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Cypress in Element Web

Scope of this Document

This doc is about our Cypress tests in Element Web and how we use Cypress to write tests. It aims to cover:

  • How to run the tests yourself
  • How the tests work
  • How to write great Cypress tests
  • Visual testing

Running the Tests

Our Cypress tests run automatically as part of our CI along with our other tests, on every pull request and on every merge to develop & master.

However the Cypress tests are run, an element-web must be running on http://localhost:8080 (this is configured in cypress.json) - this is what will be tested. When running Cypress tests yourself, the standard yarn start from the element-web project is fine: leave it running it a different terminal as you would when developing.

The tests use Docker to launch Homeserver (Synapse or Dendrite) instances to test against, so you'll also need to have Docker installed and working in order to run the Cypress tests.

There are a few different ways to run the tests yourself. The simplest is to run:

yarn run test:cypress

This will run the Cypress tests once, non-interactively.

You can also run individual tests this way too, as you'd expect:

yarn run test:cypress --spec cypress/e2e/1-register/register.spec.ts

Cypress also has its own UI that you can use to run and debug the tests. To launch it:

yarn run test:cypress:open

Running with Rust cryptography

matrix-js-sdk is currently in the process of being updated to replace its end-to-end encryption implementation to use the Matrix Rust SDK. This is not currently enabled by default, but it is possible to have Cypress configure Element to use the Rust crypto implementation by setting the environment variable CYPRESS_RUST_CRYPTO=1.

How the Tests Work

Everything Cypress-related lives in the cypress/ subdirectory of react-sdk as is typical for Cypress tests. Likewise, tests live in cypress/e2e.

cypress/plugins/synapsedocker contains a Cypress plugin that starts instances of Synapse in Docker containers. These synapses are what Element-web runs against in the Cypress tests.

Synapse can be launched with different configurations in order to test element in different configurations. cypress/plugins/synapsedocker/templates contains template configuration files for each different configuration.

Each test suite can then launch whatever Synapse instances it needs in whatever configurations.

Note that although tests should stop the Homeserver instances after running and the plugin also stop any remaining instances after all tests have run, it is possible to be left with some stray containers if, for example, you terminate a test such that the after() does not run and also exit Cypress uncleanly. All the containers it starts are prefixed, so they are easy to recognise. They can be removed safely.

After each test run, logs from the Synapse instances are saved in cypress/synapselogs with each instance in a separate directory named after its ID. These logs are removed at the start of each test run.

Writing Tests

Mostly this is the same advice as for writing any other Cypress test: the Cypress docs are well worth a read if you're not already familiar with Cypress testing, eg. https://docs.cypress.io/guides/references/best-practices. To avoid your tests being flaky it is also recommended to give https://docs.cypress.io/guides/core-concepts/retry-ability a read.

Getting a Synapse

The key difference is in starting Synapse instances. Tests use this plugin via cy.startHomeserver() to provide a Homeserver instance to log into:

cy.startHomeserver("consent").then((result) => {
    homeserver = result;
});

This returns an object with information about the Homeserver instance, including what port it was started on and the ID that needs to be passed to shut it down again. It also returns the registration shared secret (registrationSecret) that can be used to register users via the REST API. The Homeserver has been ensured ready to go by awaiting its internal health-check.

Homeserver instances should be reasonably cheap to start (you may see the first one take a while as it pulls the Docker image), so it's generally expected that tests will start a Homeserver instance for each test suite, i.e. in before(), and then tear it down in after().

To later destroy your Homeserver you should call stopHomeserver, passing the HomeserverInstance object you received when starting it.

cy.stopHomeserver(homeserver);

Synapse Config Templates

When a Synapse instance is started, it's given a config generated from one of the config templates in cypress/plugins/synapsedocker/templates. There are a couple of special files in these templates:

  • homeserver.yaml: Template substitution happens in this file. Template variables are:
    • REGISTRATION_SECRET: The secret used to register users via the REST API.
    • MACAROON_SECRET_KEY: Generated each time for security
    • FORM_SECRET: Generated each time for security
    • PUBLIC_BASEURL: The localhost url + port combination the synapse is accessible at
  • localhost.signing.key: A signing key is auto-generated and saved to this file. Config templates should not contain a signing key and instead assume that one will exist in this file.

