Add navigateToNthPage function

src/adapt/epub.js

@@ -1,6 +1,7 @@
 /**
  * Copyright 2013 Google, Inc.
  * Copyright 2015 Trim-marks Inc.
+ * Copyright 2018 Vivliostyle Foundation
  *
  * Vivliostyle.js is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -1576,6 +1577,38 @@ adapt.epub.OPFView.prototype.previousSpread = function(position) {
     }
 };
 
+/**
+ * Move to the Nth page and render it.
+ * @param {number} nthPage
+ * @param {!adapt.epub.Position} position
+ * @return {!adapt.task.Result.<?adapt.epub.PageAndPosition>}
+ */
+adapt.epub.OPFView.prototype.navigateToNthPage = function(nthPage, position) {
+    if (nthPage < 1) {
+        return adapt.task.newResult(/** @type {?adapt.epub.PageAndPosition} */ (null));
+    }
+    let countPages = 0;
+    let pageIndex = -1;
+    let spineIndex = 0;
+    for (let item of this.spineItems) {
+        const findPageIndex = nthPage - countPages - 1;
+        if (findPageIndex < item.pages.length) {
+            pageIndex = findPageIndex;
+            break;
+        }
+        countPages += item.pages.length;
+        spineIndex++;
+    }
+    if (pageIndex === -1) {
+        return adapt.task.newResult(/** @type {?adapt.epub.PageAndPosition} */ (null));
+    }
+    return this.findPage({
+        spineIndex,
+        pageIndex,
+        offsetInItem: -1
+    });
+};
+
 /**
  * Move to the epage specified by the given number (zero-based) and render it.
  * @param {number} epage

src/adapt/viewer.js

@@ -1,6 +1,7 @@
 /**
  * Copyright 2013 Google, Inc.
  * Copyright 2015 Trim-marks Inc.
+ * Copyright 2018 Vivliostyle Foundation
  *
  * Vivliostyle.js is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -917,6 +918,9 @@ adapt.viewer.Viewer.prototype.moveTo = function(command) {
             const m = method;
             method = () => m.call(self.opfView, self.pagePosition);
         }
+    } else if (typeof command["nthPage"] == "number") {
+        const nthPage = /** @type {number} */ (command["nthPage"]);
+        method = () => self.opfView.navigateToNthPage(nthPage, self.pagePosition);
     } else if (typeof command["epage"] == "number") {
         const epage = /** @type {number} */ (command["epage"]);
         method = () => self.opfView.navigateToEPage(epage);

src/vivliostyle/viewer.js

@@ -1,5 +1,6 @@
 /**
  * Copyright 2015 Trim-marks Inc.
+ * Copyright 2018 Vivliostyle Foundation
  *
  * Vivliostyle.js is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -364,6 +365,14 @@ goog.scope(() => {
         this.adaptViewer.sendCommand({"a": "moveTo", "where": this.resolveNavigation(nav)});
     };
 
+    /**
+     * Navigate to the Nth page.
+     * @param {number} nthPage
+     */
+    Viewer.prototype.navigateToNthPage = function(nthPage) {
+        this.adaptViewer.sendCommand({"a": "moveTo", "nthPage": nthPage});
+    };
+
     /**
      * Navigate to the specified internal URL.
      * @param {string} url
@@ -407,6 +416,7 @@ goog.scope(() => {
     goog.exportProperty(Viewer.prototype, "loadEPUB", Viewer.prototype.loadEPUB);
     goog.exportProperty(Viewer.prototype, "getCurrentPageProgression", Viewer.prototype.getCurrentPageProgression);
     goog.exportProperty(Viewer.prototype, "navigateToPage", Viewer.prototype.navigateToPage);
+    goog.exportProperty(Viewer.prototype, "navigateToNthPage", Viewer.prototype.navigateToNthPage);
     goog.exportProperty(Viewer.prototype, "navigateToInternalUrl", Viewer.prototype.navigateToInternalUrl);
     goog.exportProperty(Viewer.prototype, "queryZoomFactor", Viewer.prototype.queryZoomFactor);
     goog.exportProperty(Viewer.prototype, "getPageSizes", Viewer.prototype.getPageSizes);