Context.unstable_read

unstable_read can be called anywhere within the render phase. That
includes the render method, getDerivedStateFromProps, constructors,
functional components, and context consumer render props.

If it's called outside the render phase, an error is thrown.

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -239,26 +239,33 @@ function markRef(current: Fiber | null, workInProgress: Fiber) {
   }
 }
 
-function updateFunctionalComponent(current, workInProgress) {
+function updateFunctionalComponent(
+  current,
+  workInProgress,
+  renderExpirationTime,
+) {
   const fn = workInProgress.type;
   const nextProps = workInProgress.pendingProps;
 
-  if (hasLegacyContextChanged()) {
-    // Normally we can bail out on props equality but if context has changed
-    // we don't do the bailout and we have to reuse existing props instead.
-  } else {
-    if (workInProgress.memoizedProps === nextProps) {
-      return bailoutOnAlreadyFinishedWork(current, workInProgress);
+  if (!checkForPendingContext(workInProgress, renderExpirationTime)) {
+    if (hasLegacyContextChanged()) {
+      // Normally we can bail out on props equality but if context has changed
+      // we don't do the bailout and we have to reuse existing props instead.
+    } else {
+      if (workInProgress.memoizedProps === nextProps) {
+        return bailoutOnAlreadyFinishedWork(current, workInProgress);
+      }
+      // TODO: consider bringing fn.shouldComponentUpdate() back.
+      // It used to be here.
     }
-    // TODO: consider bringing fn.shouldComponentUpdate() back.
-    // It used to be here.
   }
 
   const unmaskedContext = getUnmaskedContext(workInProgress);
   const context = getMaskedContext(workInProgress, unmaskedContext);
 
   let nextChildren;
 
+  prepareToReadContext();
   if (__DEV__) {
     ReactCurrentOwner.current = workInProgress;
     ReactDebugCurrentFiber.setCurrentPhase('render');
@@ -267,6 +274,8 @@ function updateFunctionalComponent(current, workInProgress) {
   } else {
     nextChildren = fn(nextProps, context);
   }
+  workInProgress.firstContextReader = finishReadingContext();
+
   // React DevTools reads this flag.
   workInProgress.effectTag |= PerformedWork;
   reconcileChildren(current, workInProgress, nextChildren);
@@ -283,6 +292,8 @@ function updateClassComponent(
   // During mounting we don't know the child context yet as the instance doesn't exist.
   // We will invalidate the child context in finishClassComponent() right after rendering.
   const hasContext = pushLegacyContextProvider(workInProgress);
+  prepareToReadContext();
+
   let shouldUpdate;
   if (current === null) {
     if (workInProgress.stateNode === null) {
@@ -377,6 +388,8 @@ function finishClassComponent(
     }
   }
 
+  workInProgress.firstContextReader = finishReadingContext();
+
   // React DevTools reads this flag.
   workInProgress.effectTag |= PerformedWork;
   if (didCaptureError) {
@@ -582,6 +595,8 @@ function mountIndeterminateComponent(
   const unmaskedContext = getUnmaskedContext(workInProgress);
   const context = getMaskedContext(workInProgress, unmaskedContext);
 
+  prepareToReadContext();
+
   let value;
 
   if (__DEV__) {
@@ -612,6 +627,8 @@ function mountIndeterminateComponent(
   // React DevTools reads this flag.
   workInProgress.effectTag |= PerformedWork;
 
