I copied the set up we use for React.
In the www-variant test job, the Scheduler `__VARIANT__` flags will be
`true`. When writing a test, we can read the value of the flag with the
`gate` pragma and method.
Note: Since these packages are currently released in lockstep, maybe we
should remove SchedulerFeatureFlags and use ReactFeatureFlags for both.
Currently, in a React 18 root, `act` only works if you mock the
Scheduler package. This was because we didn't want to add additional
checks at runtime.
But now that the `act` testing API is dev-only, we can simplify its
implementation.
Now when an update is wrapped with `act`, React will bypass Scheduler
entirely and push its tasks onto a special internal queue. Then, when
the outermost `act` scope exists, we'll flush that queue.
I also removed the "wrong act" warning, because the plan is to move
`act` to an isomorphic entry point, simlar to `startTransition`. That's
not directly related to this PR, but I didn't want to bother
re-implementing that warning only to immediately remove it.
I'll add the isomorphic API in a follow up.
Note that the internal version of `act` that we use in our own tests
still depends on mocking the Scheduler package, because it needs to work
in production. I'm planning to move that implementation to a shared
(internal) module, too.
* Define global __WWW__ = true flag during www tests
We already do that for __PERSISTENT__.
* Use @gate www in ReactSuspenseCallback
This allows it to not be internal anymore. We test it against the www build.
In CI, we run our test suite against multiple build configurations. For
example, we run our tests in both dev and prod, and in both the
experimental and stable release channels. This is to prevent accidental
deviations in behavior between the different builds. If there's an
intentional deviation in behavior, the test author must account
for them.
However, we currently don't run tests against the www builds. That's
a problem, because it's common for features to land in www before they
land anywhere else, including the experimental release channel.
Typically we do this so we can gradually roll out the feature behind
a flag before deciding to enable it.
The way we test those features today is by mutating the
`shared/ReactFeatureFlags` module. There are a few downsides to this
approach, though. The flag is only overridden for the specific tests or
test suites where you apply the override. But usually what you want is
to run *all* tests with the flag enabled, to protect against unexpected
regressions.
Also, mutating the feature flags module only works when running the
tests against source, not against the final build artifacts, because the
ReactFeatureFlags module is inlined by the build script.
Instead, we should run the test suite against the www configuration,
just like we do for prod, experimental, and so on. I've added a new
command, `yarn test-www`. It automatically runs in CI.
Some of the www feature flags are dynamic; that is, they depend on
a runtime condition (i.e. a GK). These flags are imported from an
external module that lives in www. Those flags will be enabled for some
clients and disabled for others, so we should run the tests against
*both* modes.
So I've added a new global `__VARIANT__`, and a new test command `yarn
test-www-variant`. `__VARIANT__` is set to false by default; when
running `test-www-variant`, it's set to true.
If we were going for *really* comprehensive coverage, we would run the
tests against every possible configuration of feature flags: 2 ^
numberOfFlags total combinations. That's not practical, though, so
instead we only run against two combinations: once with `__VARIANT__`
set to `true`, and once with it set to `false`. We generally assume that
flags can be toggled independently, so in practice this should
be enough.
You can also refer to `__VARIANT__` in tests to detect which mode you're
running in. Or, you can import `shared/ReactFeatureFlags` and read the
specific flag you can about. However, we should stop mutating that
module going forward. Treat it as read-only.
In this commit, I have only setup the www tests to run against source.
I'll leave running against build for a follow up.
Many of our tests currently assume they run only in the default
configuration, and break when certain flags are toggled. Rather than fix
these all up front, I've hard-coded the relevant flags to the default
values. We can incrementally migrate those tests later.
