindcontrolstate", webViewerUpdateFindControlState);
+    eventBus._on("resize", webViewerResize);
+    eventBus._on("hashchange", webViewerHashchange);
+    eventBus._on("beforeprint", _boundEvents.beforePrint);
+    eventBus._on("afterprint", _boundEvents.afterPrint);
+    eventBus._on("pagerendered", webViewerPageRendered);
+    eventBus._on("updateviewarea", webViewerUpdateViewarea);
+    eventBus._on("pagechanging", webViewerPageChanging);
+    eventBus._on("scalechanging", webViewerScaleChanging);
+    eventBus._on("rotationchanging", webViewerRotationChanging);
+    eventBus._on("sidebarviewchanged", webViewerSidebarViewChanged);
+    eventBus._on("pagemode", webViewerPageMode);
+    eventBus._on("namedaction", webViewerNamedAction);
+    eventBus._on("presentationmodechanged", webViewerPresentationModeChanged);
+    eventBus._on("presentationmode", webViewerPresentationMode);
+    eventBus._on("openfile", webViewerOpenFile);
+    eventBus._on("print", webViewerPrint);
+    eventBus._on("download", webViewerDownload);
+    eventBus._on("firstpage", webViewerFirstPage);
+    eventBus._on("lastpage", webViewerLastPage);
+    eventBus._on("nextpage", webViewerNextPage);
+    eventBus._on("previouspage", webViewerPreviousPage);
+    eventBus._on("zoomin", webViewerZoomIn);
+    eventBus._on("zoomout", webViewerZoomOut);
+    eventBus._on("zoomreset", webViewerZoomReset);
+    eventBus._on("pagenumberchanged", webViewerPageNumberChanged);
+    eventBus._on("scalechanged", webViewerScaleChanged);
+    eventBus._on("rotatecw", webViewerRotateCw);
+    eventBus._on("rotateccw", webViewerRotateCcw);
+    eventBus._on("switchscrollmode", webViewerSwitchScrollMode);
+    eventBus._on("scrollmodechanged", webViewerScrollModeChanged);
+    eventBus._on("switchspreadmode", webViewerSwitchSpreadMode);
+    eventBus._on("spreadmodechanged", webViewerSpreadModeChanged);
+    eventBus._on("documentproperties", webViewerDocumentProperties);
+    eventBus._on("find", webViewerFind);
+    eventBus._on("findfromurlhash", webViewerFindFromUrlHash);
+    eventBus._on("updatefindmatchescount", webViewerUpdateFindMatchesCount);
+    eventBus._on("updatefindcontrolstate", webViewerUpdateFindControlState);
     if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
-      eventBus.on("fileinputchange", webViewerFileInputChange);
+      eventBus._on("fileinputchange", webViewerFileInputChange);
     }
   },
 
@@ -1671,45 +1671,45 @@ const PDFViewerApplication = {
   unbindEvents() {
     const { eventBus, _boundEvents } = this;
 
