Local State Management

We’ve learned how to manage remote data from our GraphQL server with Apollo Angular, but what should we do with our local data? We want to be able to access boolean flags and device API results from multiple components in our app, but don’t want to maintain a separate NGRX or Redux store. Ideally, we would like the Apollo cache to be the single source of truth for all data in our client application.

Apollo Client (>= 2.5) has built-in local state handling capabilities, that allow you to store your local data inside the Apollo cache alongside your remote data. To access your local data, just query it with GraphQL. You can even request local and server data within the same query!

In this section, you’ll learn how Apollo Client can help simplify local state management in your app. We’ll cover how client-side resolvers can help us execute local queries and mutations. You’ll also learn how to query and update the cache with the @client directive.

Please note that this documentation is intended to be used to familiarize yourself with Apollo Client’s local state management capabilities, and serve as a reference guide. If you’re looking for a step-by-step tutorial outlining how to handle local state with Apollo Client (and leverage other Apollo components to build a fullstack application), please refer to the Apollo tutorial.

Setting up

First, we need to extend our module:

import { resolvers, typeDefs } from './resolvers';
 
@NgModule({
  // ...
  providers: [
    {
      provide: APOLLO_OPTIONS,
      useFactory: (httpLink: HttpLink) => {
        const http = httpLink.create({
          uri: 'https://o5x5jzoo7z.sse.codesandbox.io/graphql',
        });
 
        return {
          cache: new InMemoryCache(),
          link: local.concat(http),
          resolvers,
          typeDefs,
        };
      },
      deps: [HttpLink],
    },
  ],
  // ...
})
export class AppModule {}

The two additional options you can pass to the constructor of ApolloClient are:

resolvers: Resolvers | Resolvers[];

• A map of functions that your GraphQL queries and mutations call in order to read and write to the cache

typeDefs: string | string[] | DocumentNode | DocumentNode[]

• A string representing your client-side schema written in Schema Definition Language. This schema is not used for validation (yet!), but is used for introspection in Apollo DevTools

None of these options are required. If you don’t specify anything, you will still be able to use the @client directive to query the cache.

Updating Local Data

There are two ways to perform mutations in your local store. The first way is directly writing to the cache by calling cache.writeData within a component. Direct writes are great for one-off mutations that don’t depend on the data that’s currently in the cache, such as writing a single value. The second way is creating a component with a GraphQL mutation that calls a client-side resolver. We recommend using resolvers if your mutation depends on existing values in the cache, such as adding an item to a list or toggling a boolean. You can think of direct writes like dispatching an action in NGRX, whereas resolvers offer a bit more structure like NGRX. Let’s learn about both ways below!

Direct Writes

Direct writes to the cache do not require a GraphQL mutation or a resolver function. They access your Apollo Client instance directly by using the Apollo.getClient() method. We recommend using this strategy for simple writes, such as writing a string, or one-off writes. It’s important to note that direct writes are not implemented as GraphQL mutations under the hood, so you shouldn’t include them in your schema. They also do not validate that the data you’re writing to the cache is in the shape of valid GraphQL data. If either of these features are important to you, you should opt for a resolver instead.

Here’s what a direct write looks like on our Blog app:

import { Apollo } from 'apollo-angular';
import { Component, Input, OnInit } from '@angular/core';
 
@Component({
  selector: 'filter-link',
  template: `
    <button (click)="setFilter()" [disabled]="visibilityFilter === filter">
      <ng-content></ng-content>
    </button>
  `,
})
export class FilterLinkComponent implements OnInit {
  @Input()
  filter: string;
 
  constructor(private apollo: Apollo) {}
 
  setFilter() {
    this.apollo.getClient().writeData({
      data: { visibilityFilter: this.filter },
    });
  }
}

We got Apollo Client instance through Apollo.getClient() method. From the client instance, you can directly call client.writeData and pass in the data you’d like to write to the cache.

What if we want to immediately subscribe to the data we just wrote to the cache? Let’s create an active property on the link that marks the link’s filter as active if it’s the same as the current visibilityFilter in the cache. To immediately subscribe to a client-side mutation, use Apollo.watchQuery in a component.

import { Apollo } from 'apollo-angular';
import gql from 'graphql-tag';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { Component, Input, OnInit } from '@angular/core';
 
const GET_VISIBILITY_FILTER = gql`
  {
    visibilityFilter @client
  }
`;
 
@Component({
  selector: 'filter-link',
  template: `
    <button (click)="setFilter()" [disabled]="visibilityFilter === filter">
      <ng-content></ng-content>
    </button>
  `,
})
export class FilterLinkComponent implements OnInit {
  @Input()
  filter: string;
  visibilityFilter: Observable<string>;
 
  constructor(private apollo: Apollo) {}
 
  ngOnInit() {
    this.visibilityFilter = this.apollo
      .watchQuery({
        query: GET_VISIBILITY_FILTER,
      })
      .valueChanges.pipe(map(result => result.data && result.data.visibilityFilter));
  }
 
  // ...
}

You’ll notice in our query that we have an @client directive next to our visibilityFilter field. This tells Apollo Client’s network stack to fetch the query from the cache instead of sending it to our GraphQL server. Once you call client.writeData, the query result will automatically update. All cache writes and reads are synchronous, so you don’t have to worry about loading state.

