useMutableSource-test.internal.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 * @jest-environment node
 */

/* eslint-disable no-func-assign */

'use strict';

let React;
let ReactFeatureFlags;
let ReactNoop;
let Scheduler;
let useMutableSource;
let act;

function loadModules() {
  jest.resetModules();
  jest.useFakeTimers();

  ReactFeatureFlags = require('shared/ReactFeatureFlags');
  ReactFeatureFlags.debugRenderPhaseSideEffectsForStrictMode = false;
  ReactFeatureFlags.enableSchedulerTracing = true;
  ReactFeatureFlags.flushSuspenseFallbacksInTests = false;
  ReactFeatureFlags.enableProfilerTimer = true;
  React = require('react');
  ReactNoop = require('react-noop-renderer');
  Scheduler = require('scheduler');
  useMutableSource = React.useMutableSource;
  act = ReactNoop.act;
}

describe('useMutableSource', () => {
  const defaultGetSnapshot = source => source.value;
  const defaultSubscribe = (source, callback) => source.subscribe(callback);

  function createComplexSource(initialValueA, initialValueB) {
    const callbacksA = [];
    const callbacksB = [];
    let revision = 0;
    let valueA = initialValueA;
    let valueB = initialValueB;

    const subscribeHelper = (callbacks, callback) => {
      if (callbacks.indexOf(callback) < 0) {
        callbacks.push(callback);
      }
      return () => {
        const index = callbacks.indexOf(callback);
        if (index >= 0) {
          callbacks.splice(index, 1);
        }
      };
    };

    return {
      subscribeA(callback) {
        return subscribeHelper(callbacksA, callback);
      },
      subscribeB(callback) {
        return subscribeHelper(callbacksB, callback);
      },

      get listenerCountA() {
        return callbacksA.length;
      },
      get listenerCountB() {
        return callbacksB.length;
      },

      set valueA(newValue) {
        revision++;
        valueA = newValue;
        callbacksA.forEach(callback => callback());
      },
      get valueA() {
        return valueA;
      },

      set valueB(newValue) {
        revision++;
        valueB = newValue;
        callbacksB.forEach(callback => callback());
      },
      get valueB() {
        return valueB;
      },

      get version() {
        return revision;
      },
    };
  }

  function createSource(initialValue) {
    const callbacks = [];
    let revision = 0;
    let value = initialValue;
    return {
      subscribe(callback) {
        if (callbacks.indexOf(callback) < 0) {
          callbacks.push(callback);
        }
        return () => {
          const index = callbacks.indexOf(callback);
          if (index >= 0) {
            callbacks.splice(index, 1);
          }
        };
      },
      get listenerCount() {
        return callbacks.length;
      },
      set value(newValue) {
        revision++;
        value = newValue;
        callbacks.forEach(callback => callback());
      },
      get value() {
        return value;
      },
      get version() {
        return revision;
      },
    };
  }

  function createMutableSource(source) {
    return React.createMutableSource(source, param => param.version);
  }

  function Component({getSnapshot, label, mutableSource, subscribe}) {
    const snapshot = useMutableSource(mutableSource, getSnapshot, subscribe);
    Scheduler.unstable_yieldValue(`${label}:${snapshot}`);
    return <div>{`${label}:${snapshot}`}</div>;
  }

  if (!__EXPERIMENTAL__) {
    it("empty test so Jest doesn't complain", () => {});
  } else {
    beforeEach(loadModules);

    it('should subscribe to a source and schedule updates when it changes', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.renderToRootWithID(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          'root',
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYieldThrough([
          'a:one',
          'b:one',
          'Sync effect',
        ]);

        // Subscriptions should be passive
        expect(source.listenerCount).toBe(0);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(2);

        // Changing values should schedule an update with React
        source.value = 'two';
        expect(Scheduler).toFlushAndYieldThrough(['a:two', 'b:two']);

        // Umounting a component should remove its subscriptino.
        ReactNoop.renderToRootWithID(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          'root',
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:two', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(1);

        // Umounting a root should remove the remaining event listeners
        ReactNoop.unmountRootWithID('root');
        expect(Scheduler).toFlushAndYield([]);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(0);

        // Changes to source should not trigger an updates or warnings.
        source.value = 'three';
        expect(Scheduler).toFlushAndYield([]);
      });
    });

    it('should restart work if a new source is mutated during render', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );

        // Do enough work to read from one component
        expect(Scheduler).toFlushAndYieldThrough(['a:one']);

