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

Core utilities for building navigators

import * as React from 'react';
/**
 * Utilites such as `getFocusedRouteNameFromRoute` need to access state.
 * So we need a way to suppress the warning for those use cases.
 * This is fine since they are internal utilities and this is not public API.
 */
export const CHILD_STATE = Symbol('CHILD_STATE');

/**
 * Hook to cache route props for each screen in the navigator.
 * This lets add warnings and modifications to the route object but keep references between renders.
 */
export default function useRouteCache(routes) {
  // Cache object which holds route objects for each screen
  const cache = React.useMemo(() => ({
    current: new Map()
  }), []);
  if (process.env.NODE_ENV === 'production') {
    // We don't want the overhead of creating extra maps every render in prod
    return routes;
  }
  cache.current = routes.reduce((acc, route) => {
    const previous = cache.current.get(route);
    if (previous) {
      // If a cached route object already exists, reuse it
      acc.set(route, previous);
    } else {
      const {
        state,
        ...proxy
      } = route;
      Object.defineProperty(proxy, CHILD_STATE, {
        enumerable: false,
        value: state
      });
      acc.set(route, proxy);
    }
    return acc;
  }, new Map());
  return Array.from(cache.current.values());
}
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