-    eventBus.off("resize", webViewerResize);
-    eventBus.off("hashchange", webViewerHashchange);
-    eventBus.off("beforeprint", _boundEvents.beforePrint);
-    eventBus.off("afterprint", _boundEvents.afterPrint);
-    eventBus.off("pagerendered", webViewerPageRendered);
-    eventBus.off("updateviewarea", webViewerUpdateViewarea);
-    eventBus.off("pagechanging", webViewerPageChanging);
-    eventBus.off("scalechanging", webViewerScaleChanging);
-    eventBus.off("rotationchanging", webViewerRotationChanging);
-    eventBus.off("sidebarviewchanged", webViewerSidebarViewChanged);
-    eventBus.off("pagemode", webViewerPageMode);
-    eventBus.off("namedaction", webViewerNamedAction);
-    eventBus.off("presentationmodechanged", webViewerPresentationModeChanged);
-    eventBus.off("presentationmode", webViewerPresentationMode);
-    eventBus.off("openfile", webViewerOpenFile);
-    eventBus.off("print", webViewerPrint);
-    eventBus.off("download", webViewerDownload);
-    eventBus.off("firstpage", webViewerFirstPage);
-    eventBus.off("lastpage", webViewerLastPage);
-    eventBus.off("nextpage", webViewerNextPage);
-    eventBus.off("previouspage", webViewerPreviousPage);
-    eventBus.off("zoomin", webViewerZoomIn);
-    eventBus.off("zoomout", webViewerZoomOut);
-    eventBus.off("zoomreset", webViewerZoomReset);
-    eventBus.off("pagenumberchanged", webViewerPageNumberChanged);
-    eventBus.off("scalechanged", webViewerScaleChanged);
-    eventBus.off("rotatecw", webViewerRotateCw);
-    eventBus.off("rotateccw", webViewerRotateCcw);
-    eventBus.off("switchscrollmode", webViewerSwitchScrollMode);
-    eventBus.off("scrollmodechanged", webViewerScrollModeChanged);
-    eventBus.off("switchspreadmode", webViewerSwitchSpreadMode);
-    eventBus.off("spreadmodechanged", webViewerSpreadModeChanged);
-    eventBus.off("documentproperties", webViewerDocumentProperties);
-    eventBus.off("find", webViewerFind);
-    eventBus.off("findfromurlhash", webViewerFindFromUrlHash);
-    eventBus.off("updatefindmatchescount", webViewerUpdateFindMatchesCount);
-    eventBus.off("updatefindcontrolstate", webViewerUpdateFindControlState);
+    eventBus._off("resize", webViewerResize);
+    eventBus._off("hashchange", webViewerHashchange);
+    eventBus._off("beforeprint", _boundEvents.beforePrint);
+    eventBus._off("afterprint", _boundEvents.afterPrint);
+    eventBus._off("pagerendered", webViewerPageRendered);
+    eventBus._off("updateviewarea", webViewerUpdateViewarea);
+    eventBus._off("pagechanging", webViewerPageChanging);
+    eventBus._off("scalechanging", webViewerScaleChanging);
+    eventBus._off("rotationchanging", webViewerRotationChanging);
+    eventBus._off("sidebarviewchanged", webViewerSidebarViewChanged);
+    eventBus._off("pagemode", webViewerPageMode);
+    eventBus._off("namedaction", webViewerNamedAction);
+    eventBus._off("presentationmodechanged", webViewerPresentationModeChanged);
+    eventBus._off("presentationmode", webViewerPresentationMode);
+    eventBus._off("openfile", webViewerOpenFile);
+    eventBus._off("print", webViewerPrint);
+    eventBus._off("download", webViewerDownload);
+    eventBus._off("firstpage", webViewerFirstPage);
+    eventBus._off("lastpage", webViewerLastPage);
+    eventBus._off("nextpage", webViewerNextPage);
+    eventBus._off("previouspage", webViewerPreviousPage);
+    eventBus._off("zoomin", webViewerZoomIn);
+    eventBus._off("zoomout", webViewerZoomOut);
+    eventBus._off("zoomreset", webViewerZoomReset);
+    eventBus._off("pagenumberchanged", webViewerPageNumberChanged);
+    eventBus._off("scalechanged", webViewerScaleChanged);
+    eventBus._off("rotatecw", webViewerRotateCw);
+    eventBus._off("rotateccw", webViewerRotateCcw);
+    eventBus._off("switchscrollmode", webViewerSwitchScrollMode);
+    eventBus._off("scrollmodechanged", webViewerScrollModeChanged);
+    eventBus._off("switchspreadmode", webViewerSwitchSpreadMode);
+    eventBus._off("spreadmodechanged", webViewerSpreadModeChanged);
+    eventBus._off("documentproperties", webViewerDocumentProperties);
+    eventBus._off("find", webViewerFind);
+    eventBus._off("findfromurlhash", webViewerFindFromUrlHash);
+    eventBus._off("updatefindmatchescount", webViewerUpdateFindMatchesCount);
+    eventBus._off("updatefindcontrolstate", webViewerUpdateFindControlState);
     if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
-      eventBus.off("fileinputchange", webViewerFileInputChange);
+      eventBus._off("fileinputchange", webViewerFileInputChange);
     }
 
     _boundEvents.beforePrint = null;

web/base_viewer.js

@@ -426,7 +426,7 @@ class BaseViewer {
       // evicted from the buffer and destroyed even if we pause its rendering.
       this._buffer.push(pageView);
     };
-    this.eventBus.on("pagerender", this._onBeforeDraw);
+    this.eventBus._on("pagerender", this._onBeforeDraw);
 
     this._onAfterDraw = evt => {
       if (evt.cssTransform || onePageRenderedCapability.settled) {
@@ -434,10 +434,10 @@ class BaseViewer {
       }
       onePageRenderedCapability.resolve();
 
