Move additional code/methods into BaseViewer and have the extending classes override/extend methods as necessary

This attempts to provide more "default" methods in the base class, in order to reduce unnecessary duplication and to improve self-documentation of the `BaseViewer` class slightly.
The following changes are made (in no particular order):
 - Have `BaseViewer` implement the `_scrollIntoView` method, and *extend* it as necessary in `PDFViewer`/`PDFSinglePageViewer`.
 - Simply inline the `BaseViewer._resizeBuffer` method, in `BaseViewer.update`, since there's only one call-site at this point.
 - Provide a default implementation of `_isScrollModeHorizontal` in `BaseViewer`, and have `PDFSinglePageViewer` override it.
 - Provide a default implementation of `_getVisiblePages`, and have `PDFViewer` extend it and `PDFSinglePageViewer` override it.

web/base_viewer.js

@@ -15,9 +15,10 @@
 
 import {
   CSS_UNITS, DEFAULT_SCALE, DEFAULT_SCALE_VALUE, getGlobalEventBus,
-  isPortraitOrientation, isValidRotation, MAX_AUTO_SCALE, moveToEndOfArray,
-  NullL10n, PresentationModeState, RendererType, SCROLLBAR_PADDING,
-  TextLayerMode, UNKNOWN_SCALE, VERTICAL_PADDING, watchScroll
+  getVisibleElements, isPortraitOrientation, isValidRotation, MAX_AUTO_SCALE,
+  moveToEndOfArray, NullL10n, PresentationModeState, RendererType,
+  SCROLLBAR_PADDING, scrollIntoView, TextLayerMode, UNKNOWN_SCALE,
+  VERTICAL_PADDING, watchScroll
 } from './ui_utils';
 import { PDFRenderingQueue, RenderingStates } from './pdf_rendering_queue';
 import { AnnotationLayerBuilder } from './annotation_layer_builder';
@@ -360,6 +361,7 @@ class BaseViewer {
   }
 
   get _setDocumentViewerElement() {
+    // In most viewers, e.g. `PDFViewer`, this should return `this.viewer`.
     throw new Error('Not implemented: _setDocumentViewerElement');
   }
 
@@ -545,7 +547,7 @@ class BaseViewer {
   }
 
   _scrollIntoView({ pageDiv, pageSpot = null, pageNumber = null, }) {
-    throw new Error('Not implemented: _scrollIntoView');
+    scrollIntoView(pageDiv, pageSpot);
   }
 
   _setScaleUpdatePages(newScale, newValue, noScroll = false, preset = false) {
@@ -784,17 +786,6 @@ class BaseViewer {
     });
   }
 
-  /**
-   * visiblePages is optional; if present, it should be an array of pages and in
-   * practice its length is going to be numVisiblePages, but this is not
-   * required. The new size of the buffer depends only on numVisiblePages.
-   */
-  _resizeBuffer(numVisiblePages, visiblePages) {
-    let suggestedCacheSize = Math.max(DEFAULT_CACHE_SIZE,
-                                      2 * numVisiblePages + 1);
-    this._buffer.resize(suggestedCacheSize, visiblePages);
-  }
-
   _updateLocation(firstPage) {
     let currentScale = this._currentScale;
     let currentScaleValue = this._currentScaleValue;
@@ -835,7 +826,8 @@ class BaseViewer {
     if (numVisiblePages === 0) {
       return;
     }
-    this._resizeBuffer(numVisiblePages, visiblePages);
+    const newCacheSize = Math.max(DEFAULT_CACHE_SIZE, 2 * numVisiblePages + 1);
+    this._buffer.resize(newCacheSize, visiblePages);
 
     this.renderingQueue.renderHighestPriority(visible);
 
