`useCheckpointListener`

The useCheckpointListener hook registers a listener function with the Checkpoints object that will be called whenever the label of a checkpoint changes.

useCheckpointListener(
  checkpointId: IdOrNull,
  listener: CheckpointListener,
  listenerDeps?: DependencyList,
  checkpointsOrCheckpointsId?: any,
): void

	Type	Description
`checkpointId`	`IdOrNull`	The `Id` of the checkpoint to listen to, or `null` as a wildcard.
`listener`	`CheckpointListener`	The function that will be called whenever the checkpoint label changes.
`listenerDeps?`	`DependencyList`	An optional array of dependencies for the `listener` function, which, if any change, result in the re-registration of the listener. This parameter defaults to an empty array.
`checkpointsOrCheckpointsId?`	`any`	The `Checkpoints` object to register the listener with: omit for the default context `Checkpoints` object, provide an `Id` for a named context `Checkpoints` object, or provide an explicit reference.
returns	`void`	This has no return value.

This hook is useful for situations where a component needs to register its own specific listener to do more than simply tracking the value (which is more easily done with the useCheckpoint hook).

You can either listen to a single checkpoint label (by specifying the checkpoint Id as the method's first parameter), or changes to any checkpoint label (by providing a null wildcard).

Unlike the addCheckpointListener method, which returns a listener Id and requires you to remove it manually, the useCheckpointListener hook manages this lifecycle for you: when the listener changes (per its listenerDeps dependencies) or the component unmounts, the listener on the underlying Checkpoints object will be deleted.

Example

This example uses the useCheckpointListener hook to create a listener that is scoped to a single component. When the component is unmounted, the listener is removed from the Store.

const App = ({checkpoints}) => (
  <Provider checkpoints={checkpoints}>
    <Pane />
  </Provider>
);
const Pane = () => {
  useCheckpointListener('0', () =>
    console.log('Checkpoint label changed'),
  );
  return <span>App</span>;
};

const store = createStore().setTables({pets: {fido: {sold: false}}});
const checkpoints = createCheckpoints(store);

const app = document.createElement('div');
const root = ReactDOMClient.createRoot(app);
root.render(<App checkpoints={checkpoints} />);
console.log(checkpoints.getListenerStats().checkpoint);
// -> 1

checkpoints.setCheckpoint('0', 'initial');
// -> 'Checkpoint label changed'

root.unmount();
console.log(checkpoints.getListenerStats().checkpoint);
// -> 0