Implement a `PDFSinglePageViewer` class (issue 8188)

The new `PDFSinglePageViewer` class extends the previously created abstract `BaseViewer` class.

There's *a lot* of existing functionality in `PDFViewer` that depends on all the pages being loaded and synchronously available, once the `setDocument` method has been called.
Given that initializing `PDFPageView` instances requires passing a DOM element to which the page is attached, the simplest solution I could come up with is to append all pages to a (hidden) document fragment and just swap them (one at a time) into the viewer when page switching occurs.

web/pdf_single_page_viewer.js

@@ -0,0 +1,149 @@
+/* Copyright 2017 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { BaseViewer } from './base_viewer';
+import { scrollIntoView } from './ui_utils';
+import { shadow } from 'pdfjs-lib';
+
+class PDFSinglePageViewer extends BaseViewer {
+  constructor(options) {
+    super(options);
+
+    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();
+    });
+  }
+
+  get _setDocumentViewerElement() {
+    // Since we only want to display *one* page at a time when using the
+    // `PDFSinglePageViewer`, we cannot append them to the `viewer` DOM element.
+    // Instead, they are placed in a `DocumentFragment`, and only the current
+    // page is displayed in the viewer (refer to `this._ensurePageViewVisible`).
+    return shadow(this, '_setDocumentViewerElement', this._shadowViewer);
+  }
+
+  _resetView() {
+    super._resetView();
+    this._previousPageNumber = 1;
+    this._shadowViewer = document.createDocumentFragment();
+  }
+
+  _ensurePageViewVisible() {
+    let pageView = this._pages[this._currentPageNumber - 1];
+    let previousPageView = this._pages[this._previousPageNumber - 1];
+
+    let viewerNodes = this.viewer.childNodes;
+    switch (viewerNodes.length) {
+      case 0: // Should *only* occur on initial loading.
+        this.viewer.appendChild(pageView.div);
+        break;
+      case 1: // The normal page-switching case.
+        if (viewerNodes[0] !== previousPageView.div) {
+          throw new Error(
+            '_ensurePageViewVisible: Unexpected previously visible page.');
+        }
+        if (pageView === previousPageView) {
+          break; // The correct page is already visible.
+        }
+        // Switch visible pages, and reset the viewerContainer scroll position.
+        this._shadowViewer.appendChild(previousPageView.div);
+        this.viewer.appendChild(pageView.div);
+
+        this.container.scrollTop = 0;
+        break;
+      default:
+        throw new Error(
+          '_ensurePageViewVisible: Only one page should be visible at a time.');
+    }
+    this._previousPageNumber = this._currentPageNumber;
+  }
+
+  _scrollUpdate() {
+    if (this._updateScrollDown) {
+      this._updateScrollDown();
+    }
+    super._scrollUpdate();
+  }
+
+  _scrollIntoView({ pageDiv, pageSpot = null, pageNumber = null, }) {
+    if (pageNumber) { // Ensure that `this._currentPageNumber` is correct.
+      this._setCurrentPageNumber(pageNumber);
+    }
+    let scrolledDown = this._currentPageNumber >= this._previousPageNumber;
+    let previousLocation = this._location;
+    this._ensurePageViewVisible();
+
+    scrollIntoView(pageDiv, pageSpot);
+
+    // Since scrolling is tracked using `requestAnimationFrame`, update the
+    // scroll direction during the next `this._scrollUpdate` invocation.
+    this._updateScrollDown = () => {
+      this.scroll.down = scrolledDown;
+      delete this._updateScrollDown;
+    };
+    // If the scroll position doesn't change as a result of the `scrollIntoView`
+    // call, ensure that rendering always occurs to avoid showing a blank page.
+    setTimeout(() => {
+      if (this._location === previousLocation) {
+        if (this._updateScrollDown) {
+          this._updateScrollDown();
+        }
+        this.update();
+      }
+    }, 0);
+  }
+
+  _getVisiblePages() {
+    if (!this.pagesCount) {
+      return { views: [], };
+    }
+    let pageView = this._pages[this._currentPageNumber - 1];
+    // NOTE: Compute the `x` and `y` properties of the current view,
+    // since `this._updateLocation` depends of them being available.
+    let element = pageView.div;
+
+    let view = {
+      id: pageView.id,
+      x: element.offsetLeft + element.clientLeft,
+      y: element.offsetTop + element.clientTop,
+      view: pageView,
+    };
+    return { first: view, last: view, views: [view], };
+  }
+
+  update() {
+    let visible = this._getVisiblePages();
+    let visiblePages = visible.views, numVisiblePages = visiblePages.length;
+
+    if (numVisiblePages === 0) {
+      return;
+    }
+    this._resizeBuffer(numVisiblePages);
+
+    this.renderingQueue.renderHighestPriority(visible);
+
+    this._updateLocation(visible.first);
+    this.eventBus.dispatch('updateviewarea', {
+      source: this,
+      location: this._location,
+    });
+  }
+}
+
+export {
+  PDFSinglePageViewer,
+};

web/pdf_viewer.component.js

@@ -17,6 +17,7 @@
 
 var pdfjsLib = require('./pdfjs.js');
 var pdfjsWebPDFViewer = require('./pdf_viewer.js');
+var pdfjsWebPDFSinglePageViewer = require('./pdf_single_page_viewer');
 var pdfjsWebPDFPageView = require('./pdf_page_view.js');
 var pdfjsWebPDFLinkService = require('./pdf_link_service.js');
 var pdfjsWebTextLayerBuilder = require('./text_layer_builder.js');
@@ -30,6 +31,7 @@ var pdfjsWebGenericL10n = require('./genericl10n.js');
 var PDFJS = pdfjsLib.PDFJS;
 
 PDFJS.PDFViewer = pdfjsWebPDFViewer.PDFViewer;
+PDFJS.PDFSinglePageViewer = pdfjsWebPDFSinglePageViewer.PDFSinglePageViewer;
 PDFJS.PDFPageView = pdfjsWebPDFPageView.PDFPageView;
 PDFJS.PDFLinkService = pdfjsWebPDFLinkService.PDFLinkService;
 PDFJS.SimpleLinkService = pdfjsWebPDFLinkService.SimpleLinkService;