@react-navigation/core/lib/module/useNavigationState.js

Core utilities for building navigators

import * as React from 'react';
import useNavigation from './useNavigation';
/**
 * Hook to get a value from the current navigation state using a selector.
 *
 * @param selector Selector function to get a value from the state.
 */
export default function useNavigationState(selector) {
  const navigation = useNavigation();

  // We don't care about the state value, we run the selector again at the end
  // The state is only to make sure that there's a re-render when we have a new value
  const [, setResult] = React.useState(() => selector(navigation.getState()));

  // We store the selector in a ref to avoid re-subscribing listeners every render
  const selectorRef = React.useRef(selector);
  React.useEffect(() => {
    selectorRef.current = selector;
  });
  React.useEffect(() => {
    const unsubscribe = navigation.addListener('state', e => {
      setResult(selectorRef.current(e.data.state));
    });
    return unsubscribe;
  }, [navigation]);
  return selector(navigation.getState());
}
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