Give transformers access to the full, combined reducer state

[dougkeen]

For example, this is useful if you have a transformer that needs to filter items in one reducer based on values in another reducer.

Let's say we're building a chat app, where we'd like the 100 most recent messages and the sender info shown right when the app starts up (without making a trip to the server). We have a users reducer, which is a bunch of user data objects keyed by userId. We also have a messages reducer, which is an ordered list of message data objects, with a senderId property that refers to the userId of the sender. We'd like to build a transformer that persists only the top 100 messages in the messages reducer, along with all the users in the user reducer that correspond to the senders of those 100 messages.

Right now, this isn't possible. When persisting users in the users reducer, the transformer isn't able to access the messages reducer to figure out which users are senders of the top 100 messages.

With this PR, that transformer can be written as:

import _ from "lodash";

const top100MessagesTransform = createTransform(
    (subState, key, fullState) => {
      if (key === "messages") {
        return _.take(subState, 100);
      } else if (key === "users") {
        const messages = _.take(fullState.messages, 100);
        const senderIdSet = _.keyBy(messages, "senderId");
        return _.pickBy(subState, (user, userId) => _.has(senderIdSet, userId));
      } else {
        return subState;
      }
    },
    (state) => state,
    {whitelist: ["messages", "users"]},
);

[jpshelley]

I have an issue currently with full state marked here - #871