        // Mutate source before continuing work
        source.value = 'two';

        // Render work should restart and the updated value should be used
        expect(Scheduler).toFlushAndYield(['a:two', 'b:two', 'Sync effect']);
      });
    });

    it('should schedule an update if a new source is mutated between render and commit (subscription)', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );

        // Finish rendering
        expect(Scheduler).toFlushAndYieldThrough([
          'a:one',
          'b:one',
          'Sync effect',
        ]);

        // Mutate source before subscriptions are attached
        expect(source.listenerCount).toBe(0);
        source.value = 'two';

        // Mutation should be detected, and a new render should be scheduled
        expect(Scheduler).toFlushAndYield(['a:two', 'b:two']);
      });
    });

    it('should unsubscribe and resubscribe if a new source is used', () => {
      const sourceA = createSource('a-one');
      const mutableSourceA = createMutableSource(sourceA);

      const sourceB = createSource('b-one');
      const mutableSourceB = createMutableSource(sourceB);

      act(() => {
        ReactNoop.render(
          <Component
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSourceA}
            subscribe={defaultSubscribe}
          />,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYieldThrough(['only:a-one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(sourceA.listenerCount).toBe(1);

        // Changing values should schedule an update with React
        sourceA.value = 'a-two';
        expect(Scheduler).toFlushAndYield(['only:a-two']);

        // If we re-render with a new source, the old one should be unsubscribed.
        ReactNoop.render(
          <Component
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSourceB}
            subscribe={defaultSubscribe}
          />,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:b-one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(sourceA.listenerCount).toBe(0);
        expect(sourceB.listenerCount).toBe(1);

        // Changing to original source should not schedule updates with React
        sourceA.value = 'a-three';
        expect(Scheduler).toFlushAndYield([]);

        // Changing new source value should schedule an update with React
        sourceB.value = 'b-two';
        expect(Scheduler).toFlushAndYield(['only:b-two']);
      });
    });

    it('should unsubscribe and resubscribe if a new subscribe function is provided', () => {
      const source = createSource('a-one');
      const mutableSource = createMutableSource(source);

      const unsubscribeA = jest.fn();
      const subscribeA = jest.fn(s => {
        const unsubscribe = defaultSubscribe(s);
        return () => {
          unsubscribe();
          unsubscribeA();
        };
      });
      const unsubscribeB = jest.fn();
      const subscribeB = jest.fn(s => {
        const unsubscribe = defaultSubscribe(s);
        return () => {
          unsubscribe();
          unsubscribeB();
        };
      });

      act(() => {
        ReactNoop.renderToRootWithID(
          <Component
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={subscribeA}
          />,
          'root',
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:a-one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(1);
        expect(subscribeA).toHaveBeenCalledTimes(1);

        // If we re-render with a new subscription function,
        // the old unsubscribe function should be called.
        ReactNoop.renderToRootWithID(
          <Component
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={subscribeB}
          />,
          'root',
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:a-one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(1);
        expect(unsubscribeA).toHaveBeenCalledTimes(1);
        expect(subscribeB).toHaveBeenCalledTimes(1);

        // Unmounting should call the newer unsunscribe.
        ReactNoop.unmountRootWithID('root');
        expect(Scheduler).toFlushAndYield([]);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(0);
        expect(unsubscribeB).toHaveBeenCalledTimes(1);
      });
    });

    it('should re-use previously read snapshot value when reading is unsafe', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);

        // Changing values should schedule an update with React.
        // Start working on this update but don't finish it.
        source.value = 'two';
        expect(Scheduler).toFlushAndYieldThrough(['a:two']);

        // Re-renders that occur before the udpate is processed
        // should reuse snapshot so long as the config has not changed
        ReactNoop.flushSync(() => {
          ReactNoop.render(
            <>
              <Component
                label="a"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
              <Component
                label="b"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
            </>,
            () => Scheduler.unstable_yieldValue('Sync effect'),
          );
        });
        expect(Scheduler).toHaveYielded(['a:one', 'b:one', 'Sync effect']);

        expect(Scheduler).toFlushAndYield(['a:two', 'b:two']);
      });
    });

    it('should read from source on newly mounted subtree if no pending updates are scheduled for source', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'Sync effect']);

        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
      });
    });

    it('should throw and restart render if source and snapshot are unavailable during an update', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();

        // Changing values should schedule an update with React.
        // Start working on this update but don't finish it.
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_LowPriority,
          () => {
            source.value = 'two';
            expect(Scheduler).toFlushAndYieldThrough(['a:two']);
          },
        );

        const newGetSnapshot = s => 'new:' + defaultGetSnapshot(s);

