`useHasValuesListener`

The useHasValuesListener hook registers a listener function with the Store that will be called when Values as a whole are added to or removed from the Store.

useHasValuesListener(
  listener: HasValuesListener,
  listenerDeps?: DependencyList,
  mutator?: boolean,
  storeOrStoreId?: any,
): void

	Type	Description
`listener`	`HasValuesListener`	The function that will be called whenever `Values` as a whole are 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 useHasValues hook).

Unlike the addHasValuesListener method, which returns a listener Id and requires you to remove it manually, the useHasValuesListener 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 useHasValuesListener 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 = () => {
  useHasValuesListener(() => console.log('Values 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().hasValues);
// -> 1

store.setValue('open', true);
// -> 'Values existence changed'

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

Since

v4.4.0