-      this.eventBus.off("pagerendered", this._onAfterDraw);
+      this.eventBus._off("pagerendered", this._onAfterDraw);
       this._onAfterDraw = null;
     };
-    this.eventBus.on("pagerendered", this._onAfterDraw);
+    this.eventBus._on("pagerendered", this._onAfterDraw);
 
     // Fetch a single page so we can get a viewport that will be the default
     // viewport for all pages
@@ -582,11 +582,11 @@ class BaseViewer {
     this._spreadMode = SpreadMode.NONE;
 
     if (this._onBeforeDraw) {
-      this.eventBus.off("pagerender", this._onBeforeDraw);
+      this.eventBus._off("pagerender", this._onBeforeDraw);
       this._onBeforeDraw = null;
     }
     if (this._onAfterDraw) {
-      this.eventBus.off("pagerendered", this._onAfterDraw);
+      this.eventBus._off("pagerendered", this._onAfterDraw);
       this._onAfterDraw = null;
     }
     // Remove the pages from the DOM...

web/pdf_attachment_viewer.js

@@ -43,7 +43,7 @@ class PDFAttachmentViewer {
 
     this.reset();
 
-    this.eventBus.on(
+    this.eventBus._on(
       "fileattachmentannotation",
       this._appendAttachment.bind(this)
     );

web/pdf_cursor_tools.js

@@ -122,11 +122,11 @@ class PDFCursorTools {
    * @private
    */
   _addEventListeners() {
-    this.eventBus.on("switchcursortool", evt => {
+    this.eventBus._on("switchcursortool", evt => {
       this.switchTool(evt.tool);
     });
 
-    this.eventBus.on("presentationmodechanged", evt => {
+    this.eventBus._on("presentationmodechanged", evt => {
       if (evt.switchInProgress) {
         return;
       }

web/pdf_document_properties.js

@@ -84,10 +84,10 @@ class PDFDocumentProperties {
     );
 
     if (eventBus) {
-      eventBus.on("pagechanging", evt => {
+      eventBus._on("pagechanging", evt => {
         this._currentPageNumber = evt.pageNumber;
       });
-      eventBus.on("rotationchanging", evt => {
+      eventBus._on("rotationchanging", evt => {
         this._pagesRotation = evt.pagesRotation;
       });
     }

web/pdf_find_bar.js

@@ -83,7 +83,7 @@ class PDFFindBar {
       this.dispatchEvent("entirewordchange");
     });
 
-    this.eventBus.on("resize", this._adjustWidth.bind(this));
+    this.eventBus._on("resize", this._adjustWidth.bind(this));
   }
 
   reset() {

web/pdf_find_controller.js

@@ -72,7 +72,7 @@ class PDFFindController {
     this._eventBus = eventBus || getGlobalEventBus();
 
     this._reset();
-    eventBus.on("findbarclose", this._onFindBarClose.bind(this));
+    eventBus._on("findbarclose", this._onFindBarClose.bind(this));
   }
 
   get highlightMatches() {

web/pdf_history.js

@@ -69,17 +69,17 @@ class PDFHistory {
     this._isViewerInPresentationMode = false;
     // Ensure that we don't miss either a 'presentationmodechanged' or a
     // 'pagesinit' event, by registering the listeners immediately.
-    this.eventBus.on("presentationmodechanged", evt => {
+    this.eventBus._on("presentationmodechanged", evt => {
       this._isViewerInPresentationMode = evt.active || evt.switchInProgress;
     });
-    this.eventBus.on("pagesinit", () => {
+    this.eventBus._on("pagesinit", () => {
       this._isPagesLoaded = false;
 
       const onPagesLoaded = evt => {
-        this.eventBus.off("pagesloaded", onPagesLoaded);
+        this.eventBus._off("pagesloaded", onPagesLoaded);
         this._isPagesLoaded = !!evt.pagesCount;
       };
-      this.eventBus.on("pagesloaded", onPagesLoaded);
+      this.eventBus._on("pagesloaded", onPagesLoaded);
     });
   }
 
@@ -684,7 +684,7 @@ class PDFHistory {
       pageHide: this._pageHide.bind(this),
     };
 
