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Support sync thenables for lazy() #14626

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merged 2 commits into from Jan 18, 2019
Merged

Support sync thenables for lazy() #14626

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0695c13
Support sync thenables for lazy()
gaearon Jan 18, 2019
7f9f852
Don't commit twice
gaearon Jan 18, 2019
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6 changes: 5 additions & 1 deletion packages/react-reconciler/src/ReactFiberLazyComponent.js
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Expand Up @@ -47,6 +47,7 @@ export function readLazyComponentType<T>(lazyComponent: LazyComponent<T>): T {
lazyComponent._status = Pending;
const ctor = lazyComponent._ctor;
const thenable = ctor();
lazyComponent._result = thenable;
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This is still needed because we read thenable from lazyComponent._result while it's pending. If we keep the assignment, it will overwrite sync thenables. But if we remove it at all, we will throw null instead of thenable. This fixes both cases.

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You still need the assignment, but I don't think you need to move it up here. If you keep it right after the early return branch you added on line 78-80, then there's always a single assignment.

if (lazyComponent._status === Resolved) {
  return lazyComponent._result;
}
lazyComponent._result = thenable;
throw thenable;

thenable.then(
moduleObject => {
if (lazyComponent._status === Pending) {
Expand All @@ -73,7 +74,10 @@ export function readLazyComponentType<T>(lazyComponent: LazyComponent<T>): T {
}
},
);
lazyComponent._result = thenable;
// Check if it resolved synchronously
if (lazyComponent._status === Resolved) {
return lazyComponent._result;
}
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We could also do the same for failure. But doesn't seem worth the extra code to me. Could be a recursive call but then there's more risk of messing it up and going into a stack overflow. Meh.

throw thenable;
}
}
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17 changes: 17 additions & 0 deletions packages/react-reconciler/src/__tests__/ReactLazy-test.internal.js
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Expand Up @@ -61,6 +61,23 @@ describe('ReactLazy', () => {
expect(root).toMatchRenderedOutput('Hi again');
});

it('can resolve synchronously without suspending', async () => {
const LazyText = lazy(() => ({
then(cb) {
cb({default: Text});
},
}));

const root = ReactTestRenderer.create(
<Suspense fallback={<Text text="Loading..." />}>
<LazyText text="Hi" />
</Suspense>,
);

expect(ReactTestRenderer).toHaveYielded(['Hi']);
expect(root).toMatchRenderedOutput('Hi');
});

it('multiple lazy components', async () => {
function Foo() {
return <Text text="Foo" />;
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