This was missed that we track the child index on the task. The
equivalent in retryRenderTask already has this.
The effect is that a lazy node that suspends gets its child index reset
to -1 even though it should resume in the index it left off.
When we postpone a prerender in the shell, we should just leave an empty
prelude and resume from the root. While preserving any options passed
in.
Since we haven't flushed anything we can't assume we've already emitted
html/body tags or any resources tracked in the resumable state. This
introduces a resetResumableState function to reset anything we didn't
flush.
This is a bit hacky. Ideally, we probably shouldn't have tracked it as
already happened until it flushed or something like that.
Basically, it's like restarting the prerender with the same options and
then immediately aborting. When we add the preload headers, we'd track
those as preload() being emitted after the reset and so they get readded
to the resumable state in that case.
This upgrade made the `React$Element` type opaque, which is good for
product code where accessing props of elements is code smell, but React
needs to use that internally. I overrode the type to restore it.
Float methods can hang on to a reference to a Request after the request
is closed due to AsyncLocalStorage. If a Float method is called at this
point we do not want to attempt to flush anything. This change updates
the closing logic to also call `stopFlowing` which will ensure that any
checks against the destination properly reflect that we cannot do any
writes. In addition it updates the enqueueFlush logic to existence check
the destination inside the work function since it can change across the
work scheduling gap if it is async.
fixes: https://github.com/facebook/react/issues/27540
This is the same problem as we had with keyPath before where if the
element itself suspends, we have to restore the replay node to what it
was before, however, if something below the element suspends we
shouldn't pop it because that will pop it back up the stack.
Instead of passing replay as an argument to every renderElement
function, I use a hack to compare if the node is still the same as the
one we tried to render, then that means we haven't stepped down into the
child yet. Maybe this is not quite correct because in theory you could
have a recursive node that just renders itself over and over until some
context bails out.
This solves an issue where if you suspended in an element it would retry
trying to replay from that element but using the postponed state from
the root.
The way we collect component stacks right now are pretty fragile.
We expect that we'll call captureBoundaryErrorDetailsDev whenever an
error happens. That resets lastBoundaryErrorComponentStackDev to null
but if we don't, it just lingers and we don't set it to anything new
then which leaks the previous component stack into the next time we have
an error. So we need to reset it in a bunch of places.
This is still broken with erroredReplay because it has the inverse
problem that abortRemainingReplayNodes can call
captureBoundaryErrorDetailsDev more than one time. So the second
boundary won't get a stack.
We probably should try to figure out an alternative way to carry along
the stack. Perhaps WeakMap keyed by the error object.
This also fixes an issue where we weren't invoking the onShellReady
event if we error a replay. That event is a bit weird for resuming
because we're probably really supposed to just invoke it immediately if
we have already flushed the shell in the prerender which is always atm.
Right now, it gets invoked later than necessary because you could have a
resumed hole ready before a sibling in the shell is ready and that's
blocked.
The `resumeElement` function wasn't actually doing the correct thing
because it was resuming the element itself but what the child slot means
is that we're supposed to resume in the direct child of the element.
This is difficult to check for since it's all the possible branches that
the element can render into, so instead we just check this in
renderNode. It means the hottest path always checks the task which is
unfortunate.
And also, resuming using the correct nextSegmentId.
Fixes two bugs surfaced by this test.
---------
Co-authored-by: Josh Story <josh.c.story@gmail.com>
This fixes so that you can postpone in a fallback. This postpones the
parent boundary. I track the fallbacks in a separate replay node so that
when we resume, we can replay the fallback itself and finish the
fallback and then possibly later the content. By doing this we also
ensure we don't complete the parent too early since now it has a render
task on it.
There is one case that this surfaces that isn't limited to
prerender/resume but also render/hydrateRoot. I left todos in the tests
for this.
If you postpone in a fallback, and suspend in the content but eventually
don't postpone in the content then we should be able to just skip
postponing since the content rendered and we no longer need the
fallback. This is a bit of a weird edge case though since fallbacks are
supposed to be very minimal.