        // Force a higher priority render with a new config.
        // This should signal that the snapshot is not safe and trigger a full re-render.
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_UserBlockingPriority,
          () => {
            ReactNoop.render(
              <>
                <Component
                  label="a"
                  getSnapshot={newGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
                <Component
                  label="b"
                  getSnapshot={newGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
              </>,
              () => Scheduler.unstable_yieldValue('Sync effect'),
            );
          },
        );
        expect(Scheduler).toFlushAndYieldThrough([
          'a:new:two',
          'b:new:two',
          'Sync effect',
        ]);
      });
    });

    it('should throw and restart render if source and snapshot are unavailable during a sync update', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();

        // Changing values should schedule an update with React.
        // Start working on this update but don't finish it.
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_LowPriority,
          () => {
            source.value = 'two';
            expect(Scheduler).toFlushAndYieldThrough(['a:two']);
          },
        );

        const newGetSnapshot = s => 'new:' + defaultGetSnapshot(s);

        // Force a higher priority render with a new config.
        // This should signal that the snapshot is not safe and trigger a full re-render.
        ReactNoop.flushSync(() => {
          ReactNoop.render(
            <>
              <Component
                label="a"
                getSnapshot={newGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
              <Component
                label="b"
                getSnapshot={newGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
            </>,
            () => Scheduler.unstable_yieldValue('Sync effect'),
          );
        });
        expect(Scheduler).toHaveYielded([
          'a:new:two',
          'b:new:two',
          'Sync effect',
        ]);
      });
    });

    it('should only update components whose subscriptions fire', () => {
      const source = createComplexSource('a:one', 'b:one');
      const mutableSource = createMutableSource(source);

      // Subscribe to part of the store.
      const getSnapshotA = s => s.valueA;
      const subscribeA = (s, callback) => s.subscribeA(callback);
      const getSnapshotB = s => s.valueB;
      const subscribeB = (s, callback) => s.subscribeB(callback);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={getSnapshotA}
              mutableSource={mutableSource}
              subscribe={subscribeA}
            />
            <Component
              label="b"
              getSnapshot={getSnapshotB}
              mutableSource={mutableSource}
              subscribe={subscribeB}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield([
          'a:a:one',
          'b:b:one',
          'Sync effect',
        ]);

        // Changes to part of the store (e.g. A) should not render other parts.
        source.valueA = 'a:two';
        expect(Scheduler).toFlushAndYield(['a:a:two']);
        source.valueB = 'b:two';
        expect(Scheduler).toFlushAndYield(['b:b:two']);
      });
    });

    it('should detect tearing in part of the store not yet subscribed to', () => {
      const source = createComplexSource('a:one', 'b:one');
      const mutableSource = createMutableSource(source);

      // Subscribe to part of the store.
      const getSnapshotA = s => s.valueA;
      const subscribeA = (s, callback) => s.subscribeA(callback);
      const getSnapshotB = s => s.valueB;
      const subscribeB = (s, callback) => s.subscribeB(callback);

      act(() => {
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={getSnapshotA}
              mutableSource={mutableSource}
              subscribe={subscribeA}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:a:one', 'Sync effect']);

        // Because the store has not chagned yet, there are no pending updates,
        // so it is considered safe to read from when we start this render.
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={getSnapshotA}
              mutableSource={mutableSource}
              subscribe={subscribeA}
            />
            <Component
              label="b"
              getSnapshot={getSnapshotB}
              mutableSource={mutableSource}
              subscribe={subscribeB}
            />
            <Component
              label="c"
              getSnapshot={getSnapshotB}
              mutableSource={mutableSource}
              subscribe={subscribeB}
            />
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYieldThrough(['a:a:one', 'b:b:one']);

        // Mutating the source should trigger a tear detection on the next read,
        // which should throw and re-render the entire tree.
        source.valueB = 'b:two';

        expect(Scheduler).toFlushAndYield([
          'a:a:one',
          'b:b:two',
          'c:b:two',
          'Sync effect',
        ]);
      });
    });

    it('does not schedule an update for subscriptions that fire with an unchanged snapshot', () => {
      const MockComponent = jest.fn(Component);

      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        ReactNoop.render(
          <MockComponent
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYieldThrough(['only:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(source.listenerCount).toBe(1);

