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

Core utilities for building navigators

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

/**
 * Hook to take care of emitting `focus` and `blur` events.
 */
export default function useFocusEvents({
  state,
  emitter
}) {
  const navigation = React.useContext(NavigationContext);
  const lastFocusedKeyRef = React.useRef();
  const currentFocusedKey = state.routes[state.index].key; // When the parent screen changes its focus state, we also need to change child's focus
  // Coz the child screen can't be focused if the parent screen is out of focus

  React.useEffect(() => navigation === null || navigation === void 0 ? void 0 : navigation.addListener('focus', () => {
    lastFocusedKeyRef.current = currentFocusedKey;
    emitter.emit({
      type: 'focus',
      target: currentFocusedKey
    });
  }), [currentFocusedKey, emitter, navigation]);
  React.useEffect(() => navigation === null || navigation === void 0 ? void 0 : navigation.addListener('blur', () => {
    lastFocusedKeyRef.current = undefined;
    emitter.emit({
      type: 'blur',
      target: currentFocusedKey
    });
  }), [currentFocusedKey, emitter, navigation]);
  React.useEffect(() => {
    const lastFocusedKey = lastFocusedKeyRef.current;
    lastFocusedKeyRef.current = currentFocusedKey; // We wouldn't have `lastFocusedKey` on initial mount
    // Fire focus event for the current route on mount if there's no parent navigator

    if (lastFocusedKey === undefined && !navigation) {
      emitter.emit({
        type: 'focus',
        target: currentFocusedKey
      });
    } // We should only emit events when the focused key changed and navigator is focused
    // When navigator is not focused, screens inside shouldn't receive focused status either


    if (lastFocusedKey === currentFocusedKey || !(navigation ? navigation.isFocused() : true)) {
      return;
    }

    if (lastFocusedKey === undefined) {
      // Only fire events after initial mount
      return;
    }

    emitter.emit({
      type: 'blur',
      target: lastFocusedKey
    });
    emitter.emit({
      type: 'focus',
      target: currentFocusedKey
    });
  }, [currentFocusedKey, emitter, navigation]);
}
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