This happens because in both cases the fallback starts rendering early
as soon as the content suspends. This also ensures that the parent
doesn't complete early by increasing the blocking tasks. Unfortunately,
the fallback will irreversibly postpone its parent boundary as soon as
it hits a postpone.
When you suspend, the same thing happens but we typically deal with this
by doing a "soft" abort on the fallback since we don't need it anymore
which unblocks the parent boundary. We can't do that with postpone right
now though since it's considered a terminal state.
I think I'll just leave this as is for now since it's an edge case but
it's an annoying exception in the model. Makes me feel I haven't quite
nailed it just yet.
If we track a postponed SuspenseBoundary parent that we have to replay
through before it has postponed and it postpones itself later, we need
to upgrade it to a postponed replay boundary instead of adding two.
We currently abort a stream either it's explicitly told to abort (e.g.
by an abortsignal). In this case we still finish writing what we have as
well as instructions for the client about what happened so it can
trigger fallback cases and log appropriately.
We also abort a request if the stream itself cancels. E.g. if you can't
write anymore. In this case we should not write anything to the outgoing
stream since it's supposed to be closed already now. However, we should
still abort the request so that more work isn't performed and so that we
can log the reason for it to the onError callback.
We should also not do any work after aborting.
There we need to stop the "flow" of bytes - so I call stopFlowing in the
cancel case before aborting.
The tests were testing this case but we had changed the implementation
to only start flowing at initial read (pull) instead of start like we
used to. As a result, it was no longer covering this case. We have to
call reader.read() in the tests to start the flow so that we need to
cancel it.
We also were missing a final assertion on the error logs and since we
were tracking them explicitly the extra error was silenced.
The key is that instead of storing different tags of resumable points,
we just store if a replay node has any resumable slots and if that's at
the root `number` or if it has resumable slots by index.
This is a simpler and more compact format because we don't have to
separate the three Resume forms.
This helps deal with Postpone in fallbacks because it doesn't just
double all the cases.
Moves writing queues to renderState.
We shouldn't need the resource tracking's value. We just need to know if
that resource has already been emitted. We can use a Set for this. To
ensure that set is directly serializable we can just use a
dictionary-like object with no value.
See individual commits for special cases.
Originally the intension was to have React assign an ID to a user
rendered DOM node inside a `fallback` while it was loading. If there
already were an explicit `id` defined on the DOM element we would reuse
that one instead. That's why this was a DOM Config option and not just
built in to Fizz.
This became tricky since it can load late and so we'd have to transfer
it down and detect it only once it finished rendering and if there is no
DOM element it doesn't work anyway. So instead, what we do in practice
is to always use a `<template>` tag with the ID. This has the downside
of an extra useless node and shifting child CSS selectors.
Maybe we'll get around to fixing this properly but it might not be worth
it.
This PR just gets rid of the SuspenseBoundaryID concept and instead we
just use the same ID number as the root segment ID of the boundary to
refer to the boundary to simplify the implementation.
This also solves the problem that SuspenseBoundaryID isn't currently
serializable (although that's easily fixable by itself if necessary).
Based on #27385.
When we error or abort during replay, that doesn't actually error the
component that errored because that has already rendered. The error only
affects any child that is not yet completed. Therefore the error kind of
gets thrown at the resumable point.
The resumable point might be a hole in the replay path, in which case
throwing there errors the parent boundary just the same as if the replay
component errored. If the hole is inside a deeper Suspense boundary
though, then it's that Suspense boundary that gets client rendered. I.e.
the child boundary. We can still finish any siblings.
In the shell all resumable points are inside a boundary since we must
have finished the shell. Therefore if you error in the root, we just
simply just turn all incomplete boundaries into client renders.
The idea for this check is that we shouldn't flush anything before we
flush the shell. That may or may not hold true in future formats like
RN.
It is a problem for resuming because with resuming it's possible to have
root tasks that are used for resuming but the shell was already flushed
so we can have completed boundaries before the shell has fully resumed.
