refactors the currentUpdatePriority apis to more clearly communicate …

…when you are reading a raw value and when you are getting a resolved value. This allows requestUpdateLane to do a single call into the renderer to get the update priority while still making a reasonable stack based updating of update priorities ergonomic. I had to expose a `peekCurrentUpdatePriority` to satisfy startTransition which needs to set a new priority only if the current one is too low. I tried to stay away from get/set naming to avoid potential issues with whether you are reading the raw value or the derived one that considers window.event

packages/react-art/src/ReactFiberConfigART.js

@@ -343,16 +343,20 @@ export function shouldSetTextContent(type, props) {
 
 let currentUpdatePriority: EventPriority = NoEventPriority;
 
-export function setCurrentUpdatePriority(newPriority: EventPriority) {
+export function swapCurrentUpdatePriority(
+  newPriority: EventPriority,
+): EventPriority {
+  const prevPriority = currentUpdatePriority;
   currentUpdatePriority = newPriority;
+  return prevPriority;
 }
 
-export function getCurrentUpdatePriority() {
+export function peekCurrentUpdatePriority(): EventPriority {
   return currentUpdatePriority;
 }
 
-export function getCurrentEventPriority() {
-  return DefaultEventPriority;
+export function resolveUpdatePriority(): EventPriority {
+  return currentUpdatePriority || DefaultEventPriority;
 }
 
 export function shouldAttemptEagerTransition() {

packages/react-dom-bindings/src/client/ReactDOMUpdatePriority.js

@@ -9,24 +9,45 @@
 
 import type {EventPriority} from 'react-reconciler/src/ReactEventPriorities';
 
+import {getEventPriority} from '../events/ReactDOMEventListener';
+import {
+  NoEventPriority,
+  DefaultEventPriority,
+} from 'react-reconciler/src/ReactEventPriorities';
+
 import ReactDOMSharedInternals from 'shared/ReactDOMSharedInternals';
 const ReactDOMCurrentUpdatePriority =
   ReactDOMSharedInternals.ReactDOMCurrentUpdatePriority;
 
-export function setCurrentUpdatePriority(newPriority: EventPriority): void {
+export function swapCurrentUpdatePriority(
+  newPriority: EventPriority,
+): EventPriority {
+  const current = ReactDOMCurrentUpdatePriority.current;
   ReactDOMCurrentUpdatePriority.current = newPriority;
+  return current;
 }
 
-export function getCurrentUpdatePriority(): EventPriority {
+export function peekCurrentUpdatePriority(): EventPriority {
   return ReactDOMCurrentUpdatePriority.current;
 }
 
+export function resolveUpdatePriority(): EventPriority {
+  const updatePriority = ReactDOMCurrentUpdatePriority.current;
+  if (updatePriority !== NoEventPriority) {
+    return updatePriority;
+  }
+  const currentEvent = window.event;
+  if (currentEvent === undefined) {
+    return DefaultEventPriority;
+  }
+  return getEventPriority(currentEvent.type);
+}
+
 export function runWithPriority<T>(priority: EventPriority, fn: () => T): T {
-  const previousPriority = getCurrentUpdatePriority();
+  const previousPriority = swapCurrentUpdatePriority(priority);
   try {
-    setCurrentUpdatePriority(priority);
     return fn();
   } finally {
-    setCurrentUpdatePriority(previousPriority);
+    swapCurrentUpdatePriority(previousPriority);
   }
 }

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -7,7 +7,6 @@
  * @flow
  */
 
-import type {EventPriority} from 'react-reconciler/src/ReactEventPriorities';
 import type {DOMEventName} from '../events/DOMEventNames';
 import type {Fiber, FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
 import type {
@@ -29,14 +28,14 @@ import type {
 
 import {NotPending} from 'react-dom-bindings/src/shared/ReactDOMFormActions';
 import {getCurrentRootHostContainer} from 'react-reconciler/src/ReactFiberHostContext';
-import {DefaultEventPriority} from 'react-reconciler/src/ReactEventPriorities';
 
 import hasOwnProperty from 'shared/hasOwnProperty';
 import {checkAttributeStringCoercion} from 'shared/CheckStringCoercion';
 
 export {
-  setCurrentUpdatePriority,
-  getCurrentUpdatePriority,
+  peekCurrentUpdatePriority,
+  swapCurrentUpdatePriority,
+  resolveUpdatePriority,
 } from './ReactDOMUpdatePriority';
 import {
   precacheFiberNode,
@@ -73,7 +72,6 @@ import {
 import {
   isEnabled as ReactBrowserEventEmitterIsEnabled,
   setEnabled as ReactBrowserEventEmitterSetEnabled,
-  getEventPriority,
 } from '../events/ReactDOMEventListener';
 import {SVG_NAMESPACE, MATH_NAMESPACE} from './DOMNamespaces';
 import {
@@ -576,14 +574,6 @@ export function createTextInstance(
   return textNode;
 }
 
