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Add supportsMicrotasks to the host config #20809

Merged
merged 2 commits into from
Feb 12, 2021
Merged

Add supportsMicrotasks to the host config #20809

merged 2 commits into from
Feb 12, 2021

Commits on Feb 12, 2021

  1. Add supportsMicrotasks to the host config 

    Only certain renderers support scheduling a microtask, so we need a
renderer specific flag that we can toggle. That way it's off for some
renderers and on for others.

I copied the approach we use for the other optional parts of the host
config, like persistent mode and test selectors.

Why isn't the feature flag sufficient?

The feature flag modules, confusingly, are not renderer-specific, at
least when running the our tests against the source files. They are
meant to correspond to a release channel, not a renderer, but we got
confused at some point and haven't cleaned it up.

For example, when we run `yarn test`, Jest loads the flags from the
default `ReactFeatureFlags.js` module, even when we import the React
Native renderer — but in the actual builds, we load a different feature
flag module, `ReactFeatureFlags.native-oss.js.` There's no way in our
current Jest load a different host config for each renderer, because
they all just import the same module. We should solve this by creating
separate Jest project for each renderer, so that the flags loaded when
running against source are the same ones that we use in the
compiled bundles.

The feature flag (`enableDiscreteMicrotasks`) still exists — it's used
to set the React DOM host config's `supportsMicrotasks` flag to `true`.
(Same for React Noop) The important part is that turning on the feature
flag does *not* affect the other renderers, like React Native.

The host config will likely outlive the feature flag, too, since the
feature flag only exists so we can gradually roll it out and measure the
impact in production; once we do, we'll remove it. Whereas the host
config flag may continue to be used to disable the discrete microtask
behavior for RN, because RN will likely use a native (non-JavaScript)
API to schedule its tasks.
    @acdlite
    acdlite committed Feb 12, 2021
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  2. Add supportsMicrotask to react-reconciler README

    @acdlite
    acdlite committed Feb 12, 2021
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