What matters is whether the parent has already flushed or not.
It's not technically necessary to bail early because there won't be
anything in partialBoundaries or completedBoundaries because nothing
gets added there unless the parent has already flushed.
It's not exactly slow to have to check the length of three arrays so
it's probably not a big deal.
Flush partials in an early preamble needs further consideration
regardless.
This forks Task into ReplayTask and RenderTask.
A RenderTask is the normal mode and it has a segment to write into.
A ReplayTask doesn't have a segment to write into because that has
already been written but instead it has a ReplayState which keeps track
of the next set of paths to follow. Once we hit a "Resume" node we
convert it into a RenderTask and continue rendering from there.
We can resume at either an Element position or a Slot position. An
Element pointing to a component doesn't mean we resume that component,
it means we resume in the child position directly below that component.
Slots are slots inside arrays.
Instead of statically forking most paths, I kept using the same path and
checked for the existence of a segment or replay state dynamically at
runtime.
However, there's still quite a bit of forking here like retryRenderTask
and retryReplayTask. Even in the new forks there's a lot of duplication
like resumeSuspenseBoundary, replaySuspenseBoundary and
renderSuspenseBoundary. There's opportunity to simplify this a bit.
This is an optimization where useFormState will only emit extra comment
markers if a form state is passed at the root. If no state is passed, we
don't need to emit anything because none of the hooks will match.
Typically we assign IDs lazily when flushing to minimize the ids we have
to assign and we try to maximize inlining.
When we prerender we could always flush into a buffer before returning
but that's not actually what we do right now. We complete rendering
before returning but we don't actually run the flush path until someone
reads the resulting stream. We assign IDs eagerly when something
postpone so that we can refer to those ids in the holes without flushing
first.
This leads to some interesting conditions that needs to consider this.
This PR also deals with rootSegmentId which is the ID of the segment
that contains the body of a Suspense boundary. The boundary needs to
know this in addition to the Suspense Boundary's ID to know how to
inject the root segment into the boundary.
Why is the suspense boundary ID and the rootSegmentID just not the same
ID? Why don't segments and suspense boundary not share ID namespace?
That's a good question and I don't really remember. It might not be a
good reason and maybe they should just be the same.
During an MPA form submission, useFormState should only reuse the form
state if same action is passed both times. (We also compare the key
paths.)
We compare the identity of the inner closure function, disregarding the
value of the bound arguments. That way you can pass an inline Server
Action closure:
```js
function FormContainer({maxLength}) {
function submitAction(prevState, formData) {
'use server'
if (formData.get('field').length > maxLength) {
return { errorMsg: 'Too many characters' };
}
// ...
}
return <Form submitAction={submitAction} />
}
```
If a Server Action is passed to useFormState, the action may be
submitted before it has hydrated. This will trigger a full page
(MPA-style) navigation. We can transfer the form state to the next page
by comparing the key path of the hook instance.
`ReactServerDOMServer.decodeFormState` is used by the server to extract
the form state from the submitted action. This value can then be passed
as an option when rendering the new page. It must be passed during both
SSR and hydration.
```js
const boundAction = await decodeAction(formData, serverManifest);
const result = await boundAction();
const formState = decodeFormState(result, formData, serverManifest);
// SSR
const response = createFromReadableStream(<App />);
const ssrStream = await renderToReadableStream(response, { formState })
// Hydration
hydrateRoot(container, <App />, { formState });
```
If the `formState` option is omitted, then the state won't be
transferred to the next page. However, it must be passed in both places,
or in neither; misconfiguring will result in a hydration mismatch.
(The `formState` option is currently prefixed with `experimental_`)
This field was not being initialized. Although the property is part of
the Flow type, the type error wasn't caught because the constructor
itself is not covered by Flow, which is unfortunate. (I assume this is
related to the dev-only componentStack property.)
Currently, if a component suspends, the keyPath has already been
modified to include the identity of the component itself; the path is
set before the component body is called (akin to the begin phase in
Fiber). An accidental consequence is that when the promise resolves and
component is retried, the identity gets appended to the keyPath again,
leading to a duplicate node in the path.
