[Fiber] Always flush Default priority in the microtask if a Transition was scheduled #33186
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rickhanlonii
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Stacked on #33159. This implements `onDefaultTransitionIndicator`. The sequence is: 1) In `markRootUpdated` we schedule Transition updates as needing `indicatorLanes` on the root. This tracks the lanes that currently need an indicator to either start or remain going until this lane commits. 2) Track mutations during any commit. We use the same hook that view transitions use here but instead of tracking it just per view transition scope, we also track a global boolean for the whole root. 3) If a sync/default commit had any mutations, then we clear the indicator lane for the `currentEventTransitionLane`. This requires that the lane is still active while we do these commits. See #33159. In other words, a sync update gets associated with the current transition and it is assumed to be rendering the loading state for that corresponding transition so we don't need a default indicator for this lane. 4) At the end of `processRootScheduleInMicrotask`, right before we're about to enter a new "event transition lane" scope, it is no longer possible to render any more loading states for the current transition lane. That's when we invoke `onDefaultTransitionIndicator` for any roots that have new indicator lanes. 5) When we commit, we remove the finished lanes from `indicatorLanes` and once that reaches zero again, then we can clean up the default indicator. This approach means that you can start multiple different transitions while an indicator is still going but it won't stop/restart each time. Instead, it'll wait until all are done before stopping. Follow ups: - [x] Default updates are currently not enough to cancel because those aren't flush in the same microtask. That's unfortunate. #33186 - [x] Handle async actions before the setState. Since these don't necessarily have a root this is tricky. #33190 - [x] Disable for `useDeferredValue`. ~Since it also goes through `markRootUpdated` and schedules a Transition lane it'll get a default indicator even though it probably shouldn't have one.~ EDIT: Turns out this just works because it doesn't go through `markRootUpdated` when work is left behind. - [x] Implement built-in DOM version by default. #33162
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Stacked on #33159. This implements `onDefaultTransitionIndicator`. The sequence is: 1) In `markRootUpdated` we schedule Transition updates as needing `indicatorLanes` on the root. This tracks the lanes that currently need an indicator to either start or remain going until this lane commits. 2) Track mutations during any commit. We use the same hook that view transitions use here but instead of tracking it just per view transition scope, we also track a global boolean for the whole root. 3) If a sync/default commit had any mutations, then we clear the indicator lane for the `currentEventTransitionLane`. This requires that the lane is still active while we do these commits. See #33159. In other words, a sync update gets associated with the current transition and it is assumed to be rendering the loading state for that corresponding transition so we don't need a default indicator for this lane. 4) At the end of `processRootScheduleInMicrotask`, right before we're about to enter a new "event transition lane" scope, it is no longer possible to render any more loading states for the current transition lane. That's when we invoke `onDefaultTransitionIndicator` for any roots that have new indicator lanes. 5) When we commit, we remove the finished lanes from `indicatorLanes` and once that reaches zero again, then we can clean up the default indicator. This approach means that you can start multiple different transitions while an indicator is still going but it won't stop/restart each time. Instead, it'll wait until all are done before stopping. Follow ups: - [x] Default updates are currently not enough to cancel because those aren't flush in the same microtask. That's unfortunate. #33186 - [x] Handle async actions before the setState. Since these don't necessarily have a root this is tricky. #33190 - [x] Disable for `useDeferredValue`. ~Since it also goes through `markRootUpdated` and schedules a Transition lane it'll get a default indicator even though it probably shouldn't have one.~ EDIT: Turns out this just works because it doesn't go through `markRootUpdated` when work is left behind. - [x] Implement built-in DOM version by default. #33162 DiffTrain build for [62d3f36](62d3f36)
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Stacked on #33159. This implements `onDefaultTransitionIndicator`. The sequence is: 1) In `markRootUpdated` we schedule Transition updates as needing `indicatorLanes` on the root. This tracks the lanes that currently need an indicator to either start or remain going until this lane commits. 2) Track mutations during any commit. We use the same hook that view transitions use here but instead of tracking it just per view transition scope, we also track a global boolean for the whole root. 3) If a sync/default commit had any mutations, then we clear the indicator lane for the `currentEventTransitionLane`. This requires that the lane is still active while we do these commits. See #33159. In other words, a sync update gets associated with the current transition and it is assumed to be rendering the loading state for that corresponding transition so we don't need a default indicator for this lane. 4) At the end of `processRootScheduleInMicrotask`, right before we're about to enter a new "event transition lane" scope, it is no longer possible to render any more loading states for the current transition lane. That's when we invoke `onDefaultTransitionIndicator` for any roots that have new indicator lanes. 5) When we commit, we remove the finished lanes from `indicatorLanes` and once that reaches zero again, then we can clean up the default indicator. This approach means that you can start multiple different transitions while an indicator is still going but it won't stop/restart each time. Instead, it'll wait until all are done before stopping. Follow ups: - [x] Default updates are currently not enough to cancel because those aren't flush in the same microtask. That's unfortunate. #33186 - [x] Handle async actions before the setState. Since these don't necessarily have a root this is tricky. #33190 - [x] Disable for `useDeferredValue`. ~Since it also goes through `markRootUpdated` and schedules a Transition lane it'll get a default indicator even though it probably shouldn't have one.~ EDIT: Turns out this just works because it doesn't go through `markRootUpdated` when work is left behind. - [x] Implement built-in DOM version by default. #33162 DiffTrain build for [62d3f36](62d3f36)