        // Notify subscribe function but don't change the value
        source.value = 'one';
        expect(Scheduler).toFlushWithoutYielding();
      });
    });

    it('should throw and restart if getSnapshot changes between scheduled update and re-render', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      const newGetSnapshot = s => 'new:' + defaultGetSnapshot(s);

      let updateGetSnapshot;

      function WrapperWithState() {
        const tuple = React.useState(() => defaultGetSnapshot);
        updateGetSnapshot = tuple[1];
        return (
          <Component
            label="only"
            getSnapshot={tuple[0]}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
        );
      }

      act(() => {
        ReactNoop.render(<WrapperWithState />, () =>
          Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();

        // Change the source (and schedule an update).
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_LowPriority,
          () => {
            source.value = 'two';
          },
        );

        // Schedule a higher priority update that changes getSnapshot.
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_UserBlockingPriority,
          () => {
            updateGetSnapshot(() => newGetSnapshot);
          },
        );

        expect(Scheduler).toFlushAndYield(['only:new:two']);
      });
    });

    it('should recover from a mutation during yield when other work is scheduled', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      act(() => {
        // Start a render that uses the mutable source.
        ReactNoop.render(
          <>
            <Component
              label="a"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
            <Component
              label="b"
              getSnapshot={defaultGetSnapshot}
              mutableSource={mutableSource}
              subscribe={defaultSubscribe}
            />
          </>,
        );
        expect(Scheduler).toFlushAndYieldThrough(['a:one']);

        // Mutate source
        source.value = 'two';

        // Now render something different.
        ReactNoop.render(<div />);
        expect(Scheduler).toFlushAndYield([]);
      });
    });

    it('should not throw if the new getSnapshot returns the same snapshot value', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      const onRenderA = jest.fn();
      const onRenderB = jest.fn();

      let updateGetSnapshot;

      function WrapperWithState() {
        const tuple = React.useState(() => defaultGetSnapshot);
        updateGetSnapshot = tuple[1];
        return (
          <Component
            label="b"
            getSnapshot={tuple[0]}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
        );
      }

      act(() => {
        ReactNoop.render(
          <>
            <React.Profiler id="a" onRender={onRenderA}>
              <Component
                label="a"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
            </React.Profiler>
            <React.Profiler id="b" onRender={onRenderB}>
              <WrapperWithState />
            </React.Profiler>
          </>,
          () => Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();
        expect(onRenderA).toHaveBeenCalledTimes(1);
        expect(onRenderB).toHaveBeenCalledTimes(1);

        // If B's getSnapshot function updates, but the snapshot it returns is the same,
        // only B should re-render (to update its state).
        updateGetSnapshot(() => s => defaultGetSnapshot(s));
        expect(Scheduler).toFlushAndYield(['b:one']);
        ReactNoop.flushPassiveEffects();
        expect(onRenderA).toHaveBeenCalledTimes(1);
        expect(onRenderB).toHaveBeenCalledTimes(2);
      });
    });

    it('should not throw if getSnapshot changes but the source can be safely read from anyway', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      const newGetSnapshot = s => 'new:' + defaultGetSnapshot(s);

      let updateGetSnapshot;

      function WrapperWithState() {
        const tuple = React.useState(() => defaultGetSnapshot);
        updateGetSnapshot = tuple[1];
        return (
          <Component
            label="only"
            getSnapshot={tuple[0]}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
        );
      }

      act(() => {
        ReactNoop.render(<WrapperWithState />, () =>
          Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();

        // Change the source (and schedule an update)
        // but also change the snapshot function too.
        ReactNoop.batchedUpdates(() => {
          source.value = 'two';
          updateGetSnapshot(() => newGetSnapshot);
        });

        expect(Scheduler).toFlushAndYield(['only:new:two']);
      });
    });

    it('should still schedule an update if an eager selector throws after a mutation', () => {
      const source = createSource({
        friends: [
          {id: 1, name: 'Foo'},
          {id: 2, name: 'Bar'},
        ],
      });
      const mutableSource = createMutableSource(source);

      function FriendsList() {
        const getSnapshot = React.useCallback(
          ({value}) => Array.from(value.friends),
          [],
        );
        const friends = useMutableSource(
          mutableSource,
          getSnapshot,
          defaultSubscribe,
        );
        return (
          <ul>
            {friends.map(friend => (
              <Friend key={friend.id} id={friend.id} />
            ))}
          </ul>
        );
      }

      function Friend({id}) {
        const getSnapshot = React.useCallback(
          ({value}) => {
            // This selector is intentionally written in a way that will throw
            // if no matching friend exists in the store.
            return value.friends.find(friend => friend.id === id).name;
          },
          [id],
        );
        const name = useMutableSource(
          mutableSource,
          getSnapshot,
          defaultSubscribe,
        );
        Scheduler.unstable_yieldValue(`${id}:${name}`);
        return <li>{name}</li>;
      }

      act(() => {
        ReactNoop.render(<FriendsList />, () =>
          Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['1:Foo', '2:Bar', 'Sync effect']);

