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Add a failing test for remounting fallback in sync mode #14078

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Add a failing test for remounting fallback in sync mode #14078

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60 changes: 60 additions & 0 deletions packages/react-reconciler/src/__tests__/ReactSuspense-test.internal.js
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Original file line number Diff line number Diff line change
Expand Up @@ -631,5 +631,65 @@ describe('ReactSuspense', () => {
expect(root).toFlushAndYield(['Step: 3']);
expect(root).toMatchRenderedOutput('Step: 3');
});

it('does not remount the fallback while suspended children resolve in sync mode', () => {
let mounts = 0;
class ShouldMountOnce extends React.Component {
componentDidMount() {
mounts++;
}
render() {
return <Text text="Loading..." />;
}
}

function App(props) {
return (
<Suspense maxDuration={10} fallback={<ShouldMountOnce />}>
<AsyncText ms={1000} text="Child 1" />
<AsyncText ms={2000} text="Child 2" />
<AsyncText ms={3000} text="Child 3" />
</Suspense>
);
}

const root = ReactTestRenderer.create(<App />);

// Initial render
expect(ReactTestRenderer).toHaveYielded([
'Suspend! [Child 1]',
'Suspend! [Child 2]',
'Suspend! [Child 3]',
'Loading...',
]);
expect(root).toFlushAndYield([]);
jest.advanceTimersByTime(1000);
expect(ReactTestRenderer).toHaveYielded([
'Promise resolved [Child 1]',
'Child 1',
'Suspend! [Child 2]',
'Suspend! [Child 3]',
'Loading...',
]);
jest.advanceTimersByTime(1000);
expect(ReactTestRenderer).toHaveYielded([
'Promise resolved [Child 2]',
'Child 2',
'Suspend! [Child 3]',
'Loading...',
'Suspend! [Child 3]', // TODO: why does this suspend twice?
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This was also weird

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It's a known flaw with sync mode. Pings aren't batched, so we get a synchronous retry for every individual ping. Added a comment in the rebased version of your test:

react/packages/react-reconciler/src/__tests__/ReactSuspense-test.internal.js

Lines 739 to 741 in 7432297

// TODO: This suspends twice because there were two pings, once for each
// time Child 2 suspended. This is only an issue in sync mode because
// pings aren't batched.

'Loading...',
]);
jest.advanceTimersByTime(1000);
expect(ReactTestRenderer).toHaveYielded([
'Promise resolved [Child 3]',
'Child 3',
]);
expect(root).toMatchRenderedOutput(
['Child 1', 'Child 2', 'Child 3'].join(''),
);
// TODO: this fails. Is it by design?
expect(mounts).toBe(1);
});
});
});