`useHasValueListener`

The useHasValueListener hook registers a listener function with the Store that will be called when a Value is added to or removed from the Store.

useHasValueListener(
  valueId: IdOrNull,
  listener: HasValueListener,
  listenerDeps?: DependencyList,
  mutator?: boolean,
  storeOrStoreId?: any,
): void

	Type	Description
`valueId`	`IdOrNull`	The `Id` of the `Value` to listen to, or `null` as a wildcard.
`listener`	`HasValueListener`	The function that will be called whenever the matching `Value` is added or removed.
`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.
`mutator?`	`boolean`	An optional boolean that indicates that the listener mutates `Store` data.
`storeOrStoreId?`	`any`	The `Store` to register the listener with: omit for the default context `Store`, provide an `Id` for a named context `Store`, 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 useHasValue hook).

You can either listen to a single Value being added or removed (by specifying the Value Id) or any Value being added or removed (by providing a null wildcard).

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

Example

This example uses the useHasValueListener 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 = ({store}) => (
  <Provider store={store}>
    <Pane />
  </Provider>
);
const Pane = () => {
  useHasValueListener('open', () =>
    console.log('Value existence changed'),
  );
  return <span>App</span>;
};

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

store.setValue('open', false);
// -> 'Value existence changed'

root.unmount();
console.log(store.getListenerStats().hasValue);
// -> 0

Since

v4.4.0