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…n was scheduled (#33186) Stacked on #33160. The purpose of this is to avoid calling `onDefaultTransitionIndicator` when a Default priority update acts as the loading indicator, but still call it when unrelated Default updates happens nearby. When we schedule Default priority work that gets batched with other events in the same frame more or less. This helps optimize by doing less work. However, that batching means that we can't separate work from one setState from another. If we would consider all Default priority work in a frame when determining whether to show the default we might never show it in cases like when you have a recurring timer updating something. This instead flushes the Default priority work eagerly along with the sync work at the end of the event, if this event scheduled any Transition work. This is then used to determine if the default indicator needs to be shown. DiffTrain build for [b480865](b480865)
sebmarkbage
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…o root associated (#33190) Stacked on #33160, #33162, #33186 and #33188. We have a special case that's awkward for default indicators. When you start a new async Transition from `React.startTransition` then there's not yet any associated root with the Transition because you haven't necessarily `setState` on anything yet until the promise resolves. That's what `entangleAsyncAction` handles by creating a lane that everything entangles with until all async actions are done. If there are no sync updates before the end of the event, we should trigger a default indicator until either the async action completes without update or if it gets entangled with some roots we should keep it going until those roots are done.
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…o root associated (#33190) Stacked on #33160, #33162, #33186 and #33188. We have a special case that's awkward for default indicators. When you start a new async Transition from `React.startTransition` then there's not yet any associated root with the Transition because you haven't necessarily `setState` on anything yet until the promise resolves. That's what `entangleAsyncAction` handles by creating a lane that everything entangles with until all async actions are done. If there are no sync updates before the end of the event, we should trigger a default indicator until either the async action completes without update or if it gets entangled with some roots we should keep it going until those roots are done. DiffTrain build for [3a5b326](3a5b326)
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…o root associated (#33190) Stacked on #33160, #33162, #33186 and #33188. We have a special case that's awkward for default indicators. When you start a new async Transition from `React.startTransition` then there's not yet any associated root with the Transition because you haven't necessarily `setState` on anything yet until the promise resolves. That's what `entangleAsyncAction` handles by creating a lane that everything entangles with until all async actions are done. If there are no sync updates before the end of the event, we should trigger a default indicator until either the async action completes without update or if it gets entangled with some roots we should keep it going until those roots are done. DiffTrain build for [3a5b326](3a5b326)
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…o root associated (facebook#33190) Stacked on facebook#33160, facebook#33162, facebook#33186 and facebook#33188. We have a special case that's awkward for default indicators. When you start a new async Transition from `React.startTransition` then there's not yet any associated root with the Transition because you haven't necessarily `setState` on anything yet until the promise resolves. That's what `entangleAsyncAction` handles by creating a lane that everything entangles with until all async actions are done. If there are no sync updates before the end of the event, we should trigger a default indicator until either the async action completes without update or if it gets entangled with some roots we should keep it going until those roots are done. DiffTrain build for [3a5b326](facebook@3a5b326)
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…o root associated (facebook#33190) Stacked on facebook#33160, facebook#33162, facebook#33186 and facebook#33188. We have a special case that's awkward for default indicators. When you start a new async Transition from `React.startTransition` then there's not yet any associated root with the Transition because you haven't necessarily `setState` on anything yet until the promise resolves. That's what `entangleAsyncAction` handles by creating a lane that everything entangles with until all async actions are done. If there are no sync updates before the end of the event, we should trigger a default indicator until either the async action completes without update or if it gets entangled with some roots we should keep it going until those roots are done. DiffTrain build for [3a5b326](facebook@3a5b326)
This was referenced May 14, 2025
This was referenced May 21, 2025
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Stacked on #33160.
The purpose of this is to avoid calling
onDefaultTransitionIndicatorwhen a Default priority update acts as the loading indicator, but still call it when unrelated Default updates happens nearby.When we schedule Default priority work that gets batched with other events in the same frame more or less. This helps optimize by doing less work. However, that batching means that we can't separate work from one setState from another. If we would consider all Default priority work in a frame when determining whether to show the default we might never show it in cases like when you have a recurring timer updating something.
This instead flushes the Default priority work eagerly along with the sync work at the end of the event, if this event scheduled any Transition work. This is then used to determine if the default indicator needs to be shown.