Support sharing context objects between concurrent renderers

packages/react-art/src/ReactART.js

@@ -478,6 +478,9 @@ const ARTRenderer = ReactFiberReconciler({
 
   now: ReactScheduler.now,
 
+  // The ART renderer is secondary to the React DOM renderer.
+  isPrimaryRenderer: false,
+
   mutation: {
     appendChild(parentInstance, child) {
       if (child.parentNode === parentInstance) {

packages/react-art/src/__tests__/ReactART-test.js

@@ -339,6 +339,60 @@ describe('ReactART', () => {
     doClick(instance);
     expect(onClick2).toBeCalled();
   });
+
+  it('can concurrently render with a "primary" renderer while sharing context', () => {
+    const CurrentRendererContext = React.createContext(null);
+
+    function Yield(props) {
+      testRenderer.unstable_yield(props.value);
+      return null;
+    }
+
+    let ops = [];
+    function LogCurrentRenderer() {
+      return (
+        <CurrentRendererContext.Consumer>
+          {currentRenderer => {
+            ops.push(currentRenderer);
+            return null;
+          }}
+        </CurrentRendererContext.Consumer>
+      );
+    }
+
+    // Using test renderer instead of the DOM renderer here because async
+    // testing APIs for the DOM renderer don't exist.
+    const testRenderer = renderer.create(
+      <CurrentRendererContext.Provider value="Test">
+        <Yield value="A" />
+        <Yield value="B" />
+        <LogCurrentRenderer />
+        <Yield value="C" />
+      </CurrentRendererContext.Provider>,
+      {
+        unstable_isAsync: true,
+      },
+    );
+
+    testRenderer.unstable_flushThrough(['A']);
+
+    ReactDOM.render(
+      <Surface>
+        <LogCurrentRenderer />
+        <CurrentRendererContext.Provider value="ART">
+          <LogCurrentRenderer />
+        </CurrentRendererContext.Provider>
+      </Surface>,
+      container,
+    );
+
+    expect(ops).toEqual([null, 'ART']);
+
+    ops = [];
+    expect(testRenderer.unstable_flushAll()).toEqual(['B', 'C']);
+
+    expect(ops).toEqual(['Test']);
+  });
 });
 
 describe('ReactARTComponents', () => {

packages/react-dom/src/client/ReactDOM.js

@@ -690,6 +690,8 @@ const DOMRenderer = ReactFiberReconciler({
 
   now: ReactScheduler.now,
 
+  isPrimaryRenderer: true,
+
   mutation: {
     commitMount(
       domElement: Instance,

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

@@ -217,6 +217,9 @@ const ReactFabricRenderer = ReactFiberReconciler({
 
   now: ReactNativeFrameScheduling.now,
 
+  // The Fabric renderer is secondary to the existing React Native renderer.
+  isPrimaryRenderer: false,
+
   prepareForCommit(): void {
     // Noop
   },

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

@@ -169,6 +169,8 @@ const NativeRenderer = ReactFiberReconciler({
 
   now: ReactNativeFrameScheduling.now,
 
+  isPrimaryRenderer: true,
+
   prepareForCommit(): void {
     // Noop
   },

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

@@ -185,6 +185,8 @@ let SharedHostConfig = {
   now(): number {
     return elapsedTimeInMs;
   },
+
+  isPrimaryRenderer: true,
 };
 
 const NoopRenderer = ReactFiberReconciler({

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -104,7 +104,11 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
 
   const {pushHostContext, pushHostContainer} = hostContext;
 
-  const {pushProvider} = newContext;
+  const {
+    pushProvider,
+    getContextCurrentValue,
+    getContextChangedBits,
+  } = newContext;
 
   const {
     markActualRenderTimeStarted,
@@ -1048,8 +1052,8 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
     const newProps = workInProgress.pendingProps;
     const oldProps = workInProgress.memoizedProps;
 
-    const newValue = context._currentValue;
-    const changedBits = context._changedBits;
+    const newValue = getContextCurrentValue(context);
+    const changedBits = getContextChangedBits(context);
 
     if (hasLegacyContextChanged()) {
       // Normally we can bail out on props equality but if context has changed

packages/react-reconciler/src/ReactFiberNewContext.js

@@ -11,14 +11,16 @@ import type {Fiber} from './ReactFiber';
 import type {ReactContext} from 'shared/ReactTypes';
 import type {StackCursor, Stack} from './ReactFiberStack';
 
-import warning from 'fbjs/lib/warning';
-
 export type NewContext = {
   pushProvider(providerFiber: Fiber): void,
   popProvider(providerFiber: Fiber): void,
+  getContextCurrentValue(context: ReactContext<any>): any,
+  getContextChangedBits(context: ReactContext<any>): number,
 };
 
-export default function(stack: Stack) {
+import warning from 'fbjs/lib/warning';
+
+export default function(stack: Stack, isPrimaryRenderer: boolean) {
   const {createCursor, push, pop} = stack;
 
   const providerCursor: StackCursor<Fiber | null> = createCursor(null);
@@ -34,21 +36,38 @@ export default function(stack: Stack) {
   function pushProvider(providerFiber: Fiber): void {
     const context: ReactContext<any> = providerFiber.type._context;
 
