feat(target): add support for target.page for 'backgroud_page' (#2600)

This patch teaches `target.page()` method to attach to extension background pages.

Fixes #2438

docs/api.md

@@ -525,7 +525,7 @@ Disconnects Puppeteer from the browser, but leaves the Chromium process running.
 Promise which resolves to a new [Page] object. The [Page] is created in a default browser context.
 
 #### browser.pages()
-- returns: <[Promise]<[Array]<[Page]>>> Promise which resolves to an array of all open pages.
+- returns: <[Promise]<[Array]<[Page]>>> Promise which resolves to an array of all open pages. Non visible pages, such as `"background_page"`, will not be listed here. You can find them using [target.page()](#targetpage).
 
 #### browser.process()
 - returns: <?[ChildProcess]> Spawned browser process. Returns `null` if the browser instance was created with [`puppeteer.connect`](#puppeteerconnectoptions) method.
@@ -2847,7 +2847,7 @@ Get the target that opened this target. Top-level targets return `null`.
 #### target.page()
 - returns: <[Promise]<?[Page]>>
 
-If the target is not of type `"page"`, returns `null`.
+If the target is not of type `"page"` or `"background_page"`, returns `null`.
 
 #### target.type()
 - returns: <[string]>

lib/Browser.js

@@ -183,7 +183,11 @@ class Browser extends EventEmitter {
    * @return {!Promise<!Array<!Puppeteer.Page>>}
    */
   async pages() {
-    const pages = await Promise.all(this.targets().map(target => target.page()));
+    const pages = await Promise.all(
+        this.targets()
+            .filter(target => target.type() === 'page')
+            .map(target => target.page())
+    );
     return pages.filter(page => !!page);
   }
 

lib/Target.js

@@ -38,7 +38,7 @@ class Target {
    * @return {!Promise<?Page>}
    */
   async page() {
-    if (this._targetInfo.type === 'page' && !this._pagePromise) {
+    if ((this._targetInfo.type === 'page' || this._targetInfo.type === 'background_page') && !this._pagePromise) {
       this._pagePromise = this._sessionFactory()
           .then(client => Page.create(client, this, this._ignoreHTTPSErrors, this._setDefaultViewport, this._screenshotTaskQueue));
     }

test/headful.spec.js

@@ -51,6 +51,14 @@ module.exports.addTests = function({testRunner, expect, PROJECT_ROOT, defaultBro
       await browserWithExtension.close();
       expect(backgroundPageTarget).toBeTruthy();
     });
+    it('target.page() should return a background_page', async({browser}) => {
+      const browserWithExtension = await puppeteer.launch(extensionOptions);
+      const targets = await browserWithExtension.targets();
+      const backgroundPageTarget = targets.find(target => target.type() === 'background_page');
+      const page = await backgroundPageTarget.page();
+      expect(await page.evaluate(() => 2 * 3)).toBe(6);
+      await browserWithExtension.close();
+    });
     it('should have default url when launching browser', async function() {
       const browser = await puppeteer.launch(extensionOptions);
       const pages = (await browser.pages()).map(page => page.url());