higher order component to plug xstream as a store with React components

yarn add react react-dom prop-types xstream @seracio/xstream-connect

The purpose here is not to provide an async middleware to a redux store with Streams, as redux-cycle-middleware and redux-observable do but to replace redux and its different slices (async middlewares, reducers and derived data) with MemoryStreams. As this, we can express each variable of the store as a function of other variables, in a clean and async way.

xstream's Streams are the perfect tool to achieve this goal, as they are hot and can easily be transformed into MemoryStreams (via the remember method). xstream is also light and fast, with a simple and comprehensive API compared to other FRP libs.

This library only exposes a component Provider and an higher order function connect to connect your store to the React layer in a react-redux fashion.

import React from 'react';
import ReactDOM from 'react-dom';
import xs from 'xstream';
import {connect, Provider} from '@seracio/xstream-connect';

// A store is just a dictionary of exposed Streams
const store = {
  count$: xs.periodic(1000).startWith(0)
};

const App = ({count}) => { 
  return <div>{this.props.count}</div>;
};

// the combinator defines which part of your store 
// will be exposed and realised the mapping from Streams to props
const combinator = state => {
  const {count$, hello$} = state;
  return xs.combine(count$).map(count => ({count}));
};

// We use a Higher order function connect to wrap our component and plug its props to the store values
const ConnectedApp = connect(combinator)(App);

ReactDOM.render(
  <Provider store={store}>
    <ConnectedApp />
  </Provider>,
  document.querySelector('#root')
);

Within this architecture, all the logic resides in Streams.
The store props of the Provide component is just a hash/dictionary of Streams (or static values) you want to expose to the React layer.

// main.js
import React from 'react';
import ReactDOM from 'react-dom';
import {Provider} from '@seracio/xstream-connect';
import * as store from './store'; // <---- your store is just a dictionnary of exposed Streams

ReactDOM.render(
  <Provider store={store}> {/* <-- plugged your store to the React layer */}
    <Connected />
  </Provider>,
  document.querySelector('#root')
);

The connect function takes a single function as param. This function, called the combinator, expressed two things:

• what parts of the store our component will subscribe to
• and when will it receive props updates

To put it another way, the combinator receives the Provider's store as param and will return a unique Stream that combine the parts of the store we want our component to be aware of. The value of this Stream will be a plain object {key => value} (as React's components are expected props).

For instance:

const combinator = state => {
  const {list$, selected$} = state;
  return xs
    .combine(list$, selected$)
    .map([list, selected] => ({list, selected}));
}

or if you only want props to update when selected$ changes, you can use a sampleCombine

const combinator = state => {
  const {list$, selected$} = state;
  return selected$
    .compose(sampleCombine(list$))
    .map([selected, list] => ({selected, list}));
}

or if you only want props to update when selected$'s id changes

const combinator = state => {
  const {list$, selected$} = state;
  return selected$
    .compose(dropRepeats(isIdEqual))
    .compose(sampleCombine(list$))
    .map([selected, list] => ({selected, list}));
}

Be aware that the HOC will wait for the first value of the combinator's Stream before rendering its component. Its render method will return null before or a default component if defined.

You can specify a Waiting component as this:

connect(combinator)(MyComponent, WaitingComponent);

The waiting component will receive all props of MyComponent that are not provided by the combinator.

Of course, you can also compose the combinator's Stream with a startWith operator:

const combinatr = state => {
  const {myStream$} = state;
  return myStream$
    .startWith(null)
    .map(data => ({data}));
}

There is no canonical way to achieve this. For instance, you can do as this:

• in your store, expose a dispatcher$ Stream of Stream, as this the React layer will have a Stream to send actions through.
• in your store, create an actions$ Stream that flatten dispatcher$

// store index.js
import _ from 'lodash/fp';
import xs from 'xstream';

// Our dispatcher is exposed 
export const dispatcher$ = xs.of(xs.create());
// actions are not
const actions$ = dispatcher$.flatten();

// little helper
// to check if an action is of a certain type
const isType = type => _.flow(_.get('type'), _.isEqual(type));

// an exposed Stream that depends on an action
export const counter$ = actions$
  .filter(isType('increment'))
  .fold(acc => acc   1, 0)
  .startWith(0)
  .remember(); 
// note that startWith() already returns a MemoryStream, remember() is not needed here 

• expose your store into the React layer context

// main.js
import {Provider} from '@seracio/xstream-connect';
import * as store from './store'; // dispatcher$ is exposed 

// ...

• via the connect method to actionsProvider$, retrieve a Stream into your component's props:

// components/MyComponent.js
import xs from 'xstream';
import sampleCombine from 'xstream/extra/sampleCombine';
import {connect} from '@seracio/xstream-connect';

const MyComponent = ({counter, dispatcher}) => {
  // here, the dispatcher's value is a Stream, so we can, shamefully, send actions through it
  const increment = () => dispatcher.shamefullySendNext({type: 'increment'}); 
  return (
    <div onClick={increment}>{counter}</div>;
  );
};

// create a combinator function
const combinator = (state) => state.counter$
  .compose(sampleCombine(state.dispatcher$))
  .map(([counter, dispatcher]) => ({counter, dispatcher}));

// plug the component with the store
export default connect(combinator)(MyComponent);

fromPromise Streams can be tricky as they will complete as soon as their inner Promise is resolved:

import {getAsyncData} from '../services';

const data$ = xs.fromPromise(getAsyncData()).remember();

This is problematic with complex async flow, especially if you use sampleCombine or combine operators. I advise you to transform fromPromise Streams as this:

const data$ = xs
  .merge(
    xs.create(), 
    xs.fromPromise(getAsyncData())
  ).remember();

We can generalize this rule with all completable Streams.

cycle.js is neat, but we kinda like the React ecosystem... and are also tired of the growing complexity of the redux layer when you want to manage asynchronicity.

MIT License

Copyright (c) 2017 serac.io

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The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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