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Allow suspending in the shell during hydration
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Builds on behavior added in facebook#23267.

Initial hydration should be allowed to suspend in the shell. In
practice, this happens because the code for the outer shell hasn't
loaded yet.

Currently if you try to do this, it errors because it expects there to
be a parent Suspense boundary, because without a fallback we can't
produce a consistent tree. However, for non-sync updates, we don't need
to produce a consistent tree immediately — we can delay the commit
until the data resolves.

In facebook#23267, I added support for suspending without a parent boundary if
the update was wrapped with `startTransition`. Here, I've expanded this
to include hydration, too.

I wonder if we should expand this even further to include all non-sync/
discrete updates.
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@acdlite
acdlite committed Feb 15, 2022
1 parent 27b5699 commit ad66295
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197 changes: 197 additions & 0 deletions packages/react-dom/src/__tests__/ReactDOMFizzShellHydration-test.js
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/**
* Copyright (c) Facebook, Inc. and its affiliates.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @emails react-core
*/

let JSDOM;
let React;
let ReactDOM;
let Scheduler;
let clientAct;
let ReactDOMFizzServer;
let Stream;
let document;
let writable;
let container;
let buffer = '';
let hasErrored = false;
let fatalError = undefined;
let textCache;

describe('useId', () => {
beforeEach(() => {
jest.resetModules();
JSDOM = require('jsdom').JSDOM;
React = require('react');
ReactDOM = require('react-dom');
Scheduler = require('scheduler');
clientAct = require('jest-react').act;
ReactDOMFizzServer = require('react-dom/server');
Stream = require('stream');

textCache = new Map();

// Test Environment
const jsdom = new JSDOM(
'<!DOCTYPE html><html><head></head><body><div id="container">',
{
runScripts: 'dangerously',
},
);
document = jsdom.window.document;
container = document.getElementById('container');

buffer = '';
hasErrored = false;

writable = new Stream.PassThrough();
writable.setEncoding('utf8');
writable.on('data', chunk => {
buffer += chunk;
});
writable.on('error', error => {
hasErrored = true;
fatalError = error;
});
});

async function serverAct(callback) {
await callback();
// Await one turn around the event loop.
// This assumes that we'll flush everything we have so far.
await new Promise(resolve => {
setImmediate(resolve);
});
if (hasErrored) {
throw fatalError;
}
// JSDOM doesn't support stream HTML parser so we need to give it a proper fragment.
// We also want to execute any scripts that are embedded.
// We assume that we have now received a proper fragment of HTML.
const bufferedContent = buffer;
buffer = '';
const fakeBody = document.createElement('body');
fakeBody.innerHTML = bufferedContent;
while (fakeBody.firstChild) {
const node = fakeBody.firstChild;
if (node.nodeName === 'SCRIPT') {
const script = document.createElement('script');
script.textContent = node.textContent;
fakeBody.removeChild(node);
container.appendChild(script);
} else {
container.appendChild(node);
}
}
}

function resolveText(text) {
const record = textCache.get(text);
if (record === undefined) {
const newRecord = {
status: 'resolved',
value: text,
};
textCache.set(text, newRecord);
} else if (record.status === 'pending') {
const thenable = record.value;
record.status = 'resolved';
record.value = text;
thenable.pings.forEach(t => t());
}
}

function readText(text) {
const record = textCache.get(text);
if (record !== undefined) {
switch (record.status) {
case 'pending':
throw record.value;
case 'rejected':
throw record.value;
case 'resolved':
return record.value;
}
} else {
Scheduler.unstable_yieldValue(`Suspend! [${text}]`);

const thenable = {
pings: [],
then(resolve) {
if (newRecord.status === 'pending') {
thenable.pings.push(resolve);
} else {
Promise.resolve().then(() => resolve(newRecord.value));
}
},
};

const newRecord = {
status: 'pending',
value: thenable,
};
textCache.set(text, newRecord);

throw thenable;
}
}

// function Text({text}) {
// Scheduler.unstable_yieldValue(text);
// return text;
// }

function AsyncText({text}) {
readText(text);
Scheduler.unstable_yieldValue(text);
return text;
}

function resetTextCache() {
textCache = new Map();
}

test('suspending in the shell', async () => {
const div = React.createRef(null);

function App() {
return (
<div ref={div}>
<AsyncText text="Shell" />
</div>
);
}

// Server render
await resolveText('Shell');
await serverAct(async () => {
const {pipe} = ReactDOMFizzServer.renderToPipeableStream(<App />);
pipe(writable);
});
expect(Scheduler).toHaveYielded(['Shell']);
const dehydratedDiv = container.getElementsByTagName('div')[0];

// Clear the cache and start rendering on the client
resetTextCache();

// Hydration suspends because the data for the shell hasn't loaded yet
await clientAct(async () => {
ReactDOM.hydrateRoot(container, <App />);
});
expect(Scheduler).toHaveYielded(['Suspend! [Shell]']);
expect(div.current).toBe(null);
expect(container.textContent).toBe('Shell');

// The shell loads and hydration finishes
await clientAct(async () => {
await resolveText('Shell');
});
expect(Scheduler).toHaveYielded(['Shell']);
expect(div.current).toBe(dehydratedDiv);
expect(container.textContent).toBe('Shell');
});
});
7 changes: 6 additions & 1 deletion packages/react-reconciler/src/ReactFiberThrow.new.js
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Expand Up @@ -483,12 +483,17 @@ function throwException(
// No boundary was found. If we're inside startTransition, this is OK.
// We can suspend and wait for more data to arrive.

if (includesOnlyTransitions(rootRenderLanes)) {
if (includesOnlyTransitions(rootRenderLanes) || getIsHydrating()) {
// This is a transition. Suspend. Since we're not activating a Suspense
// boundary, this will unwind all the way to the root without performing
// a second pass to render a fallback. (This is arguably how refresh
// transitions should work, too, since we're not going to commit the
// fallbacks anyway.)
//
// This case also applies to initial hydration.
//
// TODO: Maybe we should expand this branch to cover all non-sync
// updates, including default.
attachPingListener(root, wakeable, rootRenderLanes);
renderDidSuspendDelayIfPossible();
return;
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7 changes: 6 additions & 1 deletion packages/react-reconciler/src/ReactFiberThrow.old.js
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Original file line number Diff line number Diff line change
Expand Up @@ -483,12 +483,17 @@ function throwException(
// No boundary was found. If we're inside startTransition, this is OK.
// We can suspend and wait for more data to arrive.

if (includesOnlyTransitions(rootRenderLanes)) {
if (includesOnlyTransitions(rootRenderLanes) || getIsHydrating()) {
// This is a transition. Suspend. Since we're not activating a Suspense
// boundary, this will unwind all the way to the root without performing
// a second pass to render a fallback. (This is arguably how refresh
// transitions should work, too, since we're not going to commit the
// fallbacks anyway.)
//
// This case also applies to initial hydration.
//
// TODO: Maybe we should expand this branch to cover all non-sync
// updates, including default.
attachPingListener(root, wakeable, rootRenderLanes);
renderDidSuspendDelayIfPossible();
return;
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