Experimental support for resuming suspended hydration

[developit]

This PR adds two metadata properties to VNodes which are used to store the isHydrating flag and current DOM element (oldDom) when bailing out of diffing as a result of a suspension (or error).

This is particularly useful during hydration, as it means we can flip back into hydration mode and pick up the previously-attempted DOM element when we attempt to render the previously-suspended tree's root VNode.

The implementation here is not great in terms of filesize, and I haven't added dedicated tests for it yet. I have verified that this works with a prototype Suspense-based AsyncComponent implementation for Preact CLI (included below) that can be tested via npm link here.

experimental Suspense-based async-component.js for preact cli

The main thing worth noting here is that there is no use of internals.

How it works: the generated wrapper component (AsyncComponent) renders a child component that it knows will throw a given value (an object for simple referential equality). It then checks for that value having been thrown in componentDidCatch and absorbs the error, which has the effect of dropping its subtree's rendering on the floor. This preserves the DOM tree in-tact during the period where the lazy-loaded component is not yet available.

It perfom this throw routing for all renders up until its payload (whatever is passed to the callback given to load) becomes available. Once available, it "unsuspends" by triggering a re-render using setState({}). This is where the magic happens:

Since Preact knows the original "owner" VNode behind this component was suspended (in hydrate mode!) and has maintained a reference to its associated DOM element at time of suspension, it will immediately jump back into hydrate mode in the diff() call originating from setState and re-use that stored DOM element.

import { h, Component } from 'preact';

const PENDING = {};

function Pending() {
	throw PENDING;
}

export default function async (load) {
	let component;
	function AsyncComponent() {
		Component.call(this);
		if (!component) {
			this.componentWillMount = (() => {
				load(mod => {
					component = mod && mod.default || mod;
					this.componentDidCatch = null;
					this.setState({});
				});
			}
			),
			this.componentDidCatch = err => {
				if (err !== PENDING) throw err;
			};
			this.shouldComponentUpdate = () => component != null;
		}
		this.render = props => h(component || Pending, props);
	}
	AsyncComponent.preload = load;
	(AsyncComponent.prototype = new Component).constructor = AsyncComponent;
	return AsyncComponent;
}

[coveralls]

Coverage increased (+0.0009%) to 99.789% when pulling c8eb59f on experimental-suspense-hydration into 0da8005 on master.

[JoviDeCroock]

This looks really good to me

[JoviDeCroock]

Can be reduced to:

Suggested change

	newVNode._dom = oldDom;
	oldVNode._hydrateDom = newVNode._dom = oldDom;

This removes the need for the next line (multi-line suggestions aren't available)

[JoviDeCroock]

Suggested change

	oldDom = oldVNode._hydrateDom;
	newVNode._dom = oldDom = oldVNode._hydrateDom;

[prateekbh]

I tried reproducing the broken test in a real world app but couldn't everything worked fine there

[prateekbh]

@developit am i supposed to see this with your given instructions?

[github-actions]

Size Change: +188 B (0%)

Total Size: 38.3 kB

Filename	Size	Change
dist/preact.js	3.77 kB	+47 B (1%)
dist/preact.min.js	3.77 kB	+47 B (1%)
dist/preact.module.js	3.79 kB	+48 B (1%)
dist/preact.umd.js	3.83 kB	+46 B (1%)

ℹ️ View Unchanged

Filename	Size	Change
compat/dist/compat.js	3 kB	0 B
compat/dist/compat.module.js	3.03 kB	0 B
compat/dist/compat.umd.js	3.05 kB	0 B
debug/dist/debug.js	2.95 kB	0 B
debug/dist/debug.module.js	2.93 kB	0 B
debug/dist/debug.umd.js	3.01 kB	0 B
devtools/dist/devtools.js	175 B	0 B
devtools/dist/devtools.module.js	185 B	0 B
devtools/dist/devtools.umd.js	252 B	0 B
hooks/dist/hooks.js	1.05 kB	0 B
hooks/dist/hooks.module.js	1.08 kB	0 B
hooks/dist/hooks.umd.js	1.13 kB	0 B
test-utils/dist/testUtils.js	390 B	0 B
test-utils/dist/testUtils.module.js	392 B	0 B
test-utils/dist/testUtils.umd.js	469 B	0 B

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