To address this, we should only modify contexts after any code that may
suspend. For maximum safety, this should occur as late as possible:
right before the recursive renderNode call, before the children are
rendered.
I did not add a test yet because there's no feature that currently
observes it, but I do have tests in my other WIP PR for useFormState:
#27321
There's a subtle difference if you suspend before the first array or
after. In Fiber, we don't deal with this because we just suspend the
parent and replay it if lazy() or Usable are used in its child slots. In
Fizz we try to optimize this a bit more and enable resuming inside the
component.
Semantically, it's different if you suspend/postpone before the first
child array or inside that child array. Because when you resume the
inner result might be another array and either that's part of the parent
path or part of the inner slot.
There might be more clever way of structuring this but I just use -1 to
indicate that we're not yet inside the array and is in the root child
position. If that renders an element, then that's just the same as the 0
slot.
We need to also encode this in the resuming. I called that resuming the
element or resuming the slot.
It's possible to postpone a specific node and not using a wrapper
component. Therefore we encode the resumable slot as the index slot.
When it's a plain client component that postpones, it's encoded as the
child slot inside that component which is the one that's postponed
rather than the component itself.
Since it's possible for a child slot to suspend (e.g. React.lazy's
microtask in this case) retryTask might need to keep its index around
when it resolves.
A planned feature of useFormState is that if the page load is the result
of an MPA-style form submission — i.e. a form was submitted before it
was hydrated, using Server Actions — the state of the hook should
transfer to the next page.
I haven't implemented that part yet, but as a prerequisite, we need some
way for Fizz to indicate whether a useFormState hook was rendered using
the "postback" state. That way we can do all state matching logic on the
server without having to replicate it on the client, too.
The approach here is to emit a comment node for each useFormState hook.
We use one of two comment types: `<!--F-->` for a normal useFormState
hook, and `<!--F!-->` for a hook that was rendered using the postback
state. React will read these markers during hydration. This is similar
to how we encode Suspense boundaries.
Again, the actual matching algorithm is not yet implemented — for now,
the "not matching" marker is always emitted.
We can optimize this further by not emitting any markers for a render
that is not the result of a form postback, which I'll do in subsequent
PRs.
Just moving some internal code around again.
I originally encoded what type of work using startRender vs
startPrerender. I had intended to do more forking of the work loop but
we've decided not to go with that strategy. It also turns out that
forking when we start working is actually too late because of a subtle
thing where you can call abort before work begins. Therefore it's
important that starting the work comes later.
To generate IDs for useId, we modify a context variable whenever
multiple siblings are rendered, or when a component includes a useId
hook.
When this happens, we must ensure that the context is reset properly on
unwind if something errors or suspends. When I originally implemented
this, I did this by wrapping the child's rendering with a try/finally
block. But a better way to do this is by using the non-destructive
renderNode path instead of renderNodeDestructive.
In https://github.com/facebook/react/pull/21113 I moved this over to the
segment from the task. This partially reverts this two use two fields
instead. I was just trying to micro-optimize by reusing a single field.
This is really conceptually two different values. Task is keeping track
of the working state of the currently executing context.
The segment just needs to keep track of which parent context it was
created in so that it can be wrapped correctly when a segment is
written. We just happened to rely on the working state returning to the
top before completing.
The main motivation is that there is no `segment` for replaying.
This is basically the implementation for the prerender pass.
Instead of forking basically the whole implementation for prerender, I
just add a conditional field on the request. If it's `null` it behaves
like before. If it's non-`null` then instead of triggering client
rendered boundaries it triggers those into a "postponed" state which is
basically just a variant of "pending". It's supposed to be filled in
later.
It also builds up a serializable tree of which path can be followed to
find the holes. This is basically a reverse `KeyPath` tree.
