code · pull · May 14, 2025 · May 13, 2025 · May 13, 2025 · May 13, 2025
diff --git a/.eslintrc.js b/.eslintrc.js
@@ -579,6 +579,7 @@ module.exports = {
     JSONValue: 'readonly',
     JSResourceReference: 'readonly',
     MouseEventHandler: 'readonly',
+    NavigateEvent: 'readonly',
     PropagationPhases: 'readonly',
     PropertyDescriptor: 'readonly',
     React$AbstractComponent: 'readonly',
@@ -634,5 +635,6 @@ module.exports = {
     AsyncLocalStorage: 'readonly',
     async_hooks: 'readonly',
     globalThis: 'readonly',
+    navigation: 'readonly',
   },
 };
diff --git a/fixtures/flight/src/actions.js b/fixtures/flight/src/actions.js
@@ -2,7 +2,13 @@
 
 import {setServerState} from './ServerState.js';
 
+async function sleep(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
 export async function like() {
+  // Test loading state
+  await sleep(1000);
   setServerState('Liked!');
   return new Promise((resolve, reject) => resolve('Liked'));
 }
@@ -20,5 +26,7 @@ export async function greet(formData) {
 }
 
 export async function increment(n) {
+  // Test loading state
+  await sleep(1000);
   return n + 1;
 }
diff --git a/fixtures/view-transition/src/components/Page.js b/fixtures/view-transition/src/components/Page.js
@@ -18,6 +18,10 @@ import './Page.css';
 
 import transitions from './Transitions.module.css';
 
+async function sleep(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
 const a = (
   <div key="a">
     <ViewTransition>
@@ -106,7 +110,13 @@ export default function Page({url, navigate}) {
       document.body
     )
   ) : (
-    <button onClick={() => startTransition(() => setShowModal(true))}>
+    <button
+      onClick={() =>
+        startTransition(async () => {
+          await sleep(2000);
+          setShowModal(true);
+        })
+      }>
       Show Modal
     </button>
   );

diff --git a/packages/react-dom/src/client/ReactDOMDefaultTransitionIndicator.js b/packages/react-dom/src/client/ReactDOMDefaultTransitionIndicator.js
@@ -0,0 +1,89 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+export function defaultOnDefaultTransitionIndicator(): void | (() => void) {
+  if (typeof navigation !== 'object') {
+    // If the Navigation API is not available, then this is a noop.
+    return;
+  }
+
+  let isCancelled = false;
+  let pendingResolve: null | (() => void) = null;
+
+  function handleNavigate(event: NavigateEvent) {
+    if (event.canIntercept && event.info === 'react-transition') {
+      event.intercept({
+        handler() {
+          return new Promise(resolve => (pendingResolve = resolve));
+        },
+        focusReset: 'manual',
+        scroll: 'manual',
+      });
+    }
+  }
+
+  function handleNavigateComplete() {
+    if (pendingResolve !== null) {
+      // If this was not our navigation completing, we were probably cancelled.
+      // We'll start a new one below.
+      pendingResolve();
+      pendingResolve = null;
+    }
+    if (!isCancelled) {
+      // Some other navigation completed but we should still be running.
+      // Start another fake one to keep the loading indicator going.
+      startFakeNavigation();
+    }
+  }
+
+  // $FlowFixMe
+  navigation.addEventListener('navigate', handleNavigate);
+  // $FlowFixMe
+  navigation.addEventListener('navigatesuccess', handleNavigateComplete);
+  // $FlowFixMe
+  navigation.addEventListener('navigateerror', handleNavigateComplete);
+
+  function startFakeNavigation() {
+    if (isCancelled) {
+      // We already stopped this Transition.
+      return;
+    }
+    if (navigation.transition) {
+      // There is an on-going Navigation already happening. Let's wait for it to
+      // finish before starting our fake one.
+      return;
+    }
+    // Trigger a fake navigation to the same page
+    const currentEntry = navigation.currentEntry;
+    if (currentEntry && currentEntry.url != null) {
+      navigation.navigate(currentEntry.url, {
+        state: currentEntry.getState(),
+        info: 'react-transition', // indicator to routers to ignore this navigation
+        history: 'replace',
+      });
+    }
+  }
+
+  // Delay the start a bit in case this is a fast navigation.
+  setTimeout(startFakeNavigation, 100);
+
+  return function () {
+    isCancelled = true;
+    // $FlowFixMe
+    navigation.removeEventListener('navigate', handleNavigate);
+    // $FlowFixMe
+    navigation.removeEventListener('navigatesuccess', handleNavigateComplete);
+    // $FlowFixMe
+    navigation.removeEventListener('navigateerror', handleNavigateComplete);
+    if (pendingResolve !== null) {
+      pendingResolve();
+      pendingResolve = null;
+    }
+  };
+}
diff --git a/packages/react-dom/src/client/ReactDOMRoot.js b/packages/react-dom/src/client/ReactDOMRoot.js
@@ -95,13 +95,9 @@ import {
   defaultOnCaughtError,
   defaultOnRecoverableError,
 } from 'react-reconciler/src/ReactFiberReconciler';
+import {defaultOnDefaultTransitionIndicator} from './ReactDOMDefaultTransitionIndicator';
 import {ConcurrentRoot} from 'react-reconciler/src/ReactRootTags';
 