-    push(changedBitsCursor, context._changedBits, providerFiber);
-    push(valueCursor, context._currentValue, providerFiber);
-    push(providerCursor, providerFiber, providerFiber);
-
-    context._currentValue = providerFiber.pendingProps.value;
-    context._changedBits = providerFiber.stateNode;
-
-    if (__DEV__) {
-      warning(
-        context._currentRenderer === null ||
-          context._currentRenderer === rendererSigil,
-        'Detected multiple renderers concurrently rendering the ' +
-          'same context provider. This is currently unsupported.',
-      );
-      context._currentRenderer = rendererSigil;
+    if (isPrimaryRenderer) {
+      push(changedBitsCursor, context._changedBits, providerFiber);
+      push(valueCursor, context._currentValue, providerFiber);
+      push(providerCursor, providerFiber, providerFiber);
+
+      context._currentValue = providerFiber.pendingProps.value;
+      context._changedBits = providerFiber.stateNode;
+      if (__DEV__) {
+        warning(
+          context._currentRenderer === null ||
+            context._currentRenderer === rendererSigil,
+          'Detected multiple renderers concurrently rendering the ' +
+            'same context provider. This is currently unsupported.',
+        );
+        context._currentRenderer = rendererSigil;
+      }
+    } else {
+      push(changedBitsCursor, context._changedBits2, providerFiber);
+      push(valueCursor, context._currentValue2, providerFiber);
+      push(providerCursor, providerFiber, providerFiber);
+
+      context._currentValue2 = providerFiber.pendingProps.value;
+      context._changedBits2 = providerFiber.stateNode;
+      if (__DEV__) {
+        warning(
+          context._currentRenderer2 === null ||
+            context._currentRenderer2 === rendererSigil,
+          'Detected multiple renderers concurrently rendering the ' +
+            'same context provider. This is currently unsupported.',
+        );
+        context._currentRenderer2 = rendererSigil;
+      }
     }
   }
 
@@ -61,12 +80,27 @@ export default function(stack: Stack) {
     pop(changedBitsCursor, providerFiber);
 
     const context: ReactContext<any> = providerFiber.type._context;
-    context._currentValue = currentValue;
-    context._changedBits = changedBits;
+    if (isPrimaryRenderer) {
+      context._currentValue = currentValue;
+      context._changedBits = changedBits;
+    } else {
+      context._currentValue2 = currentValue;
+      context._changedBits2 = changedBits;
+    }
+  }
+
+  function getContextCurrentValue(context: ReactContext<any>): any {
+    return isPrimaryRenderer ? context._currentValue : context._currentValue2;
+  }
+
+  function getContextChangedBits(context: ReactContext<any>): number {
+    return isPrimaryRenderer ? context._changedBits : context._changedBits2;
   }
 
   return {
     pushProvider,
     popProvider,
+    getContextCurrentValue,
+    getContextChangedBits,
   };
 }

packages/react-reconciler/src/ReactFiberReconciler.js

@@ -95,6 +95,11 @@ export type HostConfig<T, P, I, TI, HI, PI, C, CC, CX, PL> = {
 
   now(): number,
 
+  // Temporary workaround for scenario where multiple renderers concurrently
+  // render using the same context objects. E.g. React DOM and React ART on the
+  // same page. DOM is the primary renderer; ART is the secondary renderer.
+  isPrimaryRenderer: boolean,
+
   +hydration?: HydrationHostConfig<T, P, I, TI, HI, C, CX, PL>,
 
   +mutation?: MutableUpdatesHostConfig<T, P, I, TI, C, PL>,

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -181,7 +181,7 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
   const stack = ReactFiberStack();
   const hostContext = ReactFiberHostContext(config, stack);
   const legacyContext = ReactFiberLegacyContext(stack);
-  const newContext = ReactFiberNewContext(stack);
+  const newContext = ReactFiberNewContext(stack, config.isPrimaryRenderer);
   const profilerTimer = createProfilerTimer(now);
   const {popHostContext, popHostContainer} = hostContext;
   const {

packages/react-test-renderer/src/ReactTestRenderer.js

@@ -226,6 +226,8 @@ const TestRenderer = ReactFiberReconciler({
   // Even after the reconciler has initialized and read host config values.
   now: () => nowImplementation(),
 
+  isPrimaryRenderer: true,
+
   mutation: {
     commitUpdate(
       instance: Instance,

packages/react/src/ReactContext.js

@@ -36,7 +36,14 @@ export function createContext<T>(
     _calculateChangedBits: calculateChangedBits,
     _defaultValue: defaultValue,
     _currentValue: defaultValue,
+    // As a workaround to support multiple concurrent renderers, we categorize
+    // some renderers as primary and others as secondary. We only expect
+    // there to be two concurrent renderers at most: React Native (primary) and
+    // Fabric (secondary); React DOM (primary) and React ART (secondary).
+    // Secondary renderers store their context values on separate fields.
+    _currentValue2: defaultValue,
     _changedBits: 0,
+    _changedBits2: 0,
     // These are circular
     Provider: (null: any),
     Consumer: (null: any),
@@ -50,6 +57,7 @@ export function createContext<T>(
 
   if (__DEV__) {
     context._currentRenderer = null;
+    context._currentRenderer2 = null;
   }
 
   return context;

packages/shared/ReactTypes.js

@@ -85,10 +85,13 @@ export type ReactContext<T> = {
   _defaultValue: T,
 
   _currentValue: T,
+  _currentValue2: T,
   _changedBits: number,
+  _changedBits2: number,
 
   // DEV only
   _currentRenderer?: Object | null,
+  _currentRenderer2?: Object | null,
 };
 
 export type ReactPortal = {