Bind to Circus events via an optional event handler on any custom env.

[scotthovestadt]

Summary

See #8307 for previous discussion.

This PR simplifies the implementation, exposing less API surface area and providing a clear method of implementation for the end-user.

Test plan

• All tests pass both with and without JEST_CIRCUS=1.
• Existing tests fully cover all functionality related to the event handler.

[SimenB]

Nice and clean! 😀

[SimenB]

Maybe add a test that has the new function and just logs every event name or something?

[jeysal]

Another thing that came to my mind:
The class / inheritance based test environment abstraction seems non-ideal with this extension. If I want to provide a package that does cool stuff by hooking into jest-circus but allow people to use it with both Node and JSDOM, how do I do that?

export const makeEnvironmentDoCoolStuff = (BaseEnvironment: typeof JestEnvironment) =>
  class CoolStuffEnvironment extends BaseEnvironment {
    handleTestEvent(event, state) {
      super.handleTestEvent(event, state);
      doCoolStuff(event);
    }
  }

It seems like this needs a composition mechanism.

[scotthovestadt]

@jeysal I agree with you in principle but I tried that first and it's just more prone to errors. I think if we needed to do that, it's not mutually exclusive with also having this automatically available (easy mode) on the environment.

[scotthovestadt]

@jeysal Also, to answer your core question of "how", I would probably:

1. Use the new feature I added to allow custom docblocks to be read by the custom env
2. Define a custom docblock for node vs jsdom
3. Initialize a new instance of node/jsdom within my custom env and call it's methods manually

Not the greatest but I think it may be the lesser of evils.

[codecov-io]

Codecov Report

Merging #8344 into master will decrease coverage by 0.02%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #8344      +/-   ##
==========================================
- Coverage   62.19%   62.17%   -0.03%     
==========================================
  Files         266      266              
  Lines       10701    10702       +1     
  Branches     2603     2605       +2     
==========================================
- Hits         6656     6654       -2     
- Misses       3459     3462       +3     
  Partials      586      586

Impacted Files	Coverage Δ
packages/jest-circus/src/types.ts	100% <ø> (ø)	⬆️
...circus/src/legacy-code-todo-rewrite/jestAdapter.ts	0% <ø> (ø)	⬆️
packages/jest-circus/src/globalErrorHandlers.ts	17.64% <ø> (ø)	⬆️
...us/src/legacy-code-todo-rewrite/jestAdapterInit.ts	0% <0%> (ø)	⬆️
packages/jest-circus/src/run.ts	0% <0%> (ø)	⬆️
packages/jest-circus/src/eventHandler.ts	7.05% <100%> (ø)	⬆️
packages/jest-circus/src/formatNodeAssertErrors.ts	14.28% <50%> (ø)	⬆️
packages/jest-circus/src/utils.ts	15.38% <61.53%> (ø)	⬆️
packages/jest-circus/src/index.ts	68.05% <80%> (ø)	⬆️
packages/jest-circus/src/state.ts	84.61% <85.71%> (-7.06%)	⬇️
... and 2 more

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[scotthovestadt]

@SimenB test added, will merge on green

[SimenB]

@CompuIves this might be something you can use in code sandbox?

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