# State Management Reference

## Table of Contents

- [Reactive Variables](#reactive-variables)
- [Local-Only Fields](#local-only-fields)
- [Local Field Read Functions (Type Policies)](#local-field-read-functions-type-policies)
- [Combining Remote and Local State](#combining-remote-and-local-state)
- [useReactiveVar Hook](#usereactivevar-hook)

## Reactive Variables

Reactive variables are a way to store local state outside of the Apollo Client cache while still triggering reactive updates.

**Important**: Reactive variables store a single value that notifies `ApolloClient` instances when changed. They do not have separate values per ApolloClient instance. In multi-user environments like SSR, global or module-level reactive variables could be shared between users and cause data leaks. In frameworks that use SSR, always avoid storing reactive variables as globals.

### Creating Reactive Variables

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

// Simple reactive variable
export const isLoggedInVar = makeVar<boolean>(false);

// Object reactive variable
export const cartItemsVar = makeVar<CartItem[]>([]);

// Complex state
interface AppState {
  theme: "light" | "dark";
  sidebarOpen: boolean;
  notifications: Notification[];
}

export const appStateVar = makeVar<AppState>({
  theme: "light",
  sidebarOpen: true,
  notifications: [],
});
```

### Reading Reactive Variables

```tsx
// Direct read (non-reactive)
const isLoggedIn = isLoggedInVar();

// Reactive read in component
import { useReactiveVar } from "@apollo/client/react";

function AuthButton() {
  const isLoggedIn = useReactiveVar(isLoggedInVar);

  return isLoggedIn ?
      <button onClick={() => isLoggedInVar(false)}>Logout</button>
    : <button onClick={() => isLoggedInVar(true)}>Login</button>;
}
```

### Updating Reactive Variables

```typescript
// Set new value
isLoggedInVar(true);

// Update based on current value
cartItemsVar([...cartItemsVar(), newItem]);

// Update object state
appStateVar({
  ...appStateVar(),
  theme: "dark",
});

// Helper function pattern
export function toggleSidebar() {
  const current = appStateVar();
  appStateVar({ ...current, sidebarOpen: !current.sidebarOpen });
}

export function addNotification(notification: Notification) {
  const current = appStateVar();
  appStateVar({
    ...current,
    notifications: [...current.notifications, notification],
  });
}
```

## Local-Only Fields

Local-only fields are fields defined in queries but resolved entirely on the client using the `@client` directive.

**Important**: To use any `@client` fields, you need to add `LocalState` to the `ApolloClient` initialization:

```typescript
import { ApolloClient, InMemoryCache } from "@apollo/client";
import { LocalState } from "@apollo/client/local-state";

const client = new ApolloClient({
  cache: new InMemoryCache(),
  localState: new LocalState({}),
  // ... other options
});
```

> **Note**: `LocalState` is an Apollo Client 4.x concept and did not exist as a class in previous versions. In previous versions, a `localState` option was not necessary, and local resolvers (if used) could be passed directly to the `ApolloClient` constructor.

### Basic @client Fields

```tsx
const GET_USER_WITH_LOCAL = gql`
  query GetUserWithLocal($id: ID!) {
    user(id: $id) {
      id
      name
      email
      # Local-only fields
      isSelected @client
      displayName @client
    }
  }
`;

function UserCard({ userId }: { userId: string }) {
  const { data } = useQuery(GET_USER_WITH_LOCAL, {
    variables: { id: userId },
  });

  return (
    <div className={data?.user.isSelected ? "selected" : ""}>
      <h2>{data?.user.displayName}</h2>
      <p>{data?.user.email}</p>
    </div>
  );
}
```

### Local Field Read Functions (Type Policies)

Local field `read` functions are defined in entity-level type policies. You can use reactive variables inside these `read` functions, along with other calculations or derived values:

```typescript
const cache = new InMemoryCache({
  typePolicies: {
    User: {
      fields: {
        // Simple local field from reactive variable
        isSelected: {
          read(_, { readField }) {
            const id = readField("id");
            return selectedUsersVar().includes(id);
          },
        },

        // Computed local field (derived value)
        displayName: {
          read(_, { readField }) {
            const name = readField("name");
            const email = readField("email");
            return name || email?.split("@")[0] || "Anonymous";
          },
        },
      },
    },
  },
});
```

## LocalState Resolvers

### Query-Level Local Resolvers

Query-level local fields can be defined using `LocalState` resolvers. **Note**: Do not read reactive variables inside LocalState resolvers - this is not a documented/tested feature. It might not behave as expected.