-    this.eventBus.on("updateviewarea", this._boundEvents.updateViewarea);
+    this.eventBus._on("updateviewarea", this._boundEvents.updateViewarea);
     window.addEventListener("popstate", this._boundEvents.popState);
     window.addEventListener("pagehide", this._boundEvents.pageHide);
   }
@@ -696,7 +696,7 @@ class PDFHistory {
     if (!this._boundEvents) {
       return; // The event listeners were already removed.
     }
-    this.eventBus.off("updateviewarea", this._boundEvents.updateViewarea);
+    this.eventBus._off("updateviewarea", this._boundEvents.updateViewarea);
     window.removeEventListener("popstate", this._boundEvents.popState);
     window.removeEventListener("pagehide", this._boundEvents.pageHide);
 

web/pdf_outline_viewer.js

@@ -40,7 +40,7 @@ class PDFOutlineViewer {
 
     this.reset();
 
-    eventBus.on("toggleoutlinetree", this.toggleOutlineTree.bind(this));
+    eventBus._on("toggleoutlinetree", this.toggleOutlineTree.bind(this));
   }
 
   reset() {

web/pdf_sidebar.js

@@ -430,7 +430,7 @@ class PDFSidebar {
     });
 
     // Disable/enable views.
-    this.eventBus.on("outlineloaded", evt => {
+    this.eventBus._on("outlineloaded", evt => {
       const outlineCount = evt.outlineCount;
 
       this.outlineButton.disabled = !outlineCount;
@@ -444,7 +444,7 @@ class PDFSidebar {
       }
     });
 
-    this.eventBus.on("attachmentsloaded", evt => {
+    this.eventBus._on("attachmentsloaded", evt => {
       if (evt.attachmentsCount) {
         this.attachmentsButton.disabled = false;
 
@@ -473,7 +473,7 @@ class PDFSidebar {
     });
 
     // Update the thumbnailViewer, if visible, when exiting presentation mode.
-    this.eventBus.on("presentationmodechanged", evt => {
+    this.eventBus._on("presentationmodechanged", evt => {
       if (!evt.active && !evt.switchInProgress && this.isThumbnailViewVisible) {
         this._updateThumbnailViewer();
       }

web/pdf_sidebar_resizer.js

@@ -151,11 +151,11 @@ class PDFSidebarResizer {
       window.addEventListener("mouseup", _boundEvents.mouseUp);
     });
 
-    this.eventBus.on("sidebarviewchanged", evt => {
+    this.eventBus._on("sidebarviewchanged", evt => {
       this.sidebarOpen = !!(evt && evt.view);
     });
 
-    this.eventBus.on("resize", evt => {
+    this.eventBus._on("resize", evt => {
       // When the *entire* viewer is resized, such that it becomes narrower,
       // ensure that the sidebar doesn't end up being too wide.
       if (!evt || evt.source !== window) {

web/pdf_single_page_viewer.js

@@ -20,7 +20,7 @@ class PDFSinglePageViewer extends BaseViewer {
   constructor(options) {
     super(options);
 
-    this.eventBus.on("pagesinit", evt => {
+    this.eventBus._on("pagesinit", evt => {
       // Since the pages are placed in a `DocumentFragment`, make sure that
       // the current page becomes visible upon loading of the document.
       this._ensurePageViewVisible();

web/secondary_toolbar.js

@@ -159,11 +159,11 @@ class SecondaryToolbar {
     this._bindSpreadModeListener(options);
 
     // Bind the event listener for adjusting the 'max-height' of the toolbar.
-    this.eventBus.on("resize", this._setMaxHeight.bind(this));
+    this.eventBus._on("resize", this._setMaxHeight.bind(this));
 
     // Hide the Scroll/Spread mode buttons, when they're not applicable to the
     // current `BaseViewer` instance (in particular `PDFSinglePageViewer`).
-    this.eventBus.on("baseviewerinit", evt => {
+    this.eventBus._on("baseviewerinit", evt => {
       if (evt.source instanceof PDFSinglePageViewer) {
         this.toolbarButtonContainer.classList.add(
           "hiddenScrollModeButtons",
@@ -233,7 +233,7 @@ class SecondaryToolbar {
   }
 
   _bindCursorToolsListener(buttons) {
-    this.eventBus.on("cursortoolchanged", function({ tool }) {
+    this.eventBus._on("cursortoolchanged", function({ tool }) {
       buttons.cursorSelectToolButton.classList.toggle(
         "toggled",
         tool === CursorTool.SELECT
@@ -267,9 +267,9 @@ class SecondaryToolbar {
       buttons.spreadOddButton.disabled = isScrollModeHorizontal;
       buttons.spreadEvenButton.disabled = isScrollModeHorizontal;
     }
-    this.eventBus.on("scrollmodechanged", scrollModeChanged);
+    this.eventBus._on("scrollmodechanged", scrollModeChanged);
 
