🏰 use-epic

Use RxJS Epics as state management for your React Components

What is an Epic❓

An Epic is a function which takes an Observable of actions (action$), an Observable of the current state (state$), and an object of dependencies (deps) and returns an Observable.

The idea of the Epic comes out of the fantastic redux middleware redux-observable, but a noteable difference is that, because redux-observable is redux middleware, the observable returned from the Epic emits new actions to be run through reducers to create new state, useEpic() skips the redux middleman and expects the Epic to return an observable of state updates.

function Epic(action$, state$, deps) {
  return newState$;
}

This simple idea opens up all the fantastic abilites of RxJS to your React components with a simple but powerful API.

🔎 Usage

function productEpic(action$, state$, deps) {
  const { productStore, cartObserver, props$ } = deps;
  combineLatest(action$.pipe(ofType('addToCart')), state$)
    .pipe(
      map(([productId, products]) => products.find(p => p.id === productId))
    )
    .subscribe(cartObserver);

  return props$.pipe(
    map(props => props.category),
    switchMap(category => productStore.productsByCategory(category)),
    startWith([])
  );
}

const ProductsComponent = props => {
  const [products, dispatch] = useEpic(productEpic, { props });

  // map dispatch to a component callback
  const addToCart = productId =>
    dispatch({ type: 'addToCart', payload: productId });

  return <ProductsList products={products} addToCart={addToCart} />;
};

⚒️ Installation

use-epic requires both react and rxjs as peer dependencies.

npm install use-epic rxjs react

yarn add use-epic rxjs react

🗃️ Examples

See examples locally with npm run examples

(source examples)

(source examples)

[Beer Search] *coming soon*
[Pull to Refresh] *coming soon*
[Working with simple-store] *coming soon*

📖 API

• useEpic()
• epic()
• ofType()
• <EpicDepsProvider>

useEpic()

A React hook for using RxJS Observables for state management.

const [state, dispatch] = useEpic( epic, options? );

The useEpic() hook, accepts an epic function, and an options object, and returns a tuple of state and a dispatch callback, similar to useReducer().

arguments

• epic an epic function, described below .

  function myEpic( action$, state$, deps ) { return newState$ }

  It should be noted, that only the first Epic function passed to useEpic() will be retained, so if you write your function inline like:

  const [state] = useEpic((action$, state$, deps) => {
    return action$.pipe(switchMap(action => fetchData(action.id)));
  });

  ...any variable closures used in the epic will not change, and component renders will generate a new Epic function that will merely be discared. For that reason we encourage defining Epics outside of the component.

• options ^*optional an object with some special properties:

  • deps - an object with keys, any key/values on this deps object will be available on the deps argument in the Epic function
  • props - a way to "pass" component props into the Epic, anything passed here will be emitted to the special, always available, deps.props$, in the Epic. This should be used with caution, as it limits portability, but is available for when dispatching an action is not appropriate.

const CatDetails = props => {
  const [cat] = useEpic(kittenEpic, { deps: { kittenService }, props: cat.id });
  <Details subject={cat} />;
};

epic()

An epic is a function, that accepts an Observable of actions (action$), an Observable of the current state (state$), and an object of dependencies (deps) and returns an Observable of stateUpdates$.

function myEpic( action$, state$, deps ) { return newState$ }

The epic will be called by useEpic(), passing the action$, state$ and deps arguments, and it may either return a new RxJS Observable or undefined. If an observable is returned, and values emitted from that observable are set as state, the first element of the tuple returned from useEpic().

const [state, dispatch] = useEpic(epic);

arguments passed when the epic is called

• action$ An observable of dispatched actions. The actions emitted are anything passed to the dispatch() callback returned from useEpic(). They can be anything, but by convention are often either objects with a type, payload and sometimes meta properties (e.g. { type: 'activate', payload: user }), or an array tuple with the type as the first element and the payload as the second (e.g. ['activate', user]).

