Introduce page.emulateMedia method (#383)

Emulating `screen` media would force `page.pdf()` to print with `screen`
media.

Fixes #312

docs/api.md

@@ -31,6 +31,7 @@
     + [page.click(selector[, options])](#pageclickselector-options)
     + [page.close()](#pageclose)
     + [page.emulate(options)](#pageemulateoptions)
+    + [page.emulateMedia(mediaType)](#pageemulatemediamediatype)
     + [page.evaluate(pageFunction, ...args)](#pageevaluatepagefunction-args)
     + [page.evaluateOnNewDocument(pageFunction, ...args)](#pageevaluateonnewdocumentpagefunction-args)
     + [page.exposeFunction(name, puppeteerFunction)](#pageexposefunctionname-puppeteerfunction)
@@ -160,7 +161,7 @@ This methods attaches Puppeteer to an existing Chromium instance.
 
 The method launches a browser instance with given arguments. The browser will be closed when the parent node.js process is closed.
 
-> **Note** Puppeteer works best with the version of Chromium it is bundled with. There is no guarantee it will work with any other version. Use `executablePath` option with extreme caution.  If Google Chrome (rather than Chromium) is preferred, a [Chrome Canary](https://www.google.com/chrome/browser/canary.html) or [Dev Channel](https://www.chromium.org/getting-involved/dev-channel) build is suggested.
+> **NOTE** Puppeteer works best with the version of Chromium it is bundled with. There is no guarantee it will work with any other version. Use `executablePath` option with extreme caution.  If Google Chrome (rather than Chromium) is preferred, a [Chrome Canary](https://www.google.com/chrome/browser/canary.html) or [Dev Channel](https://www.chromium.org/getting-involved/dev-channel) build is suggested.
 
 ### class: Browser
 
@@ -237,7 +238,7 @@ Emitted when a JavaScript dialog appears, such as `alert`, `prompt`, `confirm` o
 
 Emitted when the page crashes.
 
-> **Note** `error` event has a special meaning in Node, see [error events](https://nodejs.org/api/events.html#events_error_events) for details.
+> **NOTE** `error` event has a special meaning in Node, see [error events](https://nodejs.org/api/events.html#events_error_events) for details.
 
 #### event: 'frameattached'
 - <[Frame]>
@@ -348,6 +349,10 @@ puppeteer.launch().then(async browser => {
 
 List of all available devices is available in the source code: [DeviceDescriptors.js](https://github.com/GoogleChrome/puppeteer/blob/master/DeviceDescriptors.js).
 
+#### page.emulateMedia(mediaType)
+  - `mediaType` <[string]> Changes the CSS media type of the page. The only allowed values are `'screen'`, `'media'` and `null`. Passing `null` disables media emulation.
+  - returns: <[Promise]>
+
 #### page.evaluate(pageFunction, ...args)
 - `pageFunction` <[function]|[string]> Function to be evaluated in the page context
 - `...args` <...[Serializable]> Arguments to pass to `pageFunction`
@@ -546,6 +551,14 @@ Page is guaranteed to have a main frame which persists during navigations.
     - `left` <[string]> Left margin, accepts values labeled with units.
 - returns: <[Promise]<[Buffer]>> Promise which resolves with PDF buffer.
 
+`page.pdf()` generates a pdf of the page with `print` css media. To generate a pdf with `screen` media, call [page.emulateMedia('screen')](#pageemulatemediamediatype) before calling `page.pdf()`:
+
+```js
+// Generates a PDF with 'screen' media type.
+await page.emulateMedia('screen');
+await page.pdf({path: 'page.pdf'});
+```
+
 The `width`, `height`, and `margin` options accept values labeled with units. Unlabeled values are treated as pixels.
 
 A few examples:

lib/Page.js

@@ -363,6 +363,15 @@ class Page extends EventEmitter {
     ]);
   }
 
+  /**
+   * @param {?string} mediaType
+   * @return {!Promise}
+   */
+  async emulateMedia(mediaType) {
+    console.assert(mediaType === 'screen' || mediaType === 'print' || mediaType === null, 'Unsupported media type: ' + mediaType);
+    await this._client.send('Emulation.setEmulatedMedia', {media: mediaType || ''});
+  }
+
   /**
    * @param {!Page.Viewport} viewport
    * @return {!Promise}

test/test.js

@@ -1629,14 +1629,35 @@ describe('Page', function() {
       await page.setViewport({width: 100, height: 100});
       expect(await page.evaluate(() => screen.orientation.type)).toBe('portrait-primary');
     }));
-    it('should support emulate shorthand', SX(async function() {
+  });
+
+  describe('Page.emulate', function() {
+    it('should work', SX(async function() {
       await page.goto(PREFIX + '/mobile.html');
       await page.emulate(iPhone);
       expect(await page.evaluate(() => window.innerWidth)).toBe(375);
       expect(await page.evaluate(() => navigator.userAgent)).toContain('Safari');
     }));
   });
 
+  describe('Page.emulateMedia', function() {
+    it('should work', SX(async function() {
+      expect(await page.evaluate(() => window.matchMedia('screen').matches)).toBe(true);
+      expect(await page.evaluate(() => window.matchMedia('print').matches)).toBe(false);
+      await page.emulateMedia('print');
+      expect(await page.evaluate(() => window.matchMedia('screen').matches)).toBe(false);
+      expect(await page.evaluate(() => window.matchMedia('print').matches)).toBe(true);
+      await page.emulateMedia(null);
+      expect(await page.evaluate(() => window.matchMedia('screen').matches)).toBe(true);
+      expect(await page.evaluate(() => window.matchMedia('print').matches)).toBe(false);
+    }));
+    it('should throw in case of bad argument', SX(async function() {
+      let error = null;
+      await page.emulateMedia('bad').catch(e => error = e);
+      expect(error.message).toBe('Unsupported media type: bad');
+    }));
+  });
+
   describe('Page.evaluateOnNewDocument', function() {
     it('should evaluate before anything else on the page', SX(async function() {
       await page.evaluateOnNewDocument(function(){