        // This mutation will cause the "Bar" component to throw,
        // since its value will no longer be a part of the store.
        // Mutable source should still schedule an update though,
        // which should unmount "Bar" and mount "Baz".
        source.value = {
          friends: [
            {id: 1, name: 'Foo'},
            {id: 3, name: 'Baz'},
          ],
        };
        expect(Scheduler).toFlushAndYield(['1:Foo', '3:Baz']);
      });
    });

    it('should not warn about updates that fire between unmount and passive unsubcribe', () => {
      const source = createSource('one');
      const mutableSource = createMutableSource(source);

      function Wrapper() {
        React.useLayoutEffect(() => () => {
          Scheduler.unstable_yieldValue('layout unmount');
        });
        return (
          <Component
            label="only"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
        );
      }

      act(() => {
        ReactNoop.renderToRootWithID(<Wrapper />, 'root', () =>
          Scheduler.unstable_yieldValue('Sync effect'),
        );
        expect(Scheduler).toFlushAndYield(['only:one', 'Sync effect']);
        ReactNoop.flushPassiveEffects();

        // Umounting a root should remove the remaining event listeners in a passive effect
        ReactNoop.unmountRootWithID('root');
        expect(Scheduler).toFlushAndYieldThrough(['layout unmount']);

        // Changes to source should not cause a warning,
        // even though the unsubscribe hasn't run yet (since it's a pending passive effect).
        source.value = 'two';
        expect(Scheduler).toFlushAndYield([]);
      });
    });

    it('should support inline selectors and updates that are processed after selector change', async () => {
      const source = createSource({
        a: 'initial',
        b: 'initial',
      });
      const mutableSource = createMutableSource(source);

      const getSnapshotA = () => source.value.a;
      const getSnapshotB = () => source.value.b;

      function mutateB(newB) {
        source.value = {
          ...source.value,
          b: newB,
        };
      }

      function App({toggle}) {
        const state = useMutableSource(
          mutableSource,
          toggle ? getSnapshotB : getSnapshotA,
          defaultSubscribe,
        );
        const result = (toggle ? 'on: ' : 'off: ') + state;
        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(<App toggle={false} />);
      });
      expect(root).toMatchRenderedOutput('off: initial');

      await act(async () => {
        mutateB('Updated B');
        root.render(<App toggle={true} />);
      });
      expect(root).toMatchRenderedOutput('on: Updated B');

      await act(async () => {
        mutateB('Another update');
      });
      expect(root).toMatchRenderedOutput('on: Another update');
    });

    it('should clear the update queue when getSnapshot changes with pending lower priority updates', async () => {
      const source = createSource({
        a: 'initial',
        b: 'initial',
      });
      const mutableSource = createMutableSource(source);

      const getSnapshotA = () => source.value.a;
      const getSnapshotB = () => source.value.b;

      function mutateA(newA) {
        source.value = {
          ...source.value,
          a: newA,
        };
      }

      function mutateB(newB) {
        source.value = {
          ...source.value,
          b: newB,
        };
      }

      function App({toggle}) {
        const state = useMutableSource(
          mutableSource,
          toggle ? getSnapshotB : getSnapshotA,
          defaultSubscribe,
        );
        const result = (toggle ? 'B: ' : 'A: ') + state;
        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(<App toggle={false} />);
      });
      expect(root).toMatchRenderedOutput('A: initial');

      await act(async () => {
        ReactNoop.discreteUpdates(() => {
          // Update both A and B to the same value
          mutateA('Update');
          mutateB('Update');
          // Toggle to B in the same batch
          root.render(<App toggle={true} />);
        });
        // Mutate A at lower priority. This should never be rendered, because
        // by the time we get to the lower priority, we've already switched
        // to B.
        mutateA('OOPS! This mutation should be ignored');
      });
      expect(root).toMatchRenderedOutput('B: Update');
    });

