A Storage (e.g. localStorage) implementation for Atoms that is designed for persisting non-critical data such as selections made by the user over sessions.

Usage

You must first provide an Atom implementation. You can use either

import Atom from "bacon.atom"

or

import Atom from "kefir.atom"

for example. See bacon.atom and kefir.atom for details.

The default export

import Stored from "atom.storage"

is a function to create an atom whose contents are stored.

Creating a Stored atom

To create an atom whose contents are stored, you pass a key, a default value, the desired Atom constructor, and the desired Storage object to the Stored constructor. For example:

const stored = Stored({key: "my-unique-app-prefix:my-stored-model",
                       value: defaultValue,
                       Atom,
                       storage: localStorage})

The default value is used when the storage does not already contain a value for the key. Also, when a stored atom is written to with a value that is equal to the default value, the persisted value for the stored atom is removed from the storage. This avoids unnecessary use of storage space.

The value of the atom is converted to a string by calling JSON.stringify.

Note that when a stored atom is created, the (default) value is not stored. The value is stored only after a modify method call that actually results in a new value for the stored atom.

Sharing

When two (or more) stored atoms are created with the same storage and key, the implementation only actually creates an atom on the first call. This means that in

const stored1 = Stored({key: "my-unique-app-prefix:my-stored-model",
                        value: defaultValue,
                        Atom,
                        storage: localStorage})

const stored2 = Stored({key: "my-unique-app-prefix:my-stored-model",
                        value: defaultValue,
                        Atom,
                        storage: localStorage})

the objects stored1 and stored2 are the one and same object and stored1 === stored2 is true.

Full options

The full argument object to Stored can be described as follows:

{key: String,
 value: JSON,
 Atom: JSON => AbstractMutable,
 storage: Storage,
 time: Maybe Milliseconds,
 schema: Maybe JSON,
 debounce: Maybe Milliseconds}

The time, if specified, is the number of milliseconds after which the value is considered to have expired. If not specified, the value never expires.

The schema, if specified, is stored with the value, and checked when a stored atom is created. If the stored schema is not equal to the given schema, then the stored value is removed and the given default is used instead.

The debounce, if specified, is the debounce period, in milliseconds, to use for storing values. If not specified, values are stored immediately. Note that debounce: 0 is different from no debounce.

Expiring

When a value is persisted to storage, the expiration time is set to time + Date.now(). Also, when a stored atom with a particular key is first created (e.g. when the application is started), the expiration time is updated to time + Date.now(). This way a value is kept alive as long as it is being used.

The named export

import {expireNow} from "atom.storage"

is a function that takes a {storage, regex} argument object. expireNow goes through items in the storage, whose keys match the given regex, and removes items that have expired. You typically call expireNow once immediately or shortly after your app starts. For example:

expireNow({storage: localStorage, regex: /^my-unique-app-prefix:/})

Note the use of ^ in the above regex.

Combining with Undo

Even though Stored shares atoms with the same storage and key, Stored directly returns the shared object it constructs with Atom. This means that you can combine Stored with more complex ways to create atoms. In particular, you can combine Stored with Undo from atom.undo. You can decide whether you create an Undo atom with Stored:

const storesFullHistory =
  Undo({value, Atom: value => Stored({key, value, storage, Atom})})

Or a stored Atom with Undo:

const storesLatestValue =
  Stored({key, value, storage, Atom: value => Undo({value, Atom})})

Both of these combinations return an undoable atom and can be useful.