use-memory-value

Hooks for global state in memory and localstorage. It's like context, but only one state per value; allows you to subscribe and update values from far away.

Using Expo? Check out this implementation: expo-use-memory-value.

Installation

yarn add use-memory-value

It has the following peerDependencies, along with react:

yarn add localforage react-fast-compare
# assumes react is already installed

Usage

Start by creating a new MemoryValue or StoredMemoryValue. You can declare this in any file, make sure it's exported and importable from all the files you want to use the value.

import { MemoryValue } from 'use-memory-value';

interface State {
  foo: number;
  bar: string;
  baz?: boolean;
}

const INITIAL_STATE: State = {
  foo: 42,
  bar: 'yes',
};

export const MY_STATE = new MemoryValue<State>(INITIAL_STATE);

Then, where you want to use the value, import the MemoryValue and useMemoryValue:

import { useMemoryValue, useMutableMemoryValue } from 'use-memory-value';

import { MY_STATE } from '../path/to/state';

function ReadOnlyBar() {
  const state = useMemoryValue(MY_STATE);
  return <h1>foo: {state && state.foo}</h1>;
}

function CountingFoo() {
  const [state, updateState] = useMutableMemoryValue(MY_STATE);
  const increment = () =>
    updateState((prev) => ({ ...prev, foo: prev.foo + 1 }));

  return (
    <button type="button" onClick={increment}>
      Foo: {state.foo}
    </button>
  );
}

function ActivateBaz() {
  const [, updateState] = useMutableMemoryValue(MY_STATE);
  const activate = () => updateState((prev) => ({ ...prev, baz: true }));

  return (
    <button type="button" onClick={activate}>
      Activate
    </button>
  );
}

If the value should be persisted to (and initialized from) local storage, use StoredMemoryValue:

export const MY_STATE = new StoredMemoryValue<State>('local.key.name');

TypeScript warnings

It is important to use type and not interface when using this in conjunction with TypeScript. The reason for this is that interfaces are extendible and thus we can not safely say that the final resolved shape is serializable (JSON-compatible). types are fixed, and thus can be checked.

You want this because non-serializable fields would be lost during serialization/deserialization, and thus can cause run-time issues.

If you get errors that the type is not Serializable, make sure you're only using type and not interface.

Localforage configuration

You can set the instance yourself by importing

import { setLocalForageInstance } from 'expo-use-memory-value/storage.web';