Disallow reading context during useMemo etc (facebook#14648)

* Disallow reading context during useMemo etc

* Continue allowing readContext() in classes and context consumers

The previous commit check was too broad and incorrectly restricted classes from calling readContext(). This check is more precise and only targets components that are Hook-capable. It exploits the fact that `renderExpirationTime` is never `NoWork` after `renderWithHooks` -- something we already rely on.

Author: gaearon

packages/react-reconciler/src/ReactFiberClassComponent.js

@@ -20,7 +20,6 @@ import {
 import ReactStrictModeWarnings from './ReactStrictModeWarnings';
 import {isMounted} from 'react-reconciler/reflection';
 import {get as getInstance, set as setInstance} from 'shared/ReactInstanceMap';
-import ReactSharedInternals from 'shared/ReactSharedInternals';
 import shallowEqual from 'shared/shallowEqual';
 import getComponentName from 'shared/getComponentName';
 import invariant from 'shared/invariant';
@@ -48,20 +47,14 @@ import {
   hasContextChanged,
   emptyContextObject,
 } from './ReactFiberContext';
+import {readContext} from './ReactFiberNewContext';
 import {
   requestCurrentTime,
   computeExpirationForFiber,
   scheduleWork,
   flushPassiveEffects,
 } from './ReactFiberScheduler';
 
-const ReactCurrentDispatcher = ReactSharedInternals.ReactCurrentDispatcher;
-
-function readContext(contextType: any): any {
-  const dispatcher = ReactCurrentDispatcher.current;
-  return dispatcher.readContext(contextType);
-}
-
 const fakeInternalInstance = {};
 const isArray = Array.isArray;
 

packages/react-reconciler/src/ReactFiberDispatcher.js

@@ -7,8 +7,8 @@
  * @flow
  */
 
-import {readContext} from './ReactFiberNewContext';
 import {
+  readContext,
   useCallback,
   useContext,
   useEffect,

packages/react-reconciler/src/ReactFiberHooks.js

@@ -14,7 +14,7 @@ import type {HookEffectTag} from './ReactHookEffectTags';
 
 import {NoWork} from './ReactFiberExpirationTime';
 import {enableHooks} from 'shared/ReactFeatureFlags';
-import {readContext} from './ReactFiberNewContext';
+import {readContext as readContextWithoutCheck} from './ReactFiberNewContext';
 import {
   Update as UpdateEffect,
   Passive as PassiveEffect,
@@ -284,7 +284,7 @@ export function resetHooks(): void {
 
   // This is used to reset the state of this module when a component throws.
   // It's also called inside mountIndeterminateComponent if we determine the
-  // component is a module-style component.
+  // component is a module-style component, and also in readContext() above.
   renderExpirationTime = NoWork;
   currentlyRenderingFiber = null;
 
@@ -394,7 +394,7 @@ export function useContext<T>(
   // Ensure we're in a function component (class components support only the
   // .unstable_read() form)
   resolveCurrentlyRenderingFiber();
-  return readContext(context, observedBits);
+  return readContextWithoutCheck(context, observedBits);
 }
 
 export function useState<S>(
@@ -771,6 +771,29 @@ export function useMemo<T>(
   return nextValue;
 }
 
+export function readContext<T>(
+  context: ReactContext<T>,
+  observedBits: void | number | boolean,
+): T {
+  // Forbid reading context inside Hooks.
+  // The outer check tells us whether we're inside a Hook like useMemo().
+  // However, it would also be true if we're rendering a class.
+  if (currentlyRenderingFiber === null) {
+    // The inner check tells us we're currently in renderWithHooks() phase
+    // rather than, for example, in a class or a context consumer.
+    // Then we know it should be an error.
+    if (renderExpirationTime !== NoWork) {
+      invariant(
+        false,
+        'Context can only be read inside the body of a component. ' +
+          'If you read context inside a Hook like useMemo or useReducer, ' +
+          'move the call directly into the component body.',
+      );
+    }
+  }
+  return readContextWithoutCheck(context, observedBits);
+}
+
 function dispatchAction<S, A>(
   fiber: Fiber,
   queue: UpdateQueue<S, A>,

packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js

@@ -633,6 +633,88 @@ describe('ReactHooks', () => {
     expect(root.toJSON()).toEqual('123');
   });
 
+  it('throws when reading context inside useMemo', () => {
+    const {useMemo, createContext} = React;
+    const ReactCurrentDispatcher =
+      React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
+        .ReactCurrentDispatcher;
+
+    const ThemeContext = createContext('light');
+    function App() {
+      return useMemo(() => {
+        return ReactCurrentDispatcher.current.readContext(ThemeContext);
+      }, []);
+    }
+
+    expect(() => ReactTestRenderer.create(<App />)).toThrow(
+      'Context can only be read inside the body of a component',
+    );
+  });
+
+  it('throws when reading context inside useEffect', () => {
+    const {useEffect, createContext} = React;
+    const ReactCurrentDispatcher =
+      React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
+        .ReactCurrentDispatcher;
+
+    const ThemeContext = createContext('light');
+    function App() {
+      useEffect(() => {
+        ReactCurrentDispatcher.current.readContext(ThemeContext);
+      });
+      return null;
+    }
+
+    const root = ReactTestRenderer.create(<App />);
+    expect(() => root.update(<App />)).toThrow(
+      // The exact message doesn't matter, just make sure we don't allow this
+      "Cannot read property 'readContext' of null",
+    );
+  });
+
+  it('throws when reading context inside useLayoutEffect', () => {
+    const {useLayoutEffect, createContext} = React;
+    const ReactCurrentDispatcher =
+      React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
+        .ReactCurrentDispatcher;
+
+    const ThemeContext = createContext('light');
+    function App() {
+      useLayoutEffect(() => {
+        ReactCurrentDispatcher.current.readContext(ThemeContext);
+      });
+      return null;
+    }
+
+    expect(() => ReactTestRenderer.create(<App />)).toThrow(
+      // The exact message doesn't matter, just make sure we don't allow this
+      "Cannot read property 'readContext' of null",
+    );
+  });
+
+  it('throws when reading context inside useReducer', () => {
+    const {useReducer, createContext} = React;
+    const ReactCurrentDispatcher =
+      React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
+        .ReactCurrentDispatcher;
+
+    const ThemeContext = createContext('light');
+    function App() {
+      useReducer(
+        () => {
+          ReactCurrentDispatcher.current.readContext(ThemeContext);
+        },
+        null,
+        {},
+      );
+      return null;
+    }
+
+    expect(() => ReactTestRenderer.create(<App />)).toThrow(
+      'Context can only be read inside the body of a component.',
+    );
+  });
+
   it('throws when calling hooks inside useReducer', () => {
     const {useReducer, useRef} = React;
     function App() {