• state$ An observable of the current state. It can be sometimes helpful to have a reference to the current state when composing streams, say if your action.payload is an id and you'd like to map that to a state entity before further processing it. Unless the observable returned from useEpic() has initial state, from using startWith() or a BehaviorSubject, this will emit undefined to start.
  ⚠️ Caution: When using state$ it is possible to find yourself in an inifinte asynchronous loop. Take care in how it is used along with the returned newState$ observable.

• deps an object of key/value pairs provided by the options of useEpic when it is called, or from the <EpicDepsProvider> component.

  The deps argument can be very useful for provding a dependency injection point into your Epics and therefore into your components. For example, if you provide an ajax dependecy in deps, you could provide the RxJS ajax function by default, but stub out ajax for tests or demo pages by wrapping your component in an <EpicDepsProvider> component.

    const kittyEpic = (action$, state$, { ajax: rxjs.ajax }) => {
      return action$.pipe(
        switchMap(({ payload: id })=> ajax(`/api/kittens/${id}`))
      );
    }
  
    const KittyComponent = () => {
      const [kitty, dispatch] = useEpic(kittyEpic);
  
      //... render and such
    }
  
    // mocking for tests
    test('should load kitty details when clicked', async () => {
      // stub out ajax for the test
      const fakeResponse = { name: 'Snuggles', breed: 'tabby' };
      const ajaxStub = jest.fn(() => Promise.resolve(fakeResponse));
  
      const { getByLabelText, getByText } = render(
        <EpicDepsProvider ajax={ajaxStub}>
          <KittyComponent />
        </EpicDepsProvider>
      );
  
      fireEvent.click(getByLabelText(/Cat #1/i));
      const detailsName = await getByText(/^Name:/);
      expect(detailsName.textContent).toBe('Name: Snuggles')
    });

  The deps object can be good for providing "services", config, or any number of other useful features to help decouple your components from their dependecies.

  deps.props$
  There is a special property props$ which is always provided by useEpic() and is the methods in which components can pass props into the Epic. The options.props property of the useEpic() call is always emitted to the deps.props$ observable.

ofType()

A RxJS Operator for convient filtering of action$ by type

action$.pipe( ofType( type, ...moreTypes? ) );

Just a convinience operator for filtering actions by type, from either the action itself 'promote', the conventional object form { type: 'promote', payload: { id: 23 } } or array form ['promote', { id: 23 }]. The ofType() operator only filters, so your type property will still be in the emitted value for the next operator or subscription.

arguments

• type the ofType() operator can take one or more type arguments to match on, if any of the types match for the action emitted, the action will be emitted further down the stream. The type arguments are not restriced to Strings, they can be anything including symbols, functions or objects. They are matched with SameValueZero (pretty much ===) comparison.

const promotionChange$ = action$.pipe(ofType('promote', 'depromote'));

<EpicDepsProvider>

A React Provider component that supplies deps to any epic function used by the useEpic() hook, called anywhere lower in the component tree, just like Reacts context.Provider

<EpicDepsProvider kittenService={kittenService} catConfig={config}>
  <App />
</EpicDepsProvider>

// const kittyEpic = ( action$, state$, { kittenService, catConfig }) => {
//  ...
// }

Any props passed to the EpicDepsProvider component will be merged onto the deps object passed to the epic function when calling useEpic(). Any change in deps will unsubscribe from the newState$ observable, and recall the epic function, setting up new subscriptions, so try to change deps sparingly.

Testing

One benefit of using Epics for state management is that they are easy to test. Because they are just functions, you can ensure the behaviour of your Epic, just by calling it with some test observables and deps, emitting actions, and asserting on the newState$ emitted.

TODO: Create testing example
TODO: Create epic testing helper method

🌱 Contribute

Think you'd like to contribute to this project? Check out our contributing guideline and feel free to create issues and pull requests!

License

MIT © Adam L Barrett