Add new "pageopen"/"pageclose" events for usage with JavaScript actions

Having looked at the Acrobat JavaScript specification, see https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/AcrobatDC_js_api_reference.pdf#G5.1963437, I suppose that introducing these two new events is probably the easiest solution overall.

However there's a number of things that, as far as I'm concerned, will help the overall implementation:
 - Only dispatch these new events when `enableScripting = true` is set.
 - Handle them *separately* from the existing "pagechanging" event dispatching, to avoid too much clutter.
 - Don't dispatch either of the events if the page didn't actually change.
 - When waiting for pages to render, don't dispatch "pageopen" if the page is no longer active when rendering finishes.
 - Ensure that we only use *one* "pagerendered" event listener.
 - Ensure that "pageopen" is actually dispatched when the document loads.

I suppose that we *could* avoid adding the "pageclose" event, and use the existing "pagechanging" event instead, however having a separate event might allow more flexibility in the future. (E.g. I don't know if we'll possibly want to dispatch "pageclose" on document close, as mentioned briefly in the specification.)

web/base_viewer.js

@@ -214,6 +214,7 @@ class BaseViewer {
     if (this.removePageBorders) {
       this.viewer.classList.add("removePageBorders");
     }
+    this._initializeScriptingEvents();
     // Defer the dispatching of this event, to give other viewer components
     // time to initialize *and* register 'baseviewerinit' event listeners.
     Promise.resolve().then(() => {
@@ -283,12 +284,14 @@ class BaseViewer {
     if (!(0 < val && val <= this.pagesCount)) {
       return false;
     }
+    const previous = this._currentPageNumber;
     this._currentPageNumber = val;
 
     this.eventBus.dispatch("pagechanging", {
       source: this,
       pageNumber: val,
       pageLabel: this._pageLabels && this._pageLabels[val - 1],
+      previous,
     });
 
     if (resetCurrentPageView) {
@@ -649,6 +652,7 @@ class BaseViewer {
     this._pagesCapability = createPromiseCapability();
     this._scrollMode = ScrollMode.VERTICAL;
     this._spreadMode = SpreadMode.NONE;
+    this._pageOpenPendingSet = null;
 
     if (this._onBeforeDraw) {
       this.eventBus._off("pagerender", this._onBeforeDraw);
@@ -1500,6 +1504,58 @@ class BaseViewer {
     this._setCurrentPageNumber(pageNumber, /* resetCurrentPageView = */ true);
     this.update();
   }
+
+  /**
+   * @private
+   */
+  _initializeScriptingEvents() {
+    if (!this.enableScripting) {
+      return;
+    }
+    const { eventBus } = this;
+
+    const dispatchPageClose = pageNumber => {
+      eventBus.dispatch("pageclose", { source: this, pageNumber });
+    };
+    const dispatchPageOpen = (pageNumber, force = false) => {
+      const pageView = this._pages[pageNumber - 1];
+      if (force || pageView?.renderingState === RenderingStates.FINISHED) {
+        this._pageOpenPendingSet?.delete(pageNumber);
+
+        eventBus.dispatch("pageopen", { source: this, pageNumber });
+      } else {
+        if (!this._pageOpenPendingSet) {
+          this._pageOpenPendingSet = new Set();
+        }
+        this._pageOpenPendingSet.add(pageNumber);
+      }
+    };
+
+    eventBus._on("pagechanging", ({ pageNumber, previous }) => {
+      if (pageNumber === previous) {
+        return; // The active page didn't change.
+      }
+      dispatchPageClose(previous);
+      dispatchPageOpen(pageNumber);
+    });
+
+    eventBus._on("pagerendered", ({ pageNumber }) => {
+      if (!this._pageOpenPendingSet) {
+        return; // No pending "pageopen" events.
+      }
+      if (!this._pageOpenPendingSet.has(pageNumber)) {
+        return; // No pending "pageopen" event for the newly rendered page.
+      }
+      if (pageNumber !== this._currentPageNumber) {
+        return; // The newly rendered page is no longer the current one.
+      }
+      dispatchPageOpen(pageNumber, /* force = */ true);
+    });
+
+    eventBus._on("pagesinit", () => {
+      dispatchPageOpen(this._currentPageNumber);
+    });
+  }
 }
 
 export { BaseViewer };