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WebengineKiosk

CircleCI Hex version

Launch and control a fullscreen web browser from Elixir. This is intended for kiosks running Nerves but can be used anywhere you need to show a user a local web-based UI.

Here's an example run:

iex> {:ok, kiosk} = WebengineKiosk.start_link(fullscreen: false)
iex> WebengineKiosk.go_to_url(kiosk, "https://elixir-lang.org/")
iex> WebengineKiosk.run_javascript(kiosk, "window.alert('Hello, Elixir!')")
iex> WebengineKiosk.stop(kiosk)

It can also be linked into your application's supervision tree:

# Example childspecs

   [
      {WebengineKiosk, {[homepage: "https://somewhere.com", background: "black"], name: Display}}
   ]

# Somewhere else in your code

WebengineKiosk.run_javascript(Display, "window.alert('Hello, Elixir!')")

Kiosk options

The kiosk starts fullscreen and goes to a default local web page. To change this, set one or more options:

  • background_color: color - specify a background color as #RRGGBB or by name
  • blank_image: path - specify a path to an image for when the screen is blanked
  • data_dir: path - specify a writable path for data files
  • debug_keys: boolean - enable key combinations useful for debugging
  • fullscreen: boolean - show fullscreen
  • gid: gid - run the browser with this group id
  • homepage: url - load this page first. For local files, specify file:///path/to/index.html
  • http_accept_language: string - overrides the default Accept-Language string
  • http_user_agent: string - overrides the default UserAgent string
  • monitor: index - select the monitor for the web browser (0, 1, etc.)
  • opengl: "gl" | "gles" | "software" | "auto" - specify the OpenGL backend. This is only a hint.
  • progress: boolean - show a progress bar when loading pages
  • run_as_root: boolean - set to true if you really want to run Chromium as root
  • sounds: boolean - play sounds on clicks
  • uid: uid - run the browser as this user

See lib/webengine_kiosk.ex for some untested options.

Events

Process can subscribe to receive events from WebengineKiosk. Subscribe using register function like this from another process:

WebengineKiosk.register(Display, self())

If the subscribing process is GenServer you can handle events like this:

def handle_info({:webengine_kiosk, {:channel_message, message}}, state) do
  # Do something with event ...
  {:noreply, state}
end

Sending messages back to elixir

You can send data from Javascript back to elixir using QtWebChannel.

First you need to load appropriate javascript library:

Place this somewhere in <head>

<script type="text/javascript" src="qrc:///qtwebchannel/qwebchannel.js"></script>

Than in your javascript code initialize QtWebChannel:

new QWebChannel(qt.webChannelTransport, function (channel) {
  var JSobject = channel.objects.elixirJsChannel;
  window.elixirJsChannel = JSobject;
});

than anywhere in your javascript code you can call following function to send messages:

window.elixirJsChannel.send("Hello world!");

The message will appear as :channel_message event. Only text can be sent, but if you want so send objects, you can serialize the to JSON and send them as such.

Installation

WebengineKiosk requires Qt version 5.10 or later (it may work on earlier versions, but we haven't tested it). It is likely that your package manager already has a Qt package.

On Debian or Ubuntu:

sudo apt install qtwebengine5-dev qtmultimedia5-dev qt5-default

On OSX:

brew install qt

# Homebrew doesn't automatically add `qmake` to your path, so run this when
# building or add it to your .bashrc, .zshrc, etc.
export PATH="/usr/local/opt/qt/bin:$PATH"

If you are installing Qt manually, then the first time that you run mix, you'll need to point to the installation location. If you don't, you'll either get an error that qmake isn't found or you'll being using your system's version of Qt. Here's an example commandline:

QMAKE=~/Qt/5.11.1/gcc_64/bin/qmake mix compile

Finally, if you're using Nerves, you'll need a Nerves system that includes Qt. Take a look at kiosk_system_rpi3 for an example.

Once you've done all that, go ahead and add webengine_kiosk to your mix.exs dependencies like normal:

def deps do
  [
    {:webengine_kiosk, "~> 0.2"}
  ]
end

Permissions

WebengineKiosk will refuse to run as the root user. You may specify a name or number using the :uid and :gid parameters to WebengineKiosk.start_link/2. If unspecified and running as root, WebengineKiosk will try to drop to a kiosk user and kiosk group by default. If dropping privileges, then you also need to ensure that QtWebEngine has a writable data directory. Use the :data_dir option do do this.

The next set of permissions to check are platform-specific. Qt and Chromium directly access videos drivers and input devices. Usually the device files associated with those have group permissions and adding the user to the appropriate groups makes everything work. For example, on the Raspberry Pi, you'll need the kiosk user to be part of the video and input groups. You may also need to update the permissions and group ownership on /dev/vchiq if running on Nerves since it doesn't yet do that by default.

Debugging immediate exits

If the kiosk binary exits immediately then something basic is wrong. Unfortunately, errors often get printed to the terminal and don't end up in Elixir logs. If this happens to you, try running the kiosk binary by itself:

iex> path = Application.app_dir(:webengine_kiosk, "priv/kiosk")
"/srv/erlang/lib/webengine_kiosk-0.1.0/priv/kiosk"
iex> System.cmd(path, [])