Move legacy hidden API to new internal Fiber type #18782
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react/packages/react-reconciler/src/ReactFiberBeginWork.new.js Lines 3263 to 3280 in b330a29 |
lunaruan
approved these changes
Apr 30, 2020
swappsoftwarehouse
approved these changes
Apr 30, 2020
gabrieldonadel
approved these changes
Apr 30, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
This only exists so we can clean up the internal implementation of
`<div hidden={isHidden} />`, which is not a stable feature. The goal
is to move everything to the new Offscreen type instead. However,
Offscreen has different semantics, so before we can remove the legacy
API, we have to migrate our internal usage at Facebook. So we'll need
to maintain both temporarily.
In this initial commit, I've only added the type. It's not used
anywhere. The next step is to use it to implement `hidden`.
If a host component receives a `hidden` prop, we wrap its children in
an Offscreen fiber. This is similar to what we do for Suspense children.
The LegacyHidden type happens to share the same implementation as the
new Offscreen type, for now, but using separate types allows us to fork
the behavior later when we implement our planned changes to the
Offscreen API.
There are two subtle semantic changes here. One is that the children of
the host component will have their visibility toggled using the same
mechanism we use for Offscreen and Suspense: find the nearest host node
children and give them a style of `display: none`. We didn't used to do
this in the old API, because the `hidden` DOM attribute on the parent
already hides them. So with this change, we're actually "overhiding" the
children. I considered addressing this, but I figure I'll leave it as-is
in case we want to expose the LegacyHidden component type temporarily
to ease migration of Facebook's internal callers to the Offscreen type.
The other subtle semantic change is that, because of the extra fiber
that wraps around the children, this pattern will cause the children
to lose state:
```js
return isHidden ? <div hidden={true} /> : <div />;
```
The reason is that I didn't want to wrap every single host component
in an extra fiber. So I only wrap them if a `hidden` prop exists. In
the above example, that means the children are conditionally wrapped
in an extra fiber, so they don't line up during reconciliation, so
they get remounted every time `isHidden` changes.
The fix is to rewrite to:
```js
return <div hidden={isHidden} />;
```
I don't anticipate this will be a problem at Facebook, especially since
we're only supposed to use `hidden` via a userspace wrapper component.
(And since the bad pattern isn't very React-y, anyway.)
Again, the eventual goal is to delete this completely and replace it
with Offscreen.
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@sebmarkbage I'm going to go ahead and merge this so Luna can build on top. It only affects the new reconciler. If you have feedback after the fact, I'll address in a follow up. |
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Based on #18733
Diff compared to previous PR in stack
Adds a new LegacyHidden fiber type.
This only exists so we can clean up the internal implementation of
<div hidden={isHidden} />, which is not a stable feature. The goal is to move everything to the new Offscreen type instead. However, Offscreen has different semantics, so before we can remove the legacy API, we have to migrate our internal usage at Facebook. So we'll need to maintain both temporarily.If a host component receives a
hiddenprop, we wrap its children in an LegacyHidden fiber. This is similar to what we do for Suspense children.The LegacyHidden type happens to share the same implementation as the new Offscreen type, for now, but using separate types allows us to fork the behavior later when we implement our planned changes to the Offscreen API.
There are two subtle semantic changes here. One is that the children of the host component wil have their visibility toggled using the same mechanism we use for Offscreen and Suspense: find the nearest host node children and give them a style of
display: none. We didn't used to do this in the old API, because thehiddenDOM attribute on the parent already hides them. So with this change, we're actually "overhiding" the children. I considered addressing this, but I figure I'll leave it as-is in case we want to expose the LegacyHidden component type temporarily to ease migration of Facebook's internal callers to the Offscreen type.The other subtle semantic change is that, because of the extra fiber that wraps around the children, this pattern will cause the children to lose state:
The reason is that I didn't want to wrap every single host component in an extra fiber. So I only wrap them if a
hiddenprop exists. In the above example, that means the children are conditionally wrapped in an extra fiber, so they don't line up during reconciliation, so they get remounted every timeisHiddenchanges.The fix is to rewrite to:
I don't anticipate this will be a problem at Facebook, especially since we're only supposed to use
hiddenvia a userspace wrapper component. (And since the bad pattern isn't very React-y, anyway.)Again, the eventual goal is to delete this completely and replace it with Offscreen.