    it('should clear the update queue when source changes with pending lower priority updates', async () => {
      const sourceA = createSource('initial');
      const sourceB = createSource('initial');
      const mutableSourceA = createMutableSource(sourceA);
      const mutableSourceB = createMutableSource(sourceB);

      function App({toggle}) {
        const state = useMutableSource(
          toggle ? mutableSourceB : mutableSourceA,
          defaultGetSnapshot,
          defaultSubscribe,
        );
        const result = (toggle ? 'B: ' : 'A: ') + state;
        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(<App toggle={false} />);
      });
      expect(root).toMatchRenderedOutput('A: initial');

      await act(async () => {
        ReactNoop.discreteUpdates(() => {
          // Update both A and B to the same value
          sourceA.value = 'Update';
          sourceB.value = 'Update';
          // Toggle to B in the same batch
          root.render(<App toggle={true} />);
        });
        // Mutate A at lower priority. This should never be rendered, because
        // by the time we get to the lower priority, we've already switched
        // to B.
        sourceA.value = 'OOPS! This mutation should be ignored';
      });
      expect(root).toMatchRenderedOutput('B: Update');
    });

    it('should always treat reading as potentially unsafe when getSnapshot changes between renders', async () => {
      const source = createSource({
        a: 'foo',
        b: 'bar',
      });
      const mutableSource = createMutableSource(source);

      const getSnapshotA = () => source.value.a;
      const getSnapshotB = () => source.value.b;

      function mutateA(newA) {
        source.value = {
          ...source.value,
          a: newA,
        };
      }

      function App({getSnapshotFirst, getSnapshotSecond}) {
        const first = useMutableSource(
          mutableSource,
          getSnapshotFirst,
          defaultSubscribe,
        );
        const second = useMutableSource(
          mutableSource,
          getSnapshotSecond,
          defaultSubscribe,
        );

        let result = `x: ${first}, y: ${second}`;

        if (getSnapshotFirst === getSnapshotSecond) {
          // When both getSnapshot functions are equal,
          // the two values must be consistent.
          if (first !== second) {
            result = 'Oops, tearing!';
          }
        }

        React.useEffect(() => {
          Scheduler.unstable_yieldValue(result);
        }, [result]);

        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(
          <App
            getSnapshotFirst={getSnapshotA}
            getSnapshotSecond={getSnapshotB}
          />,
        );
      });
      // x and y start out reading from different parts of the store.
      expect(Scheduler).toHaveYielded(['x: foo, y: bar']);

      await act(async () => {
        ReactNoop.discreteUpdates(() => {
          // At high priority, toggle y so that it reads from A instead of B.
          // Simultaneously, mutate A.
          mutateA('high-pri baz');
          root.render(
            <App
              getSnapshotFirst={getSnapshotA}
              getSnapshotSecond={getSnapshotA}
            />,
          );

          // If this update were processed before the next mutation,
          // it would be expected to yield "high-pri baz" and "high-pri baz".
        });

        // At lower priority, mutate A again.
        // This happens to match the initial value of B.
        mutateA('bar');

        // When this update is processed,
        // it is expected to yield "bar" and "bar".
      });

      // Check that we didn't commit any inconsistent states.
      // The actual sequence of work will be:
      // 1. React renders the high-pri update, sees a new getSnapshot, detects the source has been further mutated, and throws
      // 2. React re-renders with all pending updates, including the second mutation, and renders "bar" and "bar".
      expect(Scheduler).toHaveYielded(['x: bar, y: bar']);
    });

    it('getSnapshot changes and then source is mutated in between paint and passive effect phase', async () => {
      const source = createSource({
        a: 'foo',
        b: 'bar',
      });
      const mutableSource = createMutableSource(source);

      function mutateB(newB) {
        source.value = {
          ...source.value,
          b: newB,
        };
      }

      const getSnapshotA = () => source.value.a;
      const getSnapshotB = () => source.value.b;

      function App({getSnapshot}) {
        const value = useMutableSource(
          mutableSource,
          getSnapshot,
          defaultSubscribe,
        );

