Introduce page.goBack/page.goForward (#93)

This patch introduces page.goBack/page.goForward methods
to navigate the page history.

References #89.

docs/api.md

@@ -35,6 +35,8 @@
   * [page.evaluateOnInitialized(pageFunction, ...args)](#pageevaluateoninitializedpagefunction-args)
   * [page.focus(selector)](#pagefocusselector)
   * [page.frames()](#pageframes)
+  * [page.goBack(options)](#pagegobackoptions)
+  * [page.goForward(options)](#pagegoforwardoptions)
   * [page.httpHeaders()](#pagehttpheaders)
   * [page.injectFile(filePath)](#pageinjectfilefilepath)
   * [page.keyboard](#pagekeyboard)
@@ -343,11 +345,23 @@ This is a shortcut for [page.mainFrame().evaluate()](#frameevaluatefun-args) met
 #### page.frames()
 - returns: <[Array]<[Frame]>> An array of all frames attached to the page.
 
+#### page.goBack(options)
+- `options` <[Object]> Navigation parameters, same as in [page.navigate](#pagenavigateurl-options).
+- returns: <[Promise]<[Response]>> Promise which resolves to the main resource response. In case of multiple redirects, the navigation will resolve with the response of the last redirect. If
+can not go back, resolves to null.
+
+Navigate to the previous page in history.
+
+#### page.goForward(options)
+- `options` <[Object]> Navigation parameters, same as in [page.navigate](#pagenavigateurl-options).
+- returns: <[Promise]<[Response]>> Promise which resolves to the main resource response. In case of multiple redirects, the navigation will resolve with the response of the last redirect. If
+can not go back, resolves to null.
+
+Navigate to the next page in history.
 
 #### page.httpHeaders()
 - returns: <[Object]> Key-value set of additional http headers which will be sent with every request.
 
-
 #### page.injectFile(filePath)
 - `filePath` <[string]> Path to the javascript file to be injected into page.
 - returns: <[Promise]> Promise which resolves when file gets successfully evaluated in page.

lib/Page.js

@@ -291,7 +291,7 @@ class Page extends EventEmitter {
 
   /**
    * @param {!Object=} options
-   * @return {!Promise<?Response>}
+   * @return {!Promise<!Response>}
    */
   async waitForNavigation(options) {
     const watcher = new NavigatorWatcher(this._client, this._networkManager, options);
@@ -299,6 +299,36 @@ class Page extends EventEmitter {
     return responses.get(this.mainFrame().url()) || null;
   }
 
+  /**
+   * @param {!Object=} options
+   * @return {!Promise<?Response>}
+   */
+  async goBack(options) {
+    return this._go(-1, options);
+  }
+
+  /**
+   * @param {!Object=} options
+   * @return {!Promise<?Response>}
+   */
+  async goForward(options) {
+    return this._go(+1, options);
+  }
+
+  /**
+   * @param {!Object=} options
+   * @return {!Promise<?Response>}
+   */
+  async _go(delta, options) {
+    const history = await this._client.send('Page.getNavigationHistory');
+    const entry = history.entries[history.currentIndex + delta];
+    if (!entry)
+      return null;
+    const result = this.waitForNavigation(options);
+    this._client.send('Page.navigateToHistoryEntry', {entryId: entry.id});
+    return result;
+  }
+
   /**
    * @param {!Page.Viewport} viewport
    * @return {!Promise}

test/test.js

@@ -438,6 +438,24 @@ describe('Puppeteer', function() {
     }));
   });
 
+  describe('Page.goBack', function() {
+    it('should work', SX(async function() {
+      await page.navigate(EMPTY_PAGE);
+      await page.navigate(PREFIX + '/grid.html');
+
+      let response = await page.goBack();
+      expect(response.ok).toBe(true);
+      expect(response.url).toContain(EMPTY_PAGE);
+
+      response = await page.goForward();
+      expect(response.ok).toBe(true);
+      expect(response.url).toContain('/grid.html');
+
+      response = await page.goForward();
+      expect(response).toBe(null);
+    }));
+  });
+
   describe('Page.setInPageCallback', function() {
     it('should work', SX(async function() {
       await page.setInPageCallback('callController', function(a, b) {