-export function getCurrentEventPriority(): EventPriority {
-  const currentEvent = window.event;
-  if (currentEvent === undefined) {
-    return DefaultEventPriority;
-  }
-  return getEventPriority(currentEvent.type);
-}
-
 let currentPopstateTransitionEvent: Event | null = null;
 export function shouldAttemptEagerTransition(): boolean {
   const event = window.event;

packages/react-dom-bindings/src/events/ReactDOMEventListener.js

@@ -34,10 +34,7 @@ import {
 } from '../client/ReactDOMComponentTree';
 
 import {dispatchEventForPluginEventSystem} from './DOMPluginEventSystem';
-import {
-  getCurrentUpdatePriority,
-  setCurrentUpdatePriority,
-} from '../client/ReactDOMUpdatePriority';
+import {swapCurrentUpdatePriority} from '../client/ReactDOMUpdatePriority';
 
 import {
   getCurrentPriorityLevel as getCurrentSchedulerPriorityLevel,
@@ -117,14 +114,13 @@ function dispatchDiscreteEvent(
   container: EventTarget,
   nativeEvent: AnyNativeEvent,
 ) {
-  const previousPriority = getCurrentUpdatePriority();
   const prevTransition = ReactCurrentBatchConfig.transition;
   ReactCurrentBatchConfig.transition = null;
+  const previousPriority = swapCurrentUpdatePriority(DiscreteEventPriority);
   try {
-    setCurrentUpdatePriority(DiscreteEventPriority);
     dispatchEvent(domEventName, eventSystemFlags, container, nativeEvent);
   } finally {
-    setCurrentUpdatePriority(previousPriority);
+    swapCurrentUpdatePriority(previousPriority);
     ReactCurrentBatchConfig.transition = prevTransition;
   }
 }
@@ -135,14 +131,13 @@ function dispatchContinuousEvent(
   container: EventTarget,
   nativeEvent: AnyNativeEvent,
 ) {
-  const previousPriority = getCurrentUpdatePriority();
   const prevTransition = ReactCurrentBatchConfig.transition;
   ReactCurrentBatchConfig.transition = null;
+  const previousPriority = swapCurrentUpdatePriority(ContinuousEventPriority);
   try {
-    setCurrentUpdatePriority(ContinuousEventPriority);
     dispatchEvent(domEventName, eventSystemFlags, container, nativeEvent);
   } finally {
-    setCurrentUpdatePriority(previousPriority);
+    swapCurrentUpdatePriority(previousPriority);
     ReactCurrentBatchConfig.transition = prevTransition;
   }
 }

packages/react-dom-bindings/src/events/ReactDOMEventReplaying.js

@@ -38,7 +38,7 @@ import {isHigherEventPriority} from 'react-reconciler/src/ReactEventPriorities';
 import {isRootDehydrated} from 'react-reconciler/src/ReactFiberShellHydration';
 import {dispatchReplayedFormAction} from './plugins/FormActionEventPlugin';
 import {
-  getCurrentUpdatePriority,
+  peekCurrentUpdatePriority,
   runWithPriority as attemptHydrationAtPriority,
 } from '../client/ReactDOMUpdatePriority';
 
@@ -336,7 +336,7 @@ export function queueExplicitHydrationTarget(target: Node): void {
   // TODO: This will read the priority if it's dispatched by the React
   // event system but not native events. Should read window.event.type, like
   // we do for updates (getCurrentEventPriority).
-  const updatePriority = getCurrentUpdatePriority();
+  const updatePriority = peekCurrentUpdatePriority();
   const queuedTarget: QueuedHydrationTarget = {
     blockedOn: null,
     target: target,

packages/react-native-renderer/src/ReactFiberConfigFabric.js

@@ -313,17 +313,23 @@ export function shouldSetTextContent(type: string, props: Props): boolean {
 }
 
 let currentUpdatePriority: EventPriority = NoEventPriority;
-export function setCurrentUpdatePriority(
-  newEventPriority: EventPriority,
-): void {
-  currentUpdatePriority = newEventPriority;
+export function swapCurrentUpdatePriority(
+  newPriority: EventPriority,
+): EventPriority {
+  const previousPriority = currentUpdatePriority;
+  currentUpdatePriority = newPriority;
+  return previousPriority;
 }
 
-export function getCurrentUpdatePriority(): EventPriority {
+export function peekCurrentUpdatePriority(): EventPriority {
   return currentUpdatePriority;
 }
 
-export function getCurrentEventPriority(): EventPriority {
+export function resolveUpdatePriority(): EventPriority {
+  if (currentUpdatePriority !== NoEventPriority) {
+    return currentUpdatePriority;
+  }
+
   const currentEventPriority = fabricGetCurrentEventPriority
     ? fabricGetCurrentEventPriority()
     : null;

packages/react-native-renderer/src/ReactFiberConfigNative.js

@@ -255,15 +255,22 @@ export function shouldSetTextContent(type: string, props: Props): boolean {
 }
 
 let currentUpdatePriority: EventPriority = NoEventPriority;
-export function setCurrentUpdatePriority(newPriority: EventPriority): void {
+export function swapCurrentUpdatePriority(
+  newPriority: EventPriority,
+): EventPriority {
+  const previousPriority = currentUpdatePriority;
   currentUpdatePriority = newPriority;
+  return previousPriority;
 }
 
