ChromeVox (for developers)

ChromeVox is the built-in screen reader on Chrome OS. It was originally developed as a separate extension but over time it has been incorporated into the operating system itself. Now the code lives inside of the Chromium tree and it's built and shipped as part of Chrome OS.

NOTE: ChromeVox ships also as an extension on the Chrome webstore. This version of ChromeVox is known as ChromeVox Classic and is loosely related to ChromeVox (on Chrome OS). All references to ChromeVox relate only to ChromeVox on Chrome OS.

To start or stop ChromeVox, press Ctrl+Alt+Z on your ChromeOS device at any time.

Developer Info

Code location: chrome/browser/resources/chromeos/accessibility/chromevox

Ninja target: it's built as part of “chrome”, but you can build and run browser_tests to test it (Chrome OS target only - you must have target_os = “chromeos” in your GN args first).

Developing On Linux

ChromeVox for Chrome OS development is done on Linux.

See ChromeVox on Desktop Linux for more information.

Code Structure

The chromevox/ extension directory is broken into subdirectories, based on what context the code runs in. The different contexts are as follows:

  • The background context (chromevox/background/) contains the bulk of the ChromeVox logic, and runs in the background page (soon to be a background service worker). To group code by its logical function, it has the following subdirectories:

    • chromevox/background/braille/, which contains logic around braille input/output.

    • chromevox/background/editing/, which contains the logic to handle input into text fields.

    • chromevox/background/event/, which contains the logic that handles events from the various APIs.

    • chromevox/background/logging/, which contains logic to generate the content shown on the log page.

    • chromevox/background/output/, which contains the logic to generate the text that is spoken or sent to the braille display. More details are in the file within that directory.

    • chromevox/background/panel/, which contains the logic to support the ChromeVox panel.

  • The content script context (chromevox/injected/) contains the code that is injected into web pages. At this point, this is only used to support the Google Docs workaround. When that is resolved, it is anticipated this directory will be removed.

  • The learn mode context (chromevox/learn_mode/) contains the code to render and run the ChromeVox learn mode (which is different from the tutorial).

  • The log context (chromevox/log_page/) contains the code specific to showing the ChromeVox log page.

  • The options context (chromevox/options/) contains the code for the ChromeVox settings page. There is an ongoing effort to migrate this page into the ChromeOS settings app, after which this directory will be unneeded.

  • The panel context (chromevox/panel/) contains the code that renders and performs the logic of the ChromeVox panel, shown at the top of the screen. When the onscreen command menus are shown, that is also rendered in this context.

  • The tutorial context (chromevox/tutorial/) contains resources used exclusively by the ChromeVox tutorial.

Other subdirectories also have specific purposes:

  • The common directory (chromevox/common/) contains files that can safely be shared between multiple contexts. These files must have no global state, as each context has its own global namespace. To get information between the contexts, bridge objects are used to pass structured messages. Any data passed through these bridges loses any and all class information, as it is converted to JSON in the process of being sent.

    • The subdirectory chromevox/common/braille/ contains common logic specific to braille
  • The earcons directory (chromevox/earcons/) contains the audio files for any short indicator sounds (earcons) used by ChromeVox to express information without words.

  • The images directory (chromevox/images/) contains any images used in any context.

  • The testing directory (chromevox/testing/) contains files that are used exclusively in testing.

  • The third_party directory (chromevox/third_party) contains open source code from other developers that is used in the ChromeVox extension.

  • The tools directory (chromevox/tools) contains python scrips used for building ChromeVox. Eventually these should be moved into the common accessibility directory.

Debugging ChromeVox

There are options available that may assist in debugging ChromeVox. Here are a few use cases.

Feature development

When developing a new feature, it may be helpful to save time by not having to go through a compile cycle. This can be achieved by setting chromevox_compress_js to 0 in chrome/browser/resources/chromeos/accessibility/chromevox/, or by using a debug build.

In a debug build or with chromevox_compress_js off, the unflattened files in the Chrome out directory (e.g. out/Release/resources/chromeos/accessibility/chromevox/). Now you can hack directly on the copy of ChromeVox in out/ and toggle ChromeVox to pick up your changes (via Ctrl+Alt+Z).

Fixing bugs

The easiest way to debug ChromeVox is from an external browser. Start Chrome with this command-line flag:

out/Release/chrome --remote-debugging-port=9222

Now open http://localhost:9222 in a separate instance of the browser, and debug the ChromeVox extension background page from there.

Another option is to use emacs indium (available through M-x package-list-packages).

It also talks to localhost:9222 but integrates more tightly into emacs instead.

Another option is to use the built-in developer console. Go to the ChromeVox options page with Search+Shift+o, o; then, substitute the “options.html” path with “background.html”, and then open up the inspector.

Debugging ChromeOS

To debug ChromeVox in ChromeOS, you need to add the command-line flag to the config file in device under test(DUT) instead of starting chrome from command line.

(dut) $ echo " --remote-debugging-port=9222 " >> /etc/chrome_dev.conf
(dut) $ restart ui

This is also written in Simple Chrome Workflow Doc.

You need to ssh from your development device into your DUT forwarding port 9222 to open ChromeVox extension background page in your dev device, for example

ssh my_crbook -L 3333:localhost:9222

Then open the forwarded port in the development device, http://localhost:3333 in the example.

You may need to remove rootfs verification to write to /etc/chrome_dev.conf.

(dut) $ crossystem dev_boot_signed_only=0
(dut) $ sudo /usr/share/vboot/bin/ --remove_rootfs_verification
(dut) $ reboot

See Chromium OS Doc for more information about removing rootfs verification.

Running tests

Build the browser_tests target. To run lots of tests in parallel, run it like this:

out/Release/browser_tests --test-launcher-jobs=20 --gtest_filter=ChromeVox*

Use a narrower test filter if you only want to run some of the tests. For example, most of the ChromeVox Next tests have “E2E” in them (for “end-to-end”), so to only run those:

out/Release/browser_tests --test-launcher-jobs=20 --gtest_filter="*ChromeVox*E2E*"