Local State

Local State Management

Overview

This skill provides best practices for local state management in this React Native/Expo codebase. Local state is managed using two complementary approaches:

1. Apollo Client Reactive Variables - In-memory reactive state for UI synchronization
2. React Native AsyncStorage - Persistent key-value storage for data that survives app restarts

When to Use Each Approach

Quick Reference

Creating a Reactive Variable

import { makeVar } from "@apollo/client";

interface IUserPreferences {
  readonly theme: "light" | "dark";
  readonly notifications: boolean;
}

const DEFAULT_PREFERENCES: IUserPreferences = {
  theme: "light",
  notifications: true,
};

export const userPreferencesVar =
  makeVar<IUserPreferences>(DEFAULT_PREFERENCES);

Reading in Components (Reactive)

import { useReactiveVar } from "@apollo/client";

const MyComponent = () => {
  const preferences = useReactiveVar(userPreferencesVar);
  return <Text>{preferences.theme}</Text>;
};

Updating Values (Immutably)

// Update with new object reference
userPreferencesVar({
  ...userPreferencesVar(),
  theme: "dark",
});

Persisting to AsyncStorage

import AsyncStorage from "@react-native-async-storage/async-storage";

const STORAGE_KEY = "@whatever:user-preferences";

export const savePreferences = async (
  prefs: IUserPreferences
): Promise<void> => {
  userPreferencesVar(prefs); // Update reactive variable first
  try {
    await AsyncStorage.setItem(STORAGE_KEY, JSON.stringify(prefs));
  } catch (error) {
    const message = error instanceof Error ? error.message : "Unknown error";
    console.error("Failed to save preferences:", message);
  }
};

export const loadPreferences = async (): Promise<void> => {
  try {
    const stored = await AsyncStorage.getItem(STORAGE_KEY);
    if (stored) {
      const parsed = JSON.parse(stored) as IUserPreferences;
      userPreferencesVar({ ...DEFAULT_PREFERENCES, ...parsed });
    }
  } catch (error) {
    const message = error instanceof Error ? error.message : "Unknown error";
    console.error("Failed to load preferences:", message);
  }
};

Core Rules

1. Always Create New References

Never mutate existing objects or arrays. Create new references to trigger reactivity:

// CORRECT - creates new object
userPreferencesVar({
  ...userPreferencesVar(),
  theme: "dark",
});

// INCORRECT - mutation does NOT trigger updates
const prefs = userPreferencesVar();
prefs.theme = "dark"; // This does nothing!
userPreferencesVar(prefs); // Same reference, no update

2. Use useReactiveVar for Reactive Components

Calling myVar() directly does NOT trigger re-renders. Always use the hook:

// CORRECT - component re-renders when variable changes
const theme = useReactiveVar(themeVar);

// INCORRECT - no re-renders on variable change
const theme = themeVar();

3. Encapsulate Updates in Custom Hooks

Create custom hooks for testability and encapsulation:

export const useTheme = () => {
  const preferences = useReactiveVar(userPreferencesVar);

  const setTheme = useCallback((theme: "light" | "dark") => {
    savePreferences({ ...userPreferencesVar(), theme });
  }, []);

  return { theme: preferences.theme, setTheme };
};

4. Use Namespace Prefixes for Storage Keys

Prefix all AsyncStorage keys with the app namespace:

// CORRECT
const STORAGE_KEY = "@whatever:filter-values";

// INCORRECT - collision risk
const STORAGE_KEY = "filters";

5. Always Handle AsyncStorage Errors

AsyncStorage operations can fail. Always use try/catch:

// CORRECT
try {
  await AsyncStorage.setItem(key, JSON.stringify(value));
} catch (error) {
  const message = error instanceof Error ? error.message : "Unknown error";
  console.error(`Failed to save ${key}:`, message);
}

// INCORRECT - silent failures
await AsyncStorage.setItem(key, JSON.stringify(value));

6. Never Store Sensitive Data in AsyncStorage

AsyncStorage is unencrypted. Use expo-secure-store for sensitive data:

// CORRECT - use secure store for tokens
import * as SecureStore from "expo-secure-store";
await SecureStore.setItemAsync("accessToken", token);

// INCORRECT - never store tokens in AsyncStorage
await AsyncStorage.setItem("accessToken", token);

7. Load Persisted State on App Start

Initialize persisted state early in the app lifecycle:

// In root layout or app initialization
useEffect(() => {
  loadPreferences();
}, []);

