These are based on the ReactNoop renderer, which we use to test React
itself. This gives library authors (Relay, Apollo, Redux, et al.) a way
to test their components for async compatibility.
- Pass `unstable_isAsync` to `TestRenderer.create` to create an async
renderer instance. This causes updates to be lazily flushed.
- `renderer.unstable_yield` tells React to yield execution after the
currently rendering component.
- `renderer.unstable_flushAll` flushes all pending async work, and
returns an array of yielded values.
- `renderer.unstable_flushThrough` receives an array of expected values,
begins rendering, and stops once those values have been yielded. It
returns the array of values that are actually yielded. The user should
assert that they are equal.
Although we've used this pattern successfully in our own tests, I'm not
sure if these are the final APIs we'll make public.
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