```typescript
import { LocalState } from "@apollo/client/local-state";

const client = new ApolloClient({
  cache: new InMemoryCache(),
  localState: new LocalState({
    resolvers: {
      Query: {
        // Read from localStorage
        theme: () => {
          if (typeof window !== "undefined") {
            return localStorage.getItem("theme") || "light";
          }
          return "light";
        },

        // Read from cache
        currentUser: (_, __, { cache }) => {
          if (typeof window === "undefined") return null;
          const userId = localStorage.getItem("currentUserId");
          if (!userId) return null;
          return cache.readFragment({
            id: cache.identify({ __typename: "User", id: userId }),
            fragment: gql`
              fragment CurrentUser on User {
                id
                name
                email
              }
            `,
          });
        },

        // Compute value
        isOnline: () => {
          if (typeof navigator !== "undefined") {
            return navigator.onLine;
          }
          return true;
        },
      },
    },
  }),
});
```

### Using Local Query Fields

```tsx
const GET_AUTH_STATE = gql`
  query GetAuthState {
    isLoggedIn @client
    currentUser @client {
      id
      name
      email
    }
  }
`;

function AuthStatus() {
  const { data } = useQuery(GET_AUTH_STATE);

  if (!data?.isLoggedIn) {
    return <LoginButton />;
  }

  return <UserMenu user={data.currentUser} />;
}
```

## Combining Remote and Local State

### Mixing Remote and Local Fields

```tsx
const GET_PRODUCTS = gql`
  query GetProducts {
    products {
      id
      name
      price
      # Local fields
      quantity @client
      isInCart @client
    }
  }
`;

const cache = new InMemoryCache({
  typePolicies: {
    Product: {
      fields: {
        quantity: {
          read(_, { readField }) {
            const id = readField("id");
            const cartItem = cartItemsVar().find(
              (item) => item.productId === id
            );
            return cartItem?.quantity ?? 0;
          },
        },

        isInCart: {
          read(_, { readField }) {
            const id = readField("id");
            return cartItemsVar().some((item) => item.productId === id);
          },
        },
      },
    },
  },
});
```

### Local Mutations

```tsx
import { LocalState } from "@apollo/client/local-state";

const client = new ApolloClient({
  cache: new InMemoryCache(),
  localState: new LocalState({
    resolvers: {
      Mutation: {
        addToCart: (_, { productId, quantity }, { cache }) => {
          // Read current cart from cache
          const { cart } = cache.readQuery({ query: GET_CART }) || { cart: [] };

          const existing = cart.find((item) => item.productId === productId);

          const updatedCart =
            existing ?
              cart.map((item) =>
                item.productId === productId ?
                  { ...item, quantity: item.quantity + quantity }
                : item
              )
            : [...cart, { productId, quantity, __typename: "CartItem" }];

          // Write updated cart back to cache
          cache.writeQuery({
            query: GET_CART,
            data: { cart: updatedCart },
          });

          return true;
        },
      },
    },
  }),
});

const ADD_TO_CART = gql`
  mutation AddToCart($productId: ID!, $quantity: Int!) {
    addToCart(productId: $productId, quantity: $quantity) @client
  }
`;
```

### Persisting Local State

```typescript
// Create a helper function to permanently subscribe to reactive variable changes, without creating memory leaks
function subscribeToVariable<T>(
  weakRef: WeakRef<ReactiveVar<T>>,
  listener: ReactiveListener<T>
) {
  weakRef.deref()?.onNextChange((value) => {
    listener(value);
    subscribeToVariable(weakRef, listener);
  });
}

// Create reactive variable with persistence
const persistentCartVar = makeVar<CartItem[]>(
  typeof window !== "undefined" && localStorage.getItem("cart") ?
    JSON.parse(localStorage.getItem("cart")!)
  : []
);

// Save to localStorage when reactive variable changes
subscribeToVariable(new WeakRef(persistentCartVar), (items) => {
  try {
    if (typeof window !== "undefined") {
      localStorage.setItem("cart", JSON.stringify(items));
    }
  } catch (error) {
    console.error("Failed to persist cart:", error);
  }
});
```

## useReactiveVar Hook

The `useReactiveVar` hook subscribes a component to reactive variable updates.

### Basic Usage

```tsx
import { useReactiveVar } from "@apollo/client/react";

function ThemeToggle() {
  const theme = useReactiveVar(themeVar);

  return (
    <button onClick={() => themeVar(theme === "light" ? "dark" : "light")}>
      Current: {theme}
    </button>
  );
}
```

### With Derived State

```tsx
function CartSummary() {
  const cartItems = useReactiveVar(cartItemsVar);

  // Derived values are computed on each render
  const totalItems = cartItems.reduce((sum, item) => sum + item.quantity, 0);
  const totalPrice = cartItems.reduce(
    (sum, item) => sum + item.price * item.quantity,
    0
  );

  return (
    <div>
      <p>Items: {totalItems}</p>
      <p>Total: ${totalPrice.toFixed(2)}</p>
    </div>
  );
}
```

### Multiple Reactive Variables

```tsx
function AppLayout() {
  const theme = useReactiveVar(themeVar);
  const sidebarOpen = useReactiveVar(sidebarOpenVar);
  const isLoggedIn = useReactiveVar(isLoggedInVar);

  return (
    <div className={`app ${theme}`}>
      {isLoggedIn && sidebarOpen && <Sidebar />}
      <main>
        <Outlet />
      </main>
    </div>
  );
}
```