It is unfortunate that this approach adds more code to the regular Fizz
builds but in practice. It seems like this side is not going to add much
code and we might instead just want to merge the builds so that it's
smaller when you have `prerender` and `resume` in the same bundle -
which I think will be common in practice.
This just implements the prerender side, and not the resume side, which
is why the tests have a TODO. That's in a follow up PR.
This exposes a `resume()` API to go with the `prerender()` (only in
experimental). It doesn't work yet since we don't yet emit the postponed
state so not yet tested.
The main thing this does is rename ResponseState->RenderState and
Resources->ResumableState. We separated out resources into a separate
concept preemptively since it seemed like separate enough but probably
doesn't warrant being a separate concept. The result is that we have a
per RenderState in the Config which is really just temporary state and
things that must be flushed completely in the prerender. Most things
should be ResumableState.
Most options are specified in the `prerender()` and transferred into the
`resume()` but certain options that are unique per request can't be.
Notably `nonce` is special. This means that bootstrap scripts and
external runtime can't use `nonce` in this mode. They need to have a CSP
configured to deal with external scripts, but not inline.
We need to be able to restore state of things that we've already emitted
in the prerender. We could have separate snapshot/restore methods that
does this work when it happens but that means we have to explicitly do
that work. This design is trying to keep to the principle that we just
work with resumable data structures instead so that we're designing for
it with every feature. It also makes restoring faster since it's just
straight into the data structure.
This is not yet a serializable format. That can be done in a follow up.
We also need to vet that each step makes sense. Notably stylesToHoist is
a bit unclear how it'll work.
Tracks the currently executing parent path of a task, using the name of
the component and the key or index.
This can be used to uniquely identify an instance of a component between
requests - assuming nothing in the parents has changed. Even if it has
changed, if things are properly keyed, it should still line up.
It's not used yet but we'll need this for two separate features so
should land this so we can stack on top.
Can be passed to `JSON.stringify(...)` to generate a unique key.
This adds an experimental `unstable_postpone(reason)` API.
Currently we don't have a way to model effectively an Infinite Promise.
I.e. something that suspends but never resolves. The reason this is
useful is because you might have something else that unblocks it later.
E.g. by updating in place later, or by client rendering.
On the client this works to model as an Infinite Promise (in fact,
that's what this implementation does). However, in Fizz and Flight that
doesn't work because the stream needs to end at some point. We don't
have any way of knowing that we're suspended on infinite promises. It's
not enough to tag the promises because you could await those and thus
creating new promises. The only way we really have to signal this
through a series of indirections like async functions, is by throwing.
It's not 100% safe because these values can be caught but it's the best
we can do.
Effectively `postpone(reason)` behaves like a built-in [Catch
Boundary](https://github.com/facebook/react/pull/26854). It's like
`raise(Postpone, reason)` except it's built-in so it needs to be able to
be encoded and caught by Suspense boundaries.
In Flight and Fizz these behave pretty much the same as errors. Flight
just forwards it to retrigger on the client. In Fizz they just trigger
client rendering which itself might just postpone again or fill in the
value. The difference is how they get logged.
In Flight and Fizz they log to `onPostpone(reason)` instead of
`onError(error)`. This log is meant to help find deopts on the server
like finding places where you fall back to client rendering. The reason
that you pass in is for that purpose to help the reason for any deopts.
I do track the stack trace in DEV but I don't currently expose it to
`onPostpone`. This seems like a limitation. It might be better to expose
the Postpone object which is an Error object but that's more of an
implementation detail. I could also pass it as a second argument.
On the client after hydration they don't get passed to
`onRecoverableError`. There's no global `onPostpone` API to capture
postponed things on the client just like there's no `onError`. At that
point it's just assumed to be intentional. It doesn't have any `digest`
or reason passed to the client since it's not logged.
There are some hacky solutions that currently just tries to reuse as
much of the existing code as possible but should be more properly
implemented.
- Fiber is currently just converting it to a fake Promise object so that
it behaves like an infinite Promise.
- Fizz is encoding the magic digest string `"POSTPONE"` in the HTML so
we know to ignore it but it should probably just be something neater
that doesn't share namespace with digests.
