`useMetricListener`

The useMetricListener hook registers a listener function with the Metrics object that will be called whenever the value of a specified Metric changes.

useMetricListener(
  metricId: IdOrNull,
  listener: MetricListener,
  listenerDeps?: DependencyList,
  metricsOrMetricsId?: MetricsOrMetricsId,
): void

	Type	Description
`metricId`	`IdOrNull`	The `Id` of the `Metric` to listen to, or `null` as a wildcard.
`listener`	`MetricListener`	The function that will be called whenever the `Metric` 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.
`metricsOrMetricsId?`	`MetricsOrMetricsId`	The `Metrics` object to register the listener with: omit for the default context `Metrics` object, provide an `Id` for a named context `Metrics` 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 useMetric hook).

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

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

Example

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

const App = ({metrics}) => (
  <Provider metrics={metrics}>
    <Pane />
  </Provider>
);
const Pane = () => {
  useMetricListener('highestPrice', () => console.log('Metric changed'));
  return <span>App</span>;
};

const store = createStore().setTable('species', {
  dog: {price: 5},
  cat: {price: 4},
  worm: {price: 1},
});
const metrics = createMetrics(store);
metrics.setMetricDefinition('highestPrice', 'species', 'max', 'price');

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

store.setCell('species', 'horse', 'price', 20);
// -> 'Metric changed'

root.unmount();
console.log(metrics.getListenerStats().metric);
// -> 0