The Chromium Chronicle #17: Browser Test Mixins

Episode 17: by Toby Huang in San Francisco, CA (January 2021)
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Browser tests are a form of automated testing for chromium code that are more end-to-end than unit tests. Browser tests create browser instances and test various UI elements instead of just testing pure input/output functionality like unit tests do. Integrated automated testing is important for simulating the user's end-to-end experience. When writing a browser test, you often want to programmatically perform setup actions that the user could do manually. To help you do this, mixins are a suite of tools for performing non-trivial browser test setup in a reusable way.

Examples of useful mixins:

  • GuestUserMixin for testing incognito mode
  • UserPolicyMixin for mocking user policy from the server
  • FakeGaiaMixin for mocking responses from the Gaia server

You can create your own custom mixins to serve a wide variety of purposes!

The snippet below logs in as a guest user for your test. It is important to make sure your feature works in incognito mode.

class MyGuestBrowserTestClass : public MixinBasedInProcessBrowserTest {
  GuestSessionMixin guest_session_mixin_{&mixin_host_};

LoggedInUserMixin is a convenient way to log in as a Family Link user for your test. If your feature affects anything related to child supervision, you should test your feature for supervised users. A possible bug is forgetting that child users have incognito mode disabled or can't install extensions without parent approval. LoggedInUserMixin is a compound mixin composed of other mixins.

class MyChildBrowserTestClass : public MixinBasedInProcessBrowserTest {
  void SetUpOnMainThread() override {

  LoggedInUserMixin logged_in_user_mixin_{&mixin_host_,
                                          embedded_test_server(), this};

One of the primary benefits of mixins is using composition instead of inheritance, and avoiding problems with diamond multiple inheritance. Inheriting from more than one test base class leads to the diamond problem, but you can mix and match mixins while avoiding this issue. Using mixins doesn't entirely get rid of inheritance because the browser test class still needs to inherit from MixinBasedInProcessBrowserTest, but fortunately that base class is general-purpose and lightweight.