All other files in the template are copied recursively to /data/, so the file foo.html in a template can be referenced in the config as /data/foo.html.

Logging In

There exists a basic utility to start the app with a random user already logged in:

cy.initTestUser(homeserver, "Jeff");

It takes the HomeserverInstance you received from startHomeserver and a display name for your test user. This custom command will register a random userId using the registrationSecret with a random password and the given display name. The returned Chainable will contain details about the credentials for if they are needed for User-Interactive Auth or similar but localStorage will already be seeded with them and the app loaded (path /).

The internals of how this custom command run may be swapped out later, but the signature can be maintained for simpler maintenance.

Joining a Room

Many tests will also want to start with the client in a room, ready to send & receive messages. Best way to do this may be to get an access token for the user and use this to create a room with the REST API before logging the user in. You can make use of cy.getBot(homeserver) and cy.getClient() to do this.

Convenience APIs

We should probably end up with convenience APIs that wrap the homeserver creation, logging in and room creation that can be called to set up tests.

Try to write tests from the users's perspective

Like for instance a user will not look for a button by querying a CSS selector. Instead you should work with roles / labels etc.. You can make use of cy.findBy… queries provided by Cypress Testing Library.

Using matrix-js-sdk

Due to the way we run the Cypress tests in CI, at this time you can only use the matrix-js-sdk module exposed on window.matrixcs. This has the limitation that it is only accessible with the app loaded. This may be revisited in the future.

Good Test Hygiene

This section mostly summarises general good Cypress testing practice, and should not be news to anyone already familiar with Cypress.

  1. Test a well-isolated unit of functionality. The more specific, the easier it will be to tell what's wrong when they fail.
  2. Don't depend on state from other tests: any given test should be able to run in isolation.
  3. Try to avoid driving the UI for anything other than the UI you're trying to test. e.g. if you're testing that the user can send a reaction to a message, it's best to send a message using a REST API, then react to it using the UI, rather than using the element-web UI to send the message.
  4. Avoid explicit waits. cy.get() will implicitly wait for the specified element to appear and all assertions are retired until they either pass or time out, so you should never need to manually wait for an element.
    • For example, for asserting about editing an already-edited message, you can't wait for the 'edited' element to appear as there was already one there, but you can assert that the body of the message is what is should be after the second edit and this assertion will pass once it becomes true. You can then assert that the 'edited' element is still in the DOM.
    • You can also wait for other things like network requests in the browser to complete (https://docs.cypress.io/guides/guides/network-requests#Waiting). Needing to wait for things can also be because of race conditions in the app itself, which ideally shouldn't be there!

This is a small selection - the Cypress best practices guide, linked above, has more good advice, and we should generally try to adhere to them.

Screenshot testing with Percy

We also support visual testing via Percy. Within many of our Cypress tests you can see lines calling cy.percySnapshot(). This creates a screenshot and uses Percy to check whether it has changed from the last time this test was run.

It can help to pass percyCSS in as the 2nd argument to percySnapshot to hide elements that vary (e.g. timestamps). See the existing code for examples of this. (Note: it is also possible for team members to mark certain parts of a screenshot to be ignored. This is done within the Percy UI.)

Percy screenshots are created using custom renderers based on Safari, Firefox, Chrome and Edge. Each percySnapshot actually creates 8 screenshots (4 browsers, 2 sizes). Since we have a limited budget for Percy screenshots, by default we only run Percy once per day against the develop branch, based on a nightly build at approximately 04:00 UTC every day. (The schedule is defined in element-web.yaml and the Percy tests are enabled/disabled in cypress.yaml.)

If your pull request makes visual changes, you are encouraged to request Percy to run by adding the label X-Needs-Percy to the PR, these will only run in the merge queue to save snapshots. This will help us find any visual bugs or validate visual changes at the time they are made, instead of having to figure it out later after the nightly build. If you don't have permission to add a label, please ask your reviewer to do it. Note: it's best to add this label when the change is nearly ready, because the screenshots will be re-created every time you make a change to your PR.