-function defaultOnDefaultTransitionIndicator(): void | (() => void) {
-  // TODO: Implement the default
-  return function () {};
-}
-
 // $FlowFixMe[missing-this-annot]
 function ReactDOMRoot(internalRoot: FiberRoot) {
   this._internalRoot = internalRoot;

diff --git a/packages/react-noop-renderer/src/createReactNoop.js b/packages/react-noop-renderer/src/createReactNoop.js
@@ -1142,9 +1142,7 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
     // TODO: Turn this on once tests are fixed
     // console.error(error);
   }
-  function onDefaultTransitionIndicator(): void | (() => void) {
-    // TODO: Allow this as an option.
-  }
+  function onDefaultTransitionIndicator(): void | (() => void) {}
 
   let idCounter = 0;
 

diff --git a/packages/react-reconciler/src/ReactFiberAsyncAction.js b/packages/react-reconciler/src/ReactFiberAsyncAction.js
@@ -15,7 +15,10 @@ import type {
 import type {Lane} from './ReactFiberLane';
 import type {Transition} from 'react/src/ReactStartTransition';
 
-import {requestTransitionLane} from './ReactFiberRootScheduler';
+import {
+  requestTransitionLane,
+  ensureScheduleIsScheduled,
+} from './ReactFiberRootScheduler';
 import {NoLane} from './ReactFiberLane';
 import {
   hasScheduledTransitionWork,
@@ -24,9 +27,13 @@ import {
 import {
   enableComponentPerformanceTrack,
   enableProfilerTimer,
+  enableDefaultTransitionIndicator,
 } from 'shared/ReactFeatureFlags';
 import {clearEntangledAsyncTransitionTypes} from './ReactFiberTransitionTypes';
 
+import noop from 'shared/noop';
+import reportGlobalError from 'shared/reportGlobalError';
+
 // If there are multiple, concurrent async actions, they are entangled. All
 // transition updates that occur while the async action is still in progress
 // are treated as part of the action.
@@ -46,6 +53,21 @@ let currentEntangledLane: Lane = NoLane;
 // until the async action scope has completed.
 let currentEntangledActionThenable: Thenable<void> | null = null;
 