+  workInProgress.firstContextReader = finishReadingContext();
+
   if (
     typeof value === 'object' &&
     value !== null &&
@@ -1035,7 +1052,11 @@ function beginWork(
         renderExpirationTime,
       );
     case FunctionalComponent:
-      return updateFunctionalComponent(current, workInProgress);
+      return updateFunctionalComponent(
+        current,
+        workInProgress,
+        renderExpirationTime,
+      );
     case ClassComponent:
       return updateClassComponent(
         current,

packages/react-reconciler/src/ReactFiberClassComponent.js

@@ -50,6 +50,7 @@ import {
   computeExpirationForFiber,
   scheduleWork,
 } from './ReactFiberScheduler';
+import {checkForPendingContext} from './ReactFiberNewContext';
 
 const fakeInternalInstance = {};
 const isArray = Array.isArray;
@@ -231,7 +232,7 @@ function checkShouldComponentUpdate(
   newProps,
   oldState,
   newState,
-  newContext,
+  nextLegacyContext,
 ) {
   const instance = workInProgress.stateNode;
   const ctor = workInProgress.type;
@@ -240,7 +241,7 @@ function checkShouldComponentUpdate(
     const shouldUpdate = instance.shouldComponentUpdate(
       newProps,
       newState,
-      newContext,
+      nextLegacyContext,
     );
     stopPhaseTimer();
 
@@ -616,15 +617,15 @@ function callComponentWillReceiveProps(
   workInProgress,
   instance,
   newProps,
-  newContext,
+  nextLegacyContext,
 ) {
   const oldState = instance.state;
   startPhaseTimer(workInProgress, 'componentWillReceiveProps');
   if (typeof instance.componentWillReceiveProps === 'function') {
-    instance.componentWillReceiveProps(newProps, newContext);
+    instance.componentWillReceiveProps(newProps, nextLegacyContext);
   }
   if (typeof instance.UNSAFE_componentWillReceiveProps === 'function') {
-    instance.UNSAFE_componentWillReceiveProps(newProps, newContext);
+    instance.UNSAFE_componentWillReceiveProps(newProps, nextLegacyContext);
   }
   stopPhaseTimer();
 
@@ -746,8 +747,16 @@ function resumeMountClassInstance(
   instance.props = oldProps;
 
   const oldContext = instance.context;
-  const newUnmaskedContext = getUnmaskedContext(workInProgress);
-  const newContext = getMaskedContext(workInProgress, newUnmaskedContext);
+  const nextLegacyUnmaskedContext = getUnmaskedContext(workInProgress);
+  const nextLegacyContext = getMaskedContext(
+    workInProgress,
+    nextLegacyUnmaskedContext,
+  );
+
+  const hasPendingNewContext = checkForPendingContext(
+    workInProgress,
+    renderExpirationTime,
+  );
 
   const getDerivedStateFromProps = ctor.getDerivedStateFromProps;
   const hasNewLifecycles =
@@ -765,12 +774,12 @@ function resumeMountClassInstance(
     (typeof instance.UNSAFE_componentWillReceiveProps === 'function' ||
       typeof instance.componentWillReceiveProps === 'function')
   ) {
-    if (oldProps !== newProps || oldContext !== newContext) {
+    if (oldProps !== newProps || oldContext !== nextLegacyContext) {
       callComponentWillReceiveProps(
         workInProgress,
         instance,
         newProps,
-        newContext,
+        nextLegacyContext,
       );
     }
   }
@@ -794,6 +803,7 @@ function resumeMountClassInstance(
     oldProps === newProps &&
     oldState === newState &&
     !hasContextChanged() &&
+    !hasPendingNewContext &&
     !checkHasForceUpdateAfterProcessing()
   ) {
     // If an update was already in progress, we should schedule an Update
@@ -815,13 +825,14 @@ function resumeMountClassInstance(
 
   const shouldUpdate =
     checkHasForceUpdateAfterProcessing() ||
+    hasPendingNewContext ||
     checkShouldComponentUpdate(
       workInProgress,
       oldProps,
       newProps,
       oldState,
       newState,
-      newContext,
+      nextLegacyContext,
     );
 
   if (shouldUpdate) {
@@ -861,7 +872,7 @@ function resumeMountClassInstance(
   // if shouldComponentUpdate returns false.
   instance.props = newProps;
   instance.state = newState;
-  instance.context = newContext;
+  instance.context = nextLegacyContext;
 
   return shouldUpdate;
 }
@@ -880,8 +891,16 @@ function updateClassInstance(
   instance.props = oldProps;
 