Next I plan on using this in the `/static` entry points for additional
features.
Why "postpone"? It's basically a synonym to "defer" but we plan on using
"defer" for other purposes and it's overloaded anyway.
This PR adds a preload for bootstrapScripts. preloads are captured
synchronously when you create a new Request and as such the normal logic
to check if a preload already exists is skipped.
E.g. if we suspend (throw a promise) in pushStartInstance today we might
have already pushed some chunks (or even child segments potentially). We
should revert back to where we were.
This doesn't usually happen because when we suspend in a component it
doesn't write anything itself, it'll always defer to som host instance
to do the writing.
There was a todo about this already but I'm not 100% sure it's always
safe when suspending. It should be safe when suspending just regularly
because it's just a noop. We might not even want "throwing a promise" in
this mechanism to be supported longer term but for now that's how a
suspend in internals.
Stacked on #26557
Supporting Float methods such as ReactDOM.preload() are challenging for
flight because it does not have an easy means to convey direct
executions in other environments. Because the flight wire format is a
JSON-like serialization that is expected to be rendered it currently
only describes renderable elements. We need a way to convey a function
invocation that gets run in the context of the client environment
whether that is Fizz or Fiber.
Fiber is somewhat straightforward because the HostDispatcher is always
active and we can just have the FlightClient dispatch the serialized
directive.
Fizz is much more challenging becaue the dispatcher is always scoped but
the specific request the dispatch belongs to is not readily available.
Environments that support AsyncLocalStorage (or in the future
AsyncContext) we will use this to be able to resolve directives in Fizz
to the appropriate Request. For other environments directives will be
elided. Right now this is pragmatic and non-breaking because all
directives are opportunistic and non-critical. If this changes in the
future we will need to reconsider how widespread support for async
context tracking is.
For Flight, if AsyncLocalStorage is available Float methods can be
called before and after await points and be expected to work. If
AsyncLocalStorage is not available float methods called in the sync
phase of a component render will be captured but anything after an await
point will be a noop. If a float call is dropped in this manner a DEV
warning should help you realize your code may need to be modified.
This PR also introduces a way for resources (Fizz) and hints (Flight) to
flush even if there is not active task being worked on. This will help
when Float methods are called in between async points within a function
execution but the task is blocked on the entire function finishing.
This PR also introduces deduping of Hints in Flight using the same
resource keys used in Fizz. This will help shrink payload sizes when the
same hint is attempted to emit over and over again
Added an explicit type to all $FlowFixMe suppressions to reduce
over-suppressions of new errors that might be caused on the same lines.
Also removes suppressions that aren't used (e.g. in a `@noflow` file as
they're purely misleading)
Test Plan:
yarn flow-ci
## Based on #25634
Like promises, this adds support for Context as a React node.
In this initial implementation, the context dependency is added to the
parent of child node. This allows the parent to re-reconcile its
children when the context updates, so that it can delete the old node if
the identity of the child has changed (i.e. if the key or type of an
element has changed). But it also means that the parent will replay its
entire begin phase. Ideally React would delete the old node and mount
the new node without reconciling all the children. I'll leave this for a
future optimization.
Implements Promise as a valid React node types. The idea is that any
type that can be unwrapped with `use` should also be renderable.
When the reconciler encounters a Usable in a child position, it will
transparently unwrap the value before reconciling it. The value of the
inner value will determine the identity of the child during
reconciliation, not the Usable object that wraps around it.
Unlike `use`, the reconciler will recursively unwrap the value until it
reaches a non-Usable type, e.g. `Usable<Usable<Usable<T>>>` will resolve
to T.
In this initial commit, I've added support for Promises. I will do
Context in the [next
step](https://github.com/facebook/react/pull/25641).
Being able to render a promise as a child has several interesting
implications. The Server Components response format can use this feature
in its implementation — instead of wrapping references to client
components in `React.lazy`, it can just use a promise.
