expo-use-memory-value v6.0.4

expo-use-memory-value

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

Tested against SDK 44.

If you're looking for non expo implementation: use-memory-value.

Installation

yarn add expo-use-memory-value

It has the following peerDependencies, along with react and react-native / react-native-web:

yarn expo add @react-native-async-storage/async-storage expo-secure-store localforage
# assumes already installed:
# - react
# - react-native
# - react-native-web

If you only target the web, you can use the following, and ignore the warnings about async-storage and expo-secure-store:

yarn add localforage
# assumes already installed:
# - react
# - react-native-web

If you do not target the web, you can use the following, and ignore the warnings about localforage:

yarn expo add @react-native-async-storage/async-storage expo-secure-store
# assumes already installed:
# - react
# - react-native

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

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

import { value 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.

Android/iOS secure storage

Swap StoredMemoryValue for SecureStoreMemoryValue. This will use expo-secure-storage under the hood for native platforms. On the web it falls back to localforage. Values on the web are never secure.

Note: There are limitations to storage size.

Web configuration

For the web it uses localforage under the hood. You can set the instance yourself by importing

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