react/packages/use-subscription/src/useSubscription.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

import {useDebugValue, useEffect, useState} from 'react';

// Hook used for safely managing subscriptions in concurrent mode.
//
// In order to avoid removing and re-adding subscriptions each time this hook is called,
// the parameters passed to this hook should be memoized in some way–
// either by wrapping the entire params object with useMemo()
// or by wrapping the individual callbacks with useCallback().
export function useSubscription<Value>({
  // (Synchronously) returns the current value of our subscription.
  getCurrentValue,

  // This function is passed an event handler to attach to the subscription.
  // It should return an unsubscribe function that removes the handler.
  subscribe,
}: {|
  getCurrentValue: () => Value,
  subscribe: (callback: Function) => () => void,
|}): Value {
  // Read the current value from our subscription.
  // When this value changes, we'll schedule an update with React.
  // It's important to also store the hook params so that we can check for staleness.
  // (See the comment in checkForUpdates() below for more info.)
  const [state, setState] = useState(() => ({
    getCurrentValue,
    subscribe,
    value: getCurrentValue(),
  }));

  let valueToReturn = state.value;

  // If parameters have changed since our last render, schedule an update with its current value.
  if (
    state.getCurrentValue !== getCurrentValue ||
    state.subscribe !== subscribe
  ) {
    // If the subscription has been updated, we'll schedule another update with React.
    // React will process this update immediately, so the old subscription value won't be committed.
    // It is still nice to avoid returning a mismatched value though, so let's override the return value.
    valueToReturn = getCurrentValue();

    setState({
      getCurrentValue,
      subscribe,
      value: valueToReturn,
    });
  }

  // Display the current value for this hook in React DevTools.
  useDebugValue(valueToReturn);

  // It is important not to subscribe while rendering because this can lead to memory leaks.
  // (Learn more at reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects)
  // Instead, we wait until the commit phase to attach our handler.
  //
  // We intentionally use a passive effect (useEffect) rather than a synchronous one (useLayoutEffect)
  // so that we don't stretch the commit phase.
  // This also has an added benefit when multiple components are subscribed to the same source:
  // It allows each of the event handlers to safely schedule work without potentially removing an another handler.
  // (Learn more at https://codesandbox.io/s/k0yvr5970o)
  useEffect(() => {
    let didUnsubscribe = false;

    const checkForUpdates = () => {
      // It's possible that this callback will be invoked even after being unsubscribed,
      // if it's removed as a result of a subscription event/update.
      // In this case, React will log a DEV warning about an update from an unmounted component.
      // We can avoid triggering that warning with this check.
      if (didUnsubscribe) {
        return;
      }

      // We use a state updater function to avoid scheduling work for a stale source.
      // However it's important to eagerly read the currently value,
      // so that all scheduled work shares the same value (in the event of multiple subscriptions).
      // This avoids visual "tearing" when a mutation happens during a (concurrent) render.
      const value = getCurrentValue();

      setState(prevState => {
        // Ignore values from stale sources!
        // Since we subscribe an unsubscribe in a passive effect,
        // it's possible that this callback will be invoked for a stale (previous) subscription.
        // This check avoids scheduling an update for that stale subscription.
        if (
          prevState.getCurrentValue !== getCurrentValue ||
          prevState.subscribe !== subscribe
        ) {
          return prevState;
        }

        // Some subscriptions will auto-invoke the handler, even if the value hasn't changed.
        // If the value hasn't changed, no update is needed.
        // Return state as-is so React can bail out and avoid an unnecessary render.
        if (prevState.value === value) {
          return prevState;
        }

        return {...prevState, value};
      });
    };
    const unsubscribe = subscribe(checkForUpdates);

    // Because we're subscribing in a passive effect,
    // it's possible that an update has occurred between render and our effect handler.
    // Check for this and schedule an update if work has occurred.
    checkForUpdates();

    return () => {
      didUnsubscribe = true;
      unsubscribe();
    };
  }, [getCurrentValue, subscribe]);

  // Return the current value for our caller to use while rendering.
  return valueToReturn;
}