This also fulfills one of the requirements for async components on the
client, because an async component always returns a promise for a React
node. However, we will likely warn and/or lint against this for the time
being because there are major caveats if you re-render an async
component in response to user input. (Note: async components already
work in a Server Components environment — the caveats only apply to
running them in the browser.)
To suspend, React uses the same algorithm as `use`: by throwing an
exception to unwind the stack, then replaying the begin phase once the
promise resolves. It's a little weird to suspend during reconciliation,
however, `lazy` already does this so if there were any obvious bugs
related to that we likely would have already found them.
Still, the structure is a bit unfortunate. Ideally, we shouldn't need to
replay the entire begin phase of the parent fiber in order to reconcile
the children again. This would require a somewhat significant refactor,
because reconciliation happens deep within the begin phase, and
depending on the type of work, not always at the end. We should consider
as a future improvement.
There is a problem with <style> as resource. For css-in-js libs there
may be an very large number of these hoistables being created. The
number of style tags can grow quickly and to help reduce the prevalence
of this FIzz now aggregates all style tags for a given precedence into a
single tag. The client can 'hydrate' against these compound tags but
currently on the client insertions are done individually.
additionally drops the implementation where style tags are embedding in
a template for one where `media="not all"` is set. The idea is to have
the browser construct the underlying stylesheet eagerly which does not
happen if the tag is embedded in a template
Key Decision:
One choice made in this PR is that we flush style tags eagerly even if a
boundary is blocked that is the only thing that depends on that style
rule. The reason we are starting with this implementation is that it
allows a very condensed representation of the style resources. If we
tracked which rules were used in which boundaries we would need a style
resource for every rendered <style> tag. This could be problematic for
css-in-js libs that might render hundreds or thousands of style tags.
The tradeoff here is we slightly delay content reveal in some cases (we
send extra bytes) but we have fewer DOM tags and faster SSR runtime
## Hoistables
In the original implementation of Float, all hoisted elements were
treated like Resources. They had deduplication semantics and hydrated
based on a key. This made certain kinds of hoists very challenging such
as sequences of meta tags for `og:image:...` metadata. The reason is
each tag along is not dedupable based on only it's intrinsic properties.
two identical tags may need to be included and hoisted together with
preceding meta tags that describe a semantic object with a linear set of
html nodes.
It was clear that the concept of Browser Resources (stylesheets /
scripts / preloads) did not extend universally to all hositable tags
(title, meta, other links, etc...)
Additionally while Resources benefit from deduping they suffer an
inability to update because while we may have multiple rendered elements
that refer to a single Resource it isn't unambiguous which element owns
the props on the underlying resource. We could try merging props, but
that is still really hard to reason about for authors. Instead we
restrict Resource semantics to freezing the props at the time the
Resource is first constructed and warn if you attempt to render the same
Resource with different props via another rendered element or by
updating an existing element for that Resource.
This lack of updating restriction is however way more extreme than
necessary for instances that get hoisted but otherwise do not dedupe;
where there is a well defined DOM instance for each rendered element. We
should be able to update props on these instances.
Hoistable is a generalization of what Float tries to model for hoisting.
Instead of assuming every hoistable element is a Resource we now have
two distinct categories, hoistable elements and hoistable resources. As
one might guess the former has semantics that match regular Host
Components except the placement of the node is usually in the <head>.
The latter continues to behave how the original implementation of
HostResource behaved with the first iteration of Float
### Hoistable Element
On the server hoistable elements render just like regular tags except
the output is stored in special queues that can be emitted in the stream
earlier than they otherwise would be if rendered in place. This also
allow for instance the ability to render a hoistable before even
rendering the <html> tag because the queues for hoistable elements won't
flush until after we have flushed the preamble (`<DOCTYPE
html><html><head>`).
On the client, hoistable elements largely operate like HostComponents.
The most notable difference is in the hydration strategy. If we are
hydrating and encounter a hoistable element we will look for all tags in
the document that could potentially be a match and we check whether the
attributes match the props for this particular instance. We also do this
in the commit phase rather than the render phase. The reason hydration
can be done for HostComponents in render is the instance will be removed
from the document if hydration fails so mutating it in render is safe.