File Organization

Organize reactive variables and persistence logic in dedicated store files:

features/
  my-feature/
    stores/
      featureState.ts        # Reactive variable + persistence logic
      index.ts               # Re-exports

Example store file structure:

// stores/userPreferences.ts
import { makeVar, useReactiveVar } from "@apollo/client";
import AsyncStorage from "@react-native-async-storage/async-storage";
import { useCallback } from "react";

// Types
interface IUserPreferences {
  readonly theme: "light" | "dark";
  readonly language: string;
}

// Constants
const STORAGE_KEY = "@whatever:user-preferences";
const DEFAULT_PREFERENCES: IUserPreferences = {
  theme: "light",
  language: "en",
};

// Reactive Variable
export const userPreferencesVar =
  makeVar<IUserPreferences>(DEFAULT_PREFERENCES);

// Persistence Functions
export const loadUserPreferences = async (): Promise<void> => {
  try {
    const stored = await AsyncStorage.getItem(STORAGE_KEY);
    if (stored) {
      const parsed = JSON.parse(stored) as IUserPreferences;
      userPreferencesVar({ ...DEFAULT_PREFERENCES, ...parsed });
    }
  } catch (error) {
    const message = error instanceof Error ? error.message : "Unknown error";
    console.error("Failed to load preferences:", message);
  }
};

const saveUserPreferences = async (prefs: IUserPreferences): Promise<void> => {
  userPreferencesVar(prefs);
  try {
    await AsyncStorage.setItem(STORAGE_KEY, JSON.stringify(prefs));
  } catch (error) {
    const message = error instanceof Error ? error.message : "Unknown error";
    console.error("Failed to save preferences:", message);
  }
};

// Custom Hook
export const useUserPreferences = () => {
  const preferences = useReactiveVar(userPreferencesVar);

  const setTheme = useCallback((theme: "light" | "dark") => {
    saveUserPreferences({ ...userPreferencesVar(), theme });
  }, []);

  const setLanguage = useCallback((language: string) => {
    saveUserPreferences({ ...userPreferencesVar(), language });
  }, []);

  return { preferences, setTheme, setLanguage };
};

Detailed Reference

For comprehensive patterns and examples, see the reference files:

• references/reactive-variables.md - Complete reactive variable patterns including TypeScript types, cache integration, and advanced use cases
• references/async-storage.md - AsyncStorage API patterns, error handling, and performance considerations
• references/persistence-patterns.md - Patterns for persisting reactive variables including AppState-based and immediate persistence

Anti-Patterns to Avoid

Never mutate reactive variable values

// WRONG - mutation
const filters = filtersVar();
filters.minAge = 25;
filtersVar(filters);

// CORRECT - new reference
filtersVar({ ...filtersVar(), minAge: 25 });

Never use localStorage in React Native

// WRONG - doesn't exist in React Native
localStorage.setItem("key", value);

// CORRECT - use AsyncStorage
await AsyncStorage.setItem("key", value);

Never call myVar() expecting re-renders

// WRONG - no reactivity
const Component = () => {
  const value = myVar(); // Does NOT trigger re-renders
  return <Text>{value}</Text>;
};

// CORRECT - reactive
const Component = () => {
  const value = useReactiveVar(myVar);
  return <Text>{value}</Text>;
};

Never store without serialization

// WRONG - AsyncStorage only accepts strings
await AsyncStorage.setItem("key", { name: "John" });

// CORRECT - serialize first
await AsyncStorage.setItem("key", JSON.stringify({ name: "John" }));

Never forget to load persisted state

// WRONG - state resets on app restart
export const filtersVar = makeVar<Filters>(DEFAULT_FILTERS);

// CORRECT - load on app start
export const filtersVar = makeVar<Filters>(DEFAULT_FILTERS);
export const loadFilters = async () => {
  /* ... */
};
// Call loadFilters() in app initialization

Validation Checklist

When writing or reviewing local state code, verify:

• Reactive variables use TypeScript generics: makeVar<Type>(default)
• Components use useReactiveVar() hook, not direct myVar() calls
• Updates create new object/array references (spread operator)
• AsyncStorage keys use namespace prefix: @whatever:
• All AsyncStorage operations wrapped in try/catch
• Sensitive data uses expo-secure-store, not AsyncStorage
• Persisted state loaded on app initialization
• Custom hooks encapsulate update logic
• State stored in feature-specific stores/ directory