Don't run stale listeners (#22)

Per the [Redux docs](https://redux.js.org/api/store#subscribe):

> The subscriptions are snapshotted just before every dispatch() call. If you subscribe or unsubscribe while the listeners are being invoked, this will not have any effect on the dispatch() that is currently in progress.

Since a component can be unmounted during dispatch, and Redux won't remove the listener until afterward, we need to manually guard against it being called after unsubscribing.

src/__tests__/index-test.tsx

@@ -170,6 +170,32 @@ describe('redux-react-hook', () => {
 
     expect(getText()).toBe('foo 45');
   });
+
+  it("doesn't try to update after unmounting during dispatch", () => {
+    let mapStateCalls = 0;
+    const Component = () => {
+      const mapState = React.useCallback((s: IState) => {
+        mapStateCalls++;
+        return s.foo;
+      }, []);
+      const foo = useMappedState(mapState);
+      return <div>{foo}</div>;
+    };
+
+    render(<Component />);
+
+    flushEffects();
+
+    ReactDOM.unmountComponentAtNode(reactRoot);
+
+    const consoleErrorSpy = jest.spyOn(console, 'error');
+    state = {...state, foo: 'foo'};
+    subscriberCallback();
+
+    // mapState is called during and after the first render
+    expect(mapStateCalls).toBe(2);
+    expect(consoleErrorSpy).not.toHaveBeenCalled();
+  });
 });
 
 // https://github.com/kentcdodds/react-testing-library/commit/11a41ce3ad9e9695f4b1662a5c67b890fc304894

src/index.ts

@@ -55,24 +55,36 @@ export function useMappedState<TState, TResult>(
 
   useEffect(
     () => {
+      let didUnsubscribe = false;
+
       // Run the mapState callback and if the result has changed, make the
       // component re-render with the new state.
       const checkForUpdates = () => {
+        if (didUnsubscribe) {
+          // Don't run stale listeners.
+          // Redux doesn't guarantee unsubscriptions happen until next dispatch.
+          return;
+        }
+
         const newDerivedState = runMapState();
         if (!shallowEqual(newDerivedState, lastRenderedDerivedState.current)) {
           setDerivedState(newDerivedState);
         }
       };
 
-      // Pull data from the store on first render.
+      // Pull data from the store after first render in case the store has
+      // changed since we began.
       checkForUpdates();
 
       // Subscribe to the store to be notified of subsequent changes.
       const unsubscribe = store.subscribe(checkForUpdates);
 
       // The return value of useEffect will be called when unmounting, so
       // we use it to unsubscribe from the store.
-      return unsubscribe;
+      return () => {
+        didUnsubscribe = true;
+        unsubscribe();
+      };
     },
     [store, mapState],
   );