react-navigation/packages/core/src/useFocusEffect.tsx

import * as React from 'react';
import useNavigation from './useNavigation';

type EffectCallback = () => undefined | void | (() => void);

/**
 * Hook to run an effect in a focused screen, similar to `React.useEffect`.
 * This can be used to perform side-effects such as fetching data or subscribing to events.
 * The passed callback should be wrapped in `React.useCallback` to avoid running the effect too often.
 *
 * @param callback Memoized callback containing the effect, should optionally return a cleanup function.
 */
export default function useFocusEffect(effect: EffectCallback) {
  const navigation = useNavigation();

  React.useEffect(() => {
    let isFocused = false;
    let cleanup: undefined | void | (() => void);

    const callback = () => {
      const destroy = effect();

      if (destroy === undefined || typeof destroy === 'function') {
        return destroy;
      }

      if (process.env.NODE_ENV !== 'production') {
        let message =
          'An effect function must not return anything besides a function, which is used for clean-up.';

        if (destroy === null) {
          message +=
            " You returned 'null'. If your effect does not require clean-up, return 'undefined' (or nothing).";
        } else if (typeof (destroy as any).then === 'function') {
          message +=
            "\n\nIt looks like you wrote 'useFocusEffect(async () => ...)' or returned a Promise. " +
            'Instead, write the async function inside your effect ' +
            'and call it immediately:\n\n' +
            'useFocusEffect(\n' +
            '  React.useCallback() => {\n' +
            '    async function fetchData() {\n' +
            '      // You can await here\n' +
            '      const response = await MyAPI.getData(someId);\n' +
            '      // ...\n' +
            '    }\n\n' +
            '    fetchData();\n' +
            '  }, [someId])\n' +
            '};\n\n' +
            'See usage guide: https://reactnavigation.org/docs/use-focus-effect';
        } else {
          message += ` You returned: '${JSON.stringify(destroy)}'`;
        }

        console.error(message);
      }
    };

    // We need to run the effect on intial render/dep changes if the screen is focused
    if (navigation.isFocused()) {
      cleanup = callback();
      isFocused = true;
    }

    const unsubscribeFocus = navigation.addListener('focus', () => {
      // If callback was already called for focus, avoid calling it again
      // The focus event may also fire on intial render, so we guard against runing the effect twice
      if (isFocused) {
        return;
      }

      if (cleanup !== undefined) {
        cleanup();
      }

      cleanup = callback();
      isFocused = true;
    });

    const unsubscribeBlur = navigation.addListener('blur', () => {
      if (cleanup !== undefined) {
        cleanup();
      }

      cleanup = undefined;
      isFocused = false;
    });

    return () => {
      if (cleanup !== undefined) {
        cleanup();
      }

      unsubscribeFocus();
      unsubscribeBlur();
    };
  }, [effect, navigation]);
}