   const oldContext = instance.context;
-  const newUnmaskedContext = getUnmaskedContext(workInProgress);
-  const newContext = getMaskedContext(workInProgress, newUnmaskedContext);
+  const nextLegacyUnmaskedContext = getUnmaskedContext(workInProgress);
+  const nextLegacyContext = getMaskedContext(
+    workInProgress,
+    nextLegacyUnmaskedContext,
+  );
+
+  const hasPendingNewContext = checkForPendingContext(
+    workInProgress,
+    renderExpirationTime,
+  );
 
   const getDerivedStateFromProps = ctor.getDerivedStateFromProps;
   const hasNewLifecycles =
@@ -899,12 +918,12 @@ function updateClassInstance(
     (typeof instance.UNSAFE_componentWillReceiveProps === 'function' ||
       typeof instance.componentWillReceiveProps === 'function')
   ) {
-    if (oldProps !== newProps || oldContext !== newContext) {
+    if (oldProps !== newProps || oldContext !== nextLegacyContext) {
       callComponentWillReceiveProps(
         workInProgress,
         instance,
         newProps,
-        newContext,
+        nextLegacyContext,
       );
     }
   }
@@ -929,6 +948,7 @@ function updateClassInstance(
     oldProps === newProps &&
     oldState === newState &&
     !hasContextChanged() &&
+    !hasPendingNewContext &&
     !checkHasForceUpdateAfterProcessing()
   ) {
     // If an update was already in progress, we should schedule an Update
@@ -963,13 +983,14 @@ function updateClassInstance(
 
   const shouldUpdate =
     checkHasForceUpdateAfterProcessing() ||
+    hasPendingNewContext ||
     checkShouldComponentUpdate(
       workInProgress,
       oldProps,
       newProps,
       oldState,
       newState,
-      newContext,
+      nextLegacyContext,
     );
 
   if (shouldUpdate) {
@@ -982,10 +1003,14 @@ function updateClassInstance(
     ) {
       startPhaseTimer(workInProgress, 'componentWillUpdate');
       if (typeof instance.componentWillUpdate === 'function') {
-        instance.componentWillUpdate(newProps, newState, newContext);
+        instance.componentWillUpdate(newProps, newState, nextLegacyContext);
       }
       if (typeof instance.UNSAFE_componentWillUpdate === 'function') {
-        instance.UNSAFE_componentWillUpdate(newProps, newState, newContext);
+        instance.UNSAFE_componentWillUpdate(
+          newProps,
+          newState,
+          nextLegacyContext,
+        );
       }
       stopPhaseTimer();
     }
@@ -1025,7 +1050,7 @@ function updateClassInstance(
   // if shouldComponentUpdate returns false.
   instance.props = newProps;
   instance.state = newState;
-  instance.context = newContext;
+  instance.context = nextLegacyContext;
 
   return shouldUpdate;
 }

packages/react-reconciler/src/ReactFiberDispatcher.js

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {readContext} from './ReactFiberNewContext';
+
+export const Dispatcher = {
+  readContext,
+};

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -135,6 +135,7 @@ import {
   commitAttachRef,
   commitDetachRef,
 } from './ReactFiberCommitWork';
+import {Dispatcher} from './ReactFiberDispatcher';
 
 export type Deadline = {
   timeRemaining: () => number,
@@ -985,6 +986,7 @@ function renderRoot(root: FiberRoot, isYieldy: boolean): void {
       'by a bug in React. Please file an issue.',
   );
   isWorking = true;
+  ReactCurrentOwner.currentDispatcher = Dispatcher;
 
   const expirationTime = root.nextExpirationTimeToWorkOn;
 
@@ -1071,6 +1073,7 @@ function renderRoot(root: FiberRoot, isYieldy: boolean): void {
 
   // We're done performing work. Time to clean up.
   isWorking = false;
+  ReactCurrentOwner.currentDispatcher = null;
 
   // Yield back to main thread.
   if (didFatal) {