HomeWork/react2/node_modules/react-redux/es/utils/Subscription.js

import { getBatch } from './batch'; // encapsulates the subscription logic for connecting a component to the redux store, as
// well as nesting subscriptions of descendant components, so that we can ensure the
// ancestor components re-render before descendants

function createListenerCollection() {
  var batch = getBatch();
  var first = null;
  var last = null;
  return {
    clear: function clear() {
      first = null;
      last = null;
    },
    notify: function notify() {
      batch(function () {
        var listener = first;

        while (listener) {
          listener.callback();
          listener = listener.next;
        }
      });
    },
    get: function get() {
      var listeners = [];
      var listener = first;

      while (listener) {
        listeners.push(listener);
        listener = listener.next;
      }

      return listeners;
    },
    subscribe: function subscribe(callback) {
      var isSubscribed = true;
      var listener = last = {
        callback: callback,
        next: null,
        prev: last
      };

      if (listener.prev) {
        listener.prev.next = listener;
      } else {
        first = listener;
      }

      return function unsubscribe() {
        if (!isSubscribed || first === null) return;
        isSubscribed = false;

        if (listener.next) {
          listener.next.prev = listener.prev;
        } else {
          last = listener.prev;
        }

        if (listener.prev) {
          listener.prev.next = listener.next;
        } else {
          first = listener.next;
        }
      };
    }
  };
}

var nullListeners = {
  notify: function notify() {},
  get: function get() {
    return [];
  }
};
export function createSubscription(store, parentSub) {
  var unsubscribe;
  var listeners = nullListeners;

  function addNestedSub(listener) {
    trySubscribe();
    return listeners.subscribe(listener);
  }

  function notifyNestedSubs() {
    listeners.notify();
  }

  function handleChangeWrapper() {
    if (subscription.onStateChange) {
      subscription.onStateChange();
    }
  }

  function isSubscribed() {
    return Boolean(unsubscribe);
  }

  function trySubscribe() {
    if (!unsubscribe) {
      unsubscribe = parentSub ? parentSub.addNestedSub(handleChangeWrapper) : store.subscribe(handleChangeWrapper);
      listeners = createListenerCollection();
    }
  }

  function tryUnsubscribe() {
    if (unsubscribe) {
      unsubscribe();
      unsubscribe = undefined;
      listeners.clear();
      listeners = nullListeners;
    }
  }

  var subscription = {
    addNestedSub: addNestedSub,
    notifyNestedSubs: notifyNestedSubs,
    handleChangeWrapper: handleChangeWrapper,
    isSubscribed: isSubscribed,
    trySubscribe: trySubscribe,
    tryUnsubscribe: tryUnsubscribe,
    getListeners: function getListeners() {
      return listeners;
    }
  };
  return subscription;
}