For hoistables the nodes are not in a hydration boundary (Root or
SuspenseBoundary at time of writing) and thus if hydration fails and we
may have an instance marked as bound to some Fiber when that Fiber never
commits. Moving the hydration matching to commit ensures we will always
succeed in pairing the hoisted DOM instance with a Fiber that has
committed.
### Hoistable Resource
On the server and client the semantics of Resources are largely the same
they just don't apply to title, meta, and most link tags anymore.
Resources hoist and dedupe via an `href` key and are ref counted. In a
future update we will add a garbage collector so we can clean up
Resources that no longer have any references
## `<style>` support
In earlier implementations there was no support for <style> tags. This
PR adds support for treating `<style href="..."
precedence="...">...</style>` as a Resource analagous to `<link
rel="stylesheet" href="..." precedence="..." />`
It may seem odd at first to require an href to get Resource semantics
for a style tag. The rationale is that these are for inlining of actual
external stylesheets as an optimization and for URI like scoping of
inline styles for css-in-js libraries. The href indicates that the key
space for `<style>` and `<link rel="stylesheet" />` Resources is shared.
and the precedence is there to allow for interleaving of both kinds of
Style resources. This is an advanced feature that we do not expect most
app developers to use directly but will be quite handy for various
styling libraries and for folks who want to inline as much as possible
once Fizz supports this feature.
## refactor notes
* HostResource Fiber type is renamed HostHoistable to reflect the
generalization of the concept
* The Resource object representation is modified to reduce hidden class
checks and to use less memory overall
* The thing that distinguishes a resource from an element is whether the
Fiber has a memoizedState. If it does, it will use resource semantics,
otherwise element semantics
* The time complexity of matching hositable elements for hydration
should be improved
This enables the "exact_empty_objects" setting for Flow which makes
empty objects exact instead of building up the type as properties are
added in code below. This is in preparation to Flow 191 which makes this
the default and removes the config.
More about the change in the Flow blog
[here](https://medium.com/flow-type/improved-handling-of-the-empty-object-in-flow-ead91887e40c).
This setting is an incremental path to the next Flow version enforcing
type annotations on most functions (except some inline callbacks).
Used
```
node_modules/.bin/flow codemod annotate-functions-and-classes --write .
```
to add a majority of the types with some hand cleanup when for large
inferred objects that should just be `Fiber` or weird constructs
including `any`.
Suppressed the remaining issues.
Builds on #25918
Flow introduced a new syntax to annotated the context type of a
function, this tries to update the rest and add 1 example usage.
- 2b1fb91a55 already added the changes
required for eslint.
- Jest transform is updated to use the recommended `hermes-parser` which
can parse current and Flow syntax and will be updated in the future.
- Rollup uses a new plugin to strip the flow types. This isn't ideal as
the npm module is deprecated in favor of using `hermes-parser`, but I
couldn't figure out how to integrate that with Rollup.
### Changes made:
- Running with enableFizzExternalRuntime (feature flag) and
unstable_externalRuntimeSrc (param) will generate html nodes with data
attributes that encode Fizz instructions.
```
<div
hidden data-rxi=""
data-bid="param0"
data-dgst="param1"
></div>
```
- Added an external runtime browser script
`ReactDOMServerExternalRuntime`, which processes and removes these nodes
- This runtime should be passed as to renderInto[...] via
`unstable_externalRuntimeSrc`
- Since this runtime is render blocking (for all streamed suspense
boundaries and segments), we want this to reach the client as early as
possible. By default, Fizz will send this script at the end of the shell
when it detects dynamic content (e.g. suspenseful pending tasks), but it
can be sent even earlier by calling `preinit(...)` inside a component.
- The current implementation relies on Float to dedupe sending
`unstable_externalRuntimeSrc`, so `enableFizzExternalRuntime` is only
valid when `enableFloat` is also set.