-    this.eventBus.on("secondarytoolbarreset", evt => {
+    this.eventBus._on("secondarytoolbarreset", evt => {
       if (evt.source === this) {
         scrollModeChanged({ mode: ScrollMode.VERTICAL });
       }
@@ -291,9 +291,9 @@ class SecondaryToolbar {
         mode === SpreadMode.EVEN
       );
     }
-    this.eventBus.on("spreadmodechanged", spreadModeChanged);
+    this.eventBus._on("spreadmodechanged", spreadModeChanged);
 
-    this.eventBus.on("secondarytoolbarreset", evt => {
+    this.eventBus._on("secondarytoolbarreset", evt => {
       if (evt.source === this) {
         spreadModeChanged({ mode: SpreadMode.NONE });
       }

web/text_layer_builder.js

@@ -123,7 +123,7 @@ class TextLayerBuilder {
           this._updateMatches();
         }
       };
-      this.eventBus.on(
+      this.eventBus._on(
         "updatetextlayermatches",
         this._onUpdateTextLayerMatches
       );
@@ -139,7 +139,7 @@ class TextLayerBuilder {
       this.textLayerRenderTask = null;
     }
     if (this._onUpdateTextLayerMatches) {
-      this.eventBus.off(
+      this.eventBus._off(
         "updatetextlayermatches",
         this._onUpdateTextLayerMatches
       );

web/toolbar.js

@@ -159,7 +159,7 @@ class Toolbar {
     // Suppress context menus for some controls.
     scaleSelect.oncontextmenu = noContextMenuHandler;
 
-    this.eventBus.on("localized", () => {
+    this.eventBus._on("localized", () => {
       this._wasLocalized = true;
       this._adjustScaleWidth();
       this._updateUIState(true);

web/ui_utils.js

@@ -766,22 +766,20 @@ class EventBus {
     this._dispatchToDOM = dispatchToDOM === true;
   }
 
+  /**
+   * @param {string} eventName
+   * @param {function} listener
+   */
   on(eventName, listener) {
-    let eventListeners = this._listeners[eventName];
-    if (!eventListeners) {
-      eventListeners = [];
-      this._listeners[eventName] = eventListeners;
-    }
-    eventListeners.push(listener);
+    this._on(eventName, listener, { external: true });
   }
 
+  /**
+   * @param {string} eventName
+   * @param {function} listener
+   */
   off(eventName, listener) {
-    const eventListeners = this._listeners[eventName];
-    let i;
-    if (!eventListeners || (i = eventListeners.indexOf(listener)) < 0) {
-      return;
-    }
-    eventListeners.splice(i, 1);
+    this._off(eventName, listener, { external: true });
   }
 
   dispatch(eventName) {
@@ -795,16 +793,62 @@ class EventBus {
     }
     // Passing all arguments after the eventName to the listeners.
     const args = Array.prototype.slice.call(arguments, 1);
+    let externalListeners;
     // Making copy of the listeners array in case if it will be modified
     // during dispatch.
-    eventListeners.slice(0).forEach(function(listener) {
+    eventListeners.slice(0).forEach(function({ listener, external }) {
+      if (external) {
+        if (!externalListeners) {
+          externalListeners = [];
+        }
+        externalListeners.push(listener);
+        return;
+      }
       listener.apply(null, args);
     });
+    // Dispatch any "external" listeners *after* the internal ones, to give the
+    // viewer components time to handle events and update their state first.
+    if (externalListeners) {
+      externalListeners.forEach(function(listener) {
+        listener.apply(null, args);
+      });
+      externalListeners = null;
+    }
     if (this._dispatchToDOM) {
       this._dispatchDOMEvent(eventName, args);
     }
   }
 
+  /**
+   * @ignore
+   */
+  _on(eventName, listener, options = null) {
+    let eventListeners = this._listeners[eventName];
+    if (!eventListeners) {
+      this._listeners[eventName] = eventListeners = [];
+    }
+    eventListeners.push({
+      listener,
+      external: options ? options.external : false,
+    });
+  }
+
+  /**
+   * @ignore
+   */
+  _off(eventName, listener, options = null) {
+    const eventListeners = this._listeners[eventName];
+    if (!eventListeners) {
+      return;
+    }
+    for (let i = 0, ii = eventListeners.length; i < ii; i++) {
+      if (eventListeners[i].listener === listener) {
+        eventListeners.splice(i, 1);
+        return;
+      }
+    }
+  }
+
   /**
    * @private
    */