        Scheduler.unstable_yieldValue('Render: ' + value);
        React.useEffect(() => {
          Scheduler.unstable_yieldValue('Commit: ' + value);
        }, [value]);

        return value;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(<App getSnapshot={getSnapshotA} />);
      });
      expect(Scheduler).toHaveYielded(['Render: foo', 'Commit: foo']);

      await act(async () => {
        // Switch getSnapshot to read from B instead
        root.render(<App getSnapshot={getSnapshotB} />);
        // Render and finish the tree, but yield right after paint, before
        // the passive effects have fired.
        expect(Scheduler).toFlushUntilNextPaint(['Render: bar']);
        // Then mutate B.
        mutateB('baz');
      });
      expect(Scheduler).toHaveYielded([
        // Fires the effect from the previous render
        'Commit: bar',
        // During that effect, it should detect that the snapshot has changed
        // and re-render.
        'Render: baz',
        'Commit: baz',
      ]);
      expect(root).toMatchRenderedOutput('baz');
    });

    it('getSnapshot changes and then source is mutated in between paint and passive effect phase, case 2', async () => {
      const source = createSource({
        a: 'foo',
        b: 'bar',
      });
      const mutableSource = createMutableSource(source);

      const getSnapshotA = () => source.value.a;
      const getSnapshotB = () => source.value.b;

      function mutateA(newA) {
        source.value = {
          ...source.value,
          a: newA,
        };
      }

      function App({getSnapshotFirst, getSnapshotSecond}) {
        const first = useMutableSource(
          mutableSource,
          getSnapshotFirst,
          defaultSubscribe,
        );
        const second = useMutableSource(
          mutableSource,
          getSnapshotSecond,
          defaultSubscribe,
        );

        let result = `x: ${first}, y: ${second}`;

        if (getSnapshotFirst === getSnapshotSecond) {
          // When both getSnapshot functions are equal,
          // the two values must be consistent.
          if (first !== second) {
            result = 'Oops, tearing!';
          }
        }

        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(
          <App
            getSnapshotFirst={getSnapshotA}
            getSnapshotSecond={getSnapshotB}
          />,
        );
      });

      await act(async () => {
        // Switch the second getSnapshot to also read from A
        root.render(
          <App
            getSnapshotFirst={getSnapshotA}
            getSnapshotSecond={getSnapshotA}
          />,
        );
        // Render and finish the tree, but yield right after paint, before
        // the passive effects have fired.
        expect(Scheduler).toFlushUntilNextPaint([]);
        // Now mutate A. Both hooks should update.
        // This is at high priority so that it doesn't get batched with default
        // priority updates that might fire during the passive effect
        ReactNoop.discreteUpdates(() => {
          mutateA('baz');
        });
        expect(Scheduler).toFlushUntilNextPaint([]);
        expect(root.getChildrenAsJSX()).not.toEqual('Oops, tearing!');
      });
    });

    it('getSnapshot changes and then source is mutated during interleaved event', async () => {
      const {useEffect} = React;

      const source = createComplexSource('1', '2');
      const mutableSource = createMutableSource(source);

      // Subscribe to part of the store.
      const getSnapshotA = s => s.valueA;
      const subscribeA = (s, callback) => s.subscribeA(callback);
      const configA = [getSnapshotA, subscribeA];

      const getSnapshotB = s => s.valueB;
      const subscribeB = (s, callback) => s.subscribeB(callback);
      const configB = [getSnapshotB, subscribeB];

      function App({parentConfig, childConfig}) {
        const [getSnapshot, subscribe] = parentConfig;
        const parentValue = useMutableSource(
          mutableSource,
          getSnapshot,
          subscribe,
        );

        Scheduler.unstable_yieldValue('Parent: ' + parentValue);

        return (
          <Child
            parentConfig={parentConfig}
            childConfig={childConfig}
            parentValue={parentValue}
          />
        );
      }

      function Child({parentConfig, childConfig, parentValue}) {
        const [getSnapshot, subscribe] = childConfig;
        const childValue = useMutableSource(
          mutableSource,
          getSnapshot,
          subscribe,
        );

        Scheduler.unstable_yieldValue('Child: ' + childValue);

        let result = `${parentValue}, ${childValue}`;

        if (parentConfig === childConfig) {
          // When both components read using the same config, the two values
          // must be consistent.
          if (parentValue !== childValue) {
            result = 'Oops, tearing!';
          }
        }

        useEffect(() => {
          Scheduler.unstable_yieldValue('Commit: ' + result);
        }, [result]);

        return result;
      }

      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(<App parentConfig={configA} childConfig={configB} />);
      });
      expect(Scheduler).toHaveYielded([
        'Parent: 1',
        'Child: 2',
        'Commit: 1, 2',
      ]);

      await act(async () => {
        // Switch the parent and the child to read using the same config
        root.render(<App parentConfig={configB} childConfig={configB} />);
        // Start rendering the parent, but yield before rendering the child
        expect(Scheduler).toFlushAndYieldThrough(['Parent: 2']);