+// Track the default indicator for every root. undefined means we haven't
+// had any roots registered yet. null means there's more than one callback.
+// If there's more than one callback we bailout to not supporting isomorphic
+// default indicators.
+let isomorphicDefaultTransitionIndicator:
+  | void
+  | null
+  | (() => void | (() => void)) = undefined;
+// The clean up function for the currently running indicator.
+let pendingIsomorphicIndicator: null | (() => void) = null;
+// The number of roots that have pending Transitions that depend on the
+// started isomorphic indicator.
+let pendingEntangledRoots: number = 0;
+let needsIsomorphicIndicator: boolean = false;
+
 export function entangleAsyncAction<S>(
   transition: Transition,
   thenable: Thenable<S>,
@@ -66,6 +88,12 @@ export function entangleAsyncAction<S>(
       },
     };
     currentEntangledActionThenable = entangledThenable;
+    if (enableDefaultTransitionIndicator) {
+      needsIsomorphicIndicator = true;
+      // We'll check if we need a default indicator in a microtask. Ensure
+      // we have this scheduled even if no root is scheduled.
+      ensureScheduleIsScheduled();
+    }
   }
   currentEntangledPendingCount++;
   thenable.then(pingEngtangledActionScope, pingEngtangledActionScope);
@@ -86,6 +114,9 @@ function pingEngtangledActionScope() {
       }
     }
     clearEntangledAsyncTransitionTypes();
+    if (pendingEntangledRoots === 0) {
+      stopIsomorphicDefaultIndicator();
+    }
     if (currentEntangledListeners !== null) {
       // All the actions have finished. Close the entangled async action scope
       // and notify all the listeners.
@@ -98,6 +129,7 @@ function pingEngtangledActionScope() {
       currentEntangledListeners = null;
       currentEntangledLane = NoLane;
       currentEntangledActionThenable = null;
+      needsIsomorphicIndicator = false;
       for (let i = 0; i < listeners.length; i++) {
         const listener = listeners[i];
         listener();
@@ -161,3 +193,71 @@ export function peekEntangledActionLane(): Lane {
 export function peekEntangledActionThenable(): Thenable<void> | null {
   return currentEntangledActionThenable;
 }
+
+export function registerDefaultIndicator(
+  onDefaultTransitionIndicator: () => void | (() => void),
+): void {
+  if (!enableDefaultTransitionIndicator) {
+    return;
+  }
+  if (isomorphicDefaultTransitionIndicator === undefined) {
+    isomorphicDefaultTransitionIndicator = onDefaultTransitionIndicator;
+  } else if (
+    isomorphicDefaultTransitionIndicator !== onDefaultTransitionIndicator
+  ) {
+    isomorphicDefaultTransitionIndicator = null;
+    // Stop any on-going indicator since it's now ambiguous.
+    stopIsomorphicDefaultIndicator();
+  }
+}
+
+export function startIsomorphicDefaultIndicatorIfNeeded() {
+  if (!enableDefaultTransitionIndicator) {
+    return;
+  }
+  if (!needsIsomorphicIndicator) {
+    return;
+  }
+  if (
+    isomorphicDefaultTransitionIndicator != null &&
+    pendingIsomorphicIndicator === null
+  ) {
+    try {
+      pendingIsomorphicIndicator =
+        isomorphicDefaultTransitionIndicator() || noop;
+    } catch (x) {
+      pendingIsomorphicIndicator = noop;
+      reportGlobalError(x);
+    }
+  }
+}
+
+function stopIsomorphicDefaultIndicator() {
+  if (!enableDefaultTransitionIndicator) {
+    return;
+  }
+  if (pendingIsomorphicIndicator !== null) {
+    const cleanup = pendingIsomorphicIndicator;
+    pendingIsomorphicIndicator = null;
+    cleanup();
+  }
+}
+
+function releaseIsomorphicIndicator() {
+  if (--pendingEntangledRoots === 0) {
+    stopIsomorphicDefaultIndicator();
+  }
+}
+
+export function hasOngoingIsomorphicIndicator(): boolean {
+  return pendingIsomorphicIndicator !== null;
+}
+
+export function retainIsomorphicIndicator(): () => void {
+  pendingEntangledRoots++;
+  return releaseIsomorphicIndicator;
+}
+
+export function markIsomorphicIndicatorHandled(): void {
+  needsIsomorphicIndicator = false;
+}
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.js b/packages/react-reconciler/src/ReactFiberCommitWork.js
@@ -20,6 +20,7 @@ import type {
 import type {Fiber, FiberRoot} from './ReactInternalTypes';
 import type {Lanes} from './ReactFiberLane';
 import {
+  includesLoadingIndicatorLanes,
   includesOnlySuspenseyCommitEligibleLanes,
   includesOnlyViewTransitionEligibleLanes,
 } from './ReactFiberLane';
@@ -60,6 +61,7 @@ import {
   enableViewTransition,
   enableFragmentRefs,
   enableEagerAlternateStateNodeCleanup,
+  enableDefaultTransitionIndicator,
 } from 'shared/ReactFeatureFlags';
 import {
   FunctionComponent,
@@ -268,13 +270,16 @@ import {
 } from './ReactFiberCommitViewTransitions';
 import {
   viewTransitionMutationContext,
+  pushRootMutationContext,
   pushMutationContext,
   popMutationContext,
+  rootMutationContext,
 } from './ReactFiberMutationTracking';
 import {
   trackNamedViewTransition,
   untrackNamedViewTransition,
 } from './ReactFiberDuplicateViewTransitions';
+import {markIndicatorHandled} from './ReactFiberRootScheduler';
 
 // Used during the commit phase to track the state of the Offscreen component stack.
 // Allows us to avoid traversing the return path to find the nearest Offscreen ancestor.
@@ -2216,6 +2221,7 @@ function commitMutationEffectsOnFiber(
     case HostRoot: {
       const prevProfilerEffectDuration = pushNestedEffectDurations();
 
+      pushRootMutationContext();
       if (supportsResources) {
         prepareToCommitHoistables();
 
@@ -2265,6 +2271,18 @@ function commitMutationEffectsOnFiber(
         );
       }
 
+      popMutationContext(false);
+
+      if (
+        enableDefaultTransitionIndicator &&
+        rootMutationContext &&
+        includesLoadingIndicatorLanes(lanes)
+      ) {
+        // This root had a mutation. Mark this root as having rendered a manual
+        // loading state.
+        markIndicatorHandled(root);
+      }
+
       break;
     }
     case HostPortal: {