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

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

/**
 * 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) {
  const navigation = useNavigation();

  if (arguments[1] !== undefined) {
    const message = "You passed a second argument to 'useFocusEffect', but it only accepts one argument. " + "If you want to pass a dependency array, you can use 'React.useCallback':\n\n" + 'useFocusEffect(\n' + '  React.useCallback(() => {\n' + '    // Your code here\n' + '  }, [depA, depB])\n' + ');\n\n' + 'See usage guide: https://reactnavigation.org/docs/use-focus-effect';
    console.error(message);
  }

  React.useEffect(() => {
    let isFocused = false;
    let cleanup;

    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.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]);
}
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