rx-atom

Reactive atom implementation in ReactiveExtensions.js. Uses Ramda compatible lenses to decompose state.

Why

Atoms provide identifier or handler to reference values changing in time. When using immutable values you separate identity and state. Atoms enable to bring those back into one place in sane way.

This implementation of atoms provide reactive interface to observe changes. Method get returns Observable from ReactiveExtensions.js library.

Atom also provides view method, which given Lens returns LensedAtom. It is a bidirectional projection of parent Atom. Meaning that, it gives access only to part of parent Atom (thanks to used Lens) and guarantees consistency with it.

Getting started

import { Atom } from 'rx-atom';

const counter = new Atom(0);

counter.get().subscribe(count => {
  console.log('Current count: ', count);
});
// logs initial value of 0

function inc() {
  counter.modify((count) => count + 1);
}

inc();
// logs modified value of 1

This is simplest usage of Atom. You can create new Atom with initial value. Method get returns Observable of values in this Atom. This observable emits new value on every change of Atom internal value. In this example we just subscribe to this Observable and log it's value to console. In inc function we modify value of Atom using method modify.

Lenses and LensedAtom

import { Atom } from 'rx-atom';
import { propLens } from 'ramda';

const state = new Atom({count: 0, name: ''});

state.get().subscribe(state => {
  console.log('debug state: ', state);
});

function nameWidget(name) {
  name.get().subscribe(name => {
    document.getElementById('name-label').textContent = name;
  });
  
  document.getElementById('name-input').addEventListener('change', function() {
  	name.set(this.value);
  });
}

const nameLens = propLens('name');
nameWidget(state.view(nameLens));

This example show how we can decompose state into smaller chunks and still have consistency and reactive nature. First, we declare global state for whole application. We also subscribe to it to provide logs for debuging. Then we define simple component which accepts Atom with string value. It is just input and text label that are synchronized. When something changes in input field, the label should reflect that change.

We define Lens that picks just name property from object. Then we use it in view method on state Atom. This atom we can pass to nameWidget. Component doesn't know how whole state looks, it gets only value viewed through nameLens (e.g. only name property). Component can modify it's value and this change will be reflected in underlying state Atom. And all subscribers to state Atom will see these changes. Because values flow down and changes start always on top, and because of nature of Observables, whole state is always consistent. If we wan't to introduce circular dependencies, we must explicity add modification of Atom inside subscription, and that's easy to spot and is considered bad practice.

How LensedAtoms works?

Each LensedAtom holds reference to parent Atom and Lens which was used to create it. In that way, Observable from get is just value of parent Atom mapped with Lens. Every modification of LensedAtoms is really modification of parent Atom through Lens This way LensedAtom doesn't really hold any value and is just projection of parent Atom. When calling view on LensedAtom, the resulting LensedAtom holds reference to original parent and Lens is just composition of Lens from parent and itself.

StoredAtom

This library provides also StoredAtom flavour. This Atom's values are stored in LocalStorage on every change, and if value exist in LocalStorage upon Atom creation - it's used as initial value.

import { StoredAtom } from 'rx-atom';

const atom = new StoredAtom('default value', {key: 'local-storage-key'});

This Atom's values will be stored under local-storage-key, and if some value existed here, it will be used instead default value.

Stored atom also accepts options to and from which are used to serialize and deserialize value to LocalStorage. That way it's possible to store only essential part of state.

Prior art

This library is heavly influenced by calmm-js.