@@ -857,7 +849,10 @@ class BaseViewer {
   }
 
   get _isScrollModeHorizontal() {
-    throw new Error('Not implemented: _isScrollModeHorizontal');
+    // Used to ensure that pre-rendering of the next/previous page works
+    // correctly, since Scroll/Spread modes are ignored in Presentation Mode.
+    return (this.isInPresentationMode ?
+            false : this._scrollMode === ScrollMode.HORIZONTAL);
   }
 
   get isInPresentationMode() {
@@ -903,7 +898,8 @@ class BaseViewer {
   }
 
   _getVisiblePages() {
-    throw new Error('Not implemented: _getVisiblePages');
+    return getVisibleElements(this.container, this._pages, true,
+                              this._isScrollModeHorizontal);
   }
 
   /**

web/pdf_single_page_viewer.js

@@ -14,7 +14,6 @@
  */
 
 import { BaseViewer } from './base_viewer';
-import { scrollIntoView } from './ui_utils';
 import { shadow } from 'pdfjs-lib';
 
 class PDFSinglePageViewer extends BaseViewer {
@@ -87,7 +86,7 @@ class PDFSinglePageViewer extends BaseViewer {
     let previousLocation = this._location;
     this._ensurePageViewVisible();
 
-    scrollIntoView(pageDiv, pageSpot);
+    super._scrollIntoView({ pageDiv, pageSpot, pageNumber, });
 
     // Since scrolling is tracked using `requestAnimationFrame`, update the
     // scroll direction during the next `this._scrollUpdate` invocation.

web/pdf_viewer.js

@@ -13,8 +13,7 @@
  * limitations under the License.
  */
 
-import { BaseViewer, ScrollMode } from './base_viewer';
-import { getVisibleElements, scrollIntoView } from './ui_utils';
+import { BaseViewer } from './base_viewer';
 import { shadow } from 'pdfjs-lib';
 
 class PDFViewer extends BaseViewer {
@@ -22,27 +21,26 @@ class PDFViewer extends BaseViewer {
     return shadow(this, '_setDocumentViewerElement', this.viewer);
   }
 
-  _scrollIntoView({ pageDiv, pageSpot = null, }) {
+  _scrollIntoView({ pageDiv, pageSpot = null, pageNumber = null, }) {
     if (!pageSpot && !this.isInPresentationMode) {
       const left = pageDiv.offsetLeft + pageDiv.clientLeft;
       const right = left + pageDiv.clientWidth;
       const { scrollLeft, clientWidth, } = this.container;
-      if (this._scrollMode === ScrollMode.HORIZONTAL ||
+      if (this._isScrollModeHorizontal ||
           left < scrollLeft || right > scrollLeft + clientWidth) {
         pageSpot = { left: 0, top: 0, };
       }
     }
-    scrollIntoView(pageDiv, pageSpot);
+    super._scrollIntoView({ pageDiv, pageSpot, pageNumber, });
   }
 
   _getVisiblePages() {
-    if (!this.isInPresentationMode) {
-      return getVisibleElements(this.container, this._pages, true,
-                                this._scrollMode === ScrollMode.HORIZONTAL);
+    if (this.isInPresentationMode) {
+      // The algorithm in `getVisibleElements` doesn't work in all browsers and
+      // configurations (e.g. Chrome) when Presentation Mode is active.
+      return this._getCurrentVisiblePage();
     }
-    // The algorithm in `getVisibleElements` doesn't work in all browsers and
-    // configurations when presentation mode is active.
-    return this._getCurrentVisiblePage();
+    return super._getVisiblePages();
   }
 
   _updateHelper(visiblePages) {
@@ -66,13 +64,6 @@ class PDFViewer extends BaseViewer {
     }
     this._setCurrentPageNumber(currentId);
   }
-
-  get _isScrollModeHorizontal() {
-    // Used to ensure that pre-rendering of the next/previous page works
-    // correctly, since Scroll/Spread modes are ignored in Presentation Mode.
-    return (this.isInPresentationMode ?
-            false : this._scrollMode === ScrollMode.HORIZONTAL);
-  }
 }
 
 export {