batchedUpdates tests

Covers some bugs I encountered while working on this feature.

Introduces a syncUpdates API to ReactNoop. Is this something we'd want
to expose to the reconciler?

src/renderers/noop/ReactNoop.js

@@ -257,6 +257,10 @@ var ReactNoop = {
     NoopRenderer.performWithPriority(AnimationPriority, fn);
   },
 
+  batchedUpdates: NoopRenderer.batchedUpdates,
+
+  syncUpdates: NoopRenderer.syncUpdates,
+
   // Logs the current state of the tree.
   dumpTree(rootID : string = DEFAULT_ROOT_ID) {
     const root = roots.get(rootID);

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -71,6 +71,7 @@ export type Reconciler<C, I, TI> = {
   /* eslint-disable no-undef */
   // FIXME: ESLint complains about type parameter
   batchedUpdates<A>(fn : () => A) : A,
+  syncUpdates<A>(fn : () => A) : A,
   /* eslint-enable no-undef */
 
   // Used to extract the return value from the initial render. Legacy API.
@@ -86,6 +87,7 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) :
     scheduleWork,
     performWithPriority,
     batchedUpdates,
+    syncUpdates,
   } = ReactFiberScheduler(config);
 
   return {
@@ -152,6 +154,8 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) :
 
     batchedUpdates,
 
+    syncUpdates,
+
     getPublicRootInstance(container : OpaqueNode) : (ReactComponent<any, any, any> | I | TI | null) {
       const root : FiberRoot = (container.stateNode : any);
       const containerFiber = root.current;

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -712,10 +712,21 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) {
     }
   }
 
+  function syncUpdates<A>(fn : () => A) : A {
+    const previousPriorityContext = priorityContext;
+    priorityContext = SynchronousPriority;
+    try {
+      return fn();
+    } finally {
+      priorityContext = previousPriorityContext;
+    }
+  }
+
   return {
     scheduleWork: scheduleWork,
     scheduleDeferredWork: scheduleDeferredWork,
     performWithPriority: performWithPriority,
     batchedUpdates: batchedUpdates,
+    syncUpdates: syncUpdates,
   };
 };

src/renderers/shared/fiber/__tests__/ReactIncremental-test.js

@@ -1386,4 +1386,83 @@ describe('ReactIncremental', () => {
     ]);
   });
 
+  it('performs batched updates at the end of the batch', () => {
+    var ops = [];
+    var instance;
+
+    class Foo extends React.Component {
+      state = { n: 0 };
+      render() {
+        instance = this;
+        return <div />;
+      }
+    }
+
+    ReactNoop.render(<Foo />);
+    ReactNoop.flush();
+    ops = [];
+
+    ReactNoop.syncUpdates(() => {
+      ReactNoop.batchedUpdates(() => {
+        instance.setState({ n: 1 }, () => ops.push('setState 1'));
+        instance.setState({ n: 2 }, () => ops.push('setState 2'));
+        ops.push('end batchedUpdates');
+      });
+      ops.push('end syncUpdates');
+    });
+
+    // ReactNoop.flush() not needed because updates are synchronous
+
+    expect(ops).toEqual([
+      'end batchedUpdates',
+      'setState 1',
+      'setState 2',
+      'end syncUpdates',
+    ]);
+    expect(instance.state.n).toEqual(2);
+  });
+
+  it('can nest batchedUpdates', () => {
+    var ops = [];
+    var instance;
+
+    class Foo extends React.Component {
+      state = { n: 0 };
+      render() {
+        instance = this;
+        return <div />;
+      }
+    }
+
+    ReactNoop.render(<Foo />);
+    ReactNoop.flush();
+    ops = [];
+
+    ReactNoop.syncUpdates(() => {
+      ReactNoop.batchedUpdates(() => {
+        instance.setState({ n: 1 }, () => ops.push('setState 1'));
+        instance.setState({ n: 2 }, () => ops.push('setState 2'));
+        ReactNoop.batchedUpdates(() => {
+          instance.setState({ n: 3 }, () => ops.push('setState 3'));
+          instance.setState({ n: 4 }, () => ops.push('setState 4'));
+          ops.push('end inner batchedUpdates');
+        });
+        ops.push('end outer batchedUpdates');
+      });
+      ops.push('end syncUpdates');
+    });
+
+    // ReactNoop.flush() not needed because updates are synchronous
+
+    expect(ops).toEqual([
+      'end inner batchedUpdates',
+      'end outer batchedUpdates',
+      'setState 1',
+      'setState 2',
+      'setState 3',
+      'setState 4',
+      'end syncUpdates',
+    ]);
+    expect(instance.state.n).toEqual(4);
+  });
 });