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How Chromecast WebDriver Server Works

The Chromecast WebDriver server is composed of two parts:

  1. A Chromecast receiver application (see receiver.html)
  2. A Cast v2 protocol client written in NodeJS (castv2)

The Chromecast receiver

Every Chromecast app is composed of a sender and a receiver. The sender is part of the web app, Android app, or iOS app, and when it begins casting, it launches a receiver app on the Chromecast device.

Receiver apps are really just web pages, and everything on the screen is implemented in HTML, CSS, and JavaScript.

The Chromecast WebDriver server's receiver app hosts an iframe which can be redirected to any URL at the client's request. This is how we load the arbitrary URL requested by a test runner like Karma without changing the receiver app's registered URL.

The Chromecast v2 client

The Chromecast WebDriver server uses the castv2 module to send commands to the Chromecast device using the Cast v2 protocol. When the WebDriver client asks for a certain URL to be loaded, the server uses Cast v2 to start the receiver app and send the desired URL in a message. The receiver app handles that message and loads the desired URL into its <iframe> element.

When the WebDriver client closes the session, the Chromecast WebDriver server uses Cast v2 to open the Chromecast home screen "app", which closes our own receiver app.

Feature policy

The <iframe> element is normally restricted from using certain sensitive features of the browser, to prevent embedded third-party content from abusing the security or privacy of the hosting app or user.

But some of these features may be interesting to you in testing, so our receiver app explicitly allows most features. For a full list, refer to the variable allowedFeatures in receiver.html.

Source: https://developer.mozilla.org/en-US/docs/Web/HTTP/Feature_Policy

Access Limitations

We show an arbitrary URL on the device by embedding it into an iframe in our Chromecast receiver app. However, sites can prevent iframe-embedding with the X-Frame-Options header.

Though this should not be an issue for a test runner, this may affect other URLs. Unfortunately, there is no way for the receiver app to detect when this has happened. See: #8

Chromecast receiver deployment

To learn how to deploy your own copy of the Chromecast Receiver, see receiver-deployment.md for details.