-export function getCurrentUpdatePriority(): EventPriority {
+export function peekCurrentUpdatePriority(): EventPriority {
   return currentUpdatePriority;
 }
 
-export function getCurrentEventPriority(): EventPriority {
+export function resolveUpdatePriority(): EventPriority {
+  if (currentUpdatePriority !== NoEventPriority) {
+    return currentUpdatePriority;
+  }
   return DefaultEventPriority;
 }
 

packages/react-noop-renderer/src/createReactNoop.js

@@ -510,10 +510,13 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
 
     resetAfterCommit(): void {},
 
-    setCurrentUpdatePriority,
-    getCurrentUpdatePriority,
+    swapCurrentUpdatePriority,
+    peekCurrentUpdatePriority,
 
-    getCurrentEventPriority() {
+    resolveUpdatePriority() {
+      if (currentUpdatePriority !== NoEventPriority) {
+        return currentUpdatePriority;
+      }
       return currentEventPriority;
     },
 
@@ -788,11 +791,15 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
   const DEFAULT_ROOT_ID = '<default>';
 
   let currentUpdatePriority = NoEventPriority;
-  function setCurrentUpdatePriority(newPriority: EventPriority): void {
+  function swapCurrentUpdatePriority(
+    newPriority: EventPriority,
+  ): EventPriority {
+    const previousPriority = currentUpdatePriority;
     currentUpdatePriority = newPriority;
+    return previousPriority;
   }
 
-  function getCurrentUpdatePriority(): EventPriority {
+  function peekCurrentUpdatePriority(): EventPriority {
     return currentUpdatePriority;
   }
 
@@ -1229,12 +1236,11 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
       priority: EventPriority,
       fn: () => T,
     ): T {
-      const previousPriority = getCurrentUpdatePriority();
+      const previousPriority = swapCurrentUpdatePriority(priority);
       try {
-        setCurrentUpdatePriority(priority);
         return fn();
       } finally {
-        setCurrentUpdatePriority(previousPriority);
+        swapCurrentUpdatePriority(previousPriority);
       }
     },
 

packages/react-reconciler/src/ReactEventPriorities.js

@@ -48,6 +48,10 @@ export function isHigherEventPriority(
   return a !== 0 && a < b;
 }
 
+export function eventPriorityToLane(updatePriority: EventPriority): Lane {
+  return updatePriority;
+}
+
 export function lanesToEventPriority(lanes: Lanes): EventPriority {
   const lane = getHighestPriorityLane(lanes);
   if (!isHigherEventPriority(DiscreteEventPriority, lane)) {

packages/react-reconciler/src/ReactFiberHooks.js

@@ -29,8 +29,8 @@ import type {TransitionStatus} from './ReactFiberConfig';
 
 import {
   NotPendingTransition as NoPendingHostTransition,
-  setCurrentUpdatePriority,
-  getCurrentUpdatePriority,
+  swapCurrentUpdatePriority,
+  peekCurrentUpdatePriority,
 } from './ReactFiberConfig';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
@@ -2790,8 +2790,8 @@ function startTransition<S>(
   callback: () => mixed,
   options?: StartTransitionOptions,
 ): void {
-  const previousPriority = getCurrentUpdatePriority();
-  setCurrentUpdatePriority(
+  const previousPriority = peekCurrentUpdatePriority();
+  swapCurrentUpdatePriority(
     higherEventPriority(previousPriority, ContinuousEventPriority),
   );
 
@@ -2877,7 +2877,7 @@ function startTransition<S>(
       throw error;
     }
   } finally {
-    setCurrentUpdatePriority(previousPriority);
+    swapCurrentUpdatePriority(previousPriority);
 
     ReactCurrentBatchConfig.transition = prevTransition;
 

packages/react-reconciler/src/ReactFiberReconciler.js

@@ -41,7 +41,7 @@ import getComponentNameFromFiber from 'react-reconciler/src/getComponentNameFrom
 import isArray from 'shared/isArray';
 import {enableSchedulingProfiler} from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
-import {getPublicInstance, getCurrentUpdatePriority} from './ReactFiberConfig';
+import {getPublicInstance} from './ReactFiberConfig';
 import {
   findCurrentUnmaskedContext,
   processChildContext,
@@ -521,8 +521,6 @@ export function attemptHydrationAtCurrentPriority(fiber: Fiber): void {
   markRetryLaneIfNotHydrated(fiber, lane);
 }
 
-export {getCurrentUpdatePriority};
-
 export {findHostInstance};
 
 export {findHostInstanceWithWarning};