        // Mutate the config. This is at lower priority so that 1) to make sure
        // it doesn't happen to get batched with the in-progress render, and 2)
        // so it doesn't interrupt the in-progress render.
        Scheduler.unstable_runWithPriority(
          Scheduler.unstable_IdlePriority,
          () => {
            source.valueB = '3';
          },
        );
      });
      expect(Scheduler).toHaveYielded([
        'TODO: This currently tears. Fill in with correct values once bug',

        // Here's the current behavior:

        // The partial render completes
        'Child: 2',
        'Commit: 2, 2',

        // Then we start rendering the low priority mutation
        'Parent: 3',
        // But the child never received a mutation event, because it hadn't
        // mounted yet. So the render tears.
        'Child: 2',
        'Commit: Oops, tearing!',

        // Eventually the child corrects itself, because of the check that
        // occurs when re-subscribing.
        'Child: 3',
        'Commit: 3, 3',
      ]);
    });

    if (__DEV__) {
      describe('dev warnings', () => {
        it('should warn if the subscribe function does not return an unsubscribe function', () => {
          const source = createSource('one');
          const mutableSource = createMutableSource(source);

          const brokenSubscribe = () => {};

          expect(() => {
            act(() => {
              ReactNoop.render(
                <Component
                  label="only"
                  getSnapshot={defaultGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={brokenSubscribe}
                />,
              );
            });
          }).toErrorDev(
            'Mutable source subscribe function must return an unsubscribe function.',
          );
        });

        it('should error if multiple renderers of the same type use a mutable source at the same time', () => {
          const source = createSource('one');
          const mutableSource = createMutableSource(source);

          act(() => {
            // Start a render that uses the mutable source.
            ReactNoop.render(
              <>
                <Component
                  label="a"
                  getSnapshot={defaultGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
                <Component
                  label="b"
                  getSnapshot={defaultGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
              </>,
            );
            expect(Scheduler).toFlushAndYieldThrough(['a:one']);

            const PrevScheduler = Scheduler;

            // Get a new copy of ReactNoop.
            loadModules();

            spyOnDev(console, 'error');

            // Use the mutablesource again but with a different renderer.
            ReactNoop.render(
              <Component
                label="c"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />,
            );
            expect(Scheduler).toFlushAndYieldThrough(['c:one']);

            expect(console.error.calls.argsFor(0)[0]).toContain(
              'Detected multiple renderers concurrently rendering the ' +
                'same mutable source. This is currently unsupported.',
            );

            // TODO (useMutableSource) Act will automatically flush remaining work from render 1,
            // but at this point something in the hooks dispatcher has been broken by jest.resetModules()
            // Figure out what this is and remove this catch.
            expect(() =>
              PrevScheduler.unstable_flushAllWithoutAsserting(),
            ).toThrow('Invalid hook call');
          });
        });

        it('should error if multiple renderers of the same type use a mutable source at the same time with mutation between', () => {
          const source = createSource('one');
          const mutableSource = createMutableSource(source);

          act(() => {
            // Start a render that uses the mutable source.
            ReactNoop.render(
              <>
                <Component
                  label="a"
                  getSnapshot={defaultGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
                <Component
                  label="b"
                  getSnapshot={defaultGetSnapshot}
                  mutableSource={mutableSource}
                  subscribe={defaultSubscribe}
                />
              </>,
            );
            expect(Scheduler).toFlushAndYieldThrough(['a:one']);

            const PrevScheduler = Scheduler;

            // Get a new copy of ReactNoop.
            loadModules();

            spyOnDev(console, 'error');

            // Mutate before the new render reads from the source.
            source.value = 'two';

            // Use the mutablesource again but with a different renderer.
            ReactNoop.render(
              <Component
                label="c"
                getSnapshot={defaultGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />,
            );
            expect(Scheduler).toFlushAndYieldThrough(['c:two']);

            expect(console.error.calls.argsFor(0)[0]).toContain(
              'Detected multiple renderers concurrently rendering the ' +
                'same mutable source. This is currently unsupported.',
            );

            // TODO (useMutableSource) Act will automatically flush remaining work from render 1,
            // but at this point something in the hooks dispatcher has been broken by jest.resetModules()
            // Figure out what this is and remove this catch.
            expect(() =>
              PrevScheduler.unstable_flushAllWithoutAsserting(),
            ).toThrow('Invalid hook call');
          });
        });
      });
    }
  }
});