Resolvers

If you’d like to implement your local state update as a GraphQL mutation, then you’ll need to specify a function in your resolver map. The resolver map is an object with resolver functions for each GraphQL object type. You can think of a GraphQL query or mutation as a tree of function calls for each field. These function calls resolve to data or another function call.

The signature of a resolver function is the exact same as resolver functions on the server built with graphql-tools. Let’s quickly recap the four parameters of a resolver function:

fieldName: (obj, args, context, info) => result;

1. obj: The object containing the result returned from the resolver on the parent field or the ROOT_QUERY object in the case of a top-level query or mutation.
2. args: An object containing all of the arguments passed into the field. For example, if you called a mutation with updateNetworkStatus(isConnected: true), the args object would be { isConnected: true }.
3. context: The context object, which is shared between your Angular components and your Apollo Client network stack. The most important thing to note here is that we’ve added the Apollo cache to the context for you, so you can manipulate the cache with readQuery, writeQuery, readFragment, writeFragment, and writeData. Learn more about those methods here.
4. info: Information about the execution state of the query. You will probably never have to use this one.

Let’s take a look at an example of a resolver where we toggle a todos completed status:

export const resolvers = {
  Mutation: {
    toggleTodo: (_, variables, { cache, getCacheKey }) => {
      const id = getCacheKey({ __typename: 'TodoItem', id: variables.id });
      const fragment = gql`
        fragment completeTodo on TodoItem {
          completed
        }
      `;
      const todo = cache.readFragment({ fragment, id });
      const data = { ...todo, completed: !todo.completed };
      cache.writeData({ id, data });
      return null;
    },
  },
};

In order to toggle the todo’s completed status, we first need to query the cache to find out what the todo’s current completed status is. We do this by reading a fragment from the cache with cache.readFragment. This function takes a fragment and an id, which corresponds to the todo item’s cache key. We get the cache key by calling the getCacheKey that’s on the context and passing in the item’s __typename and id.

Once we read the fragment, we toggle the todo’s completed status and write the updated data back to the cache. Since we don’t plan on using the mutation’s return result in our UI, we return null since all GraphQL types are nullable by default.

Let’s learn how to trigger our toggleTodo mutation from our component:

import { Apollo } from 'apollo-angular';
import gql from 'graphql-tag';
import { Component, Input } from '@angular/core';
 
const TOGGLE_TODO = gql`
  mutation ToggleTodo($id: Int!) {
    toggleTodo(id: $id) @client
  }
`;
 
@Component({
  selector: 'todo',
  template: `
    <li
      *ngIf="task"
      (click)="toggle()"
      [ngStyle]="{ textDecoration: task.completed ? 'line-through' : 'none' }"
    >
      {{ task.text }}
    </li>
  `,
})
export class TodoComponent {
  @Input()
  task: any;
 
  constructor(private apollo: Apollo) {}
 
  toggle() {
    this.apollo
      .mutate({
        mutation: TOGGLE_TODO,
        variables: {
          id: this.task.id,
        },
      })
      .subscribe();
  }
}

First, we create a GraphQL mutation that takes the todo’s id we want to toggle as its only argument. We indicate that this is a local mutation by marking the field with a @client directive. This will tell Apollo Client to call our toggleTodo mutation resolver in order to resolve the field. Then, we define Apollo.mutate in the component just as we would for a remote mutation. Finally, call in your GraphQL mutation in your component and trigger it from within the UI.

If you’d like to see an example of a local mutation adding a todo to a list, check out the TodoList component in the StackBlitz.

Querying Local Data

Querying the Apollo cache is similar to querying your GraphQL server. The only difference is that you add a @client directive on your local fields to indicate they should be resolved from the cache. Let’s look at an example:

import { Apollo } from 'apollo-angular';
import gql from 'graphql-tag';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { Component, OnInit } from '@angular/core';
 
const GET_TODOS = gql`
  {
    todos @client {
      id
      completed
      text
    }
    visibilityFilter @client
  }
`;
 
@Component({
  selector: 'todo-list',
  template: `
    <ul>
      <todo *ngFor="let task of todos | async" [task]="task"></todo>
    </ul>
  `,
})
export class TodoListComponent implements OnInit {
  todos: Observable<any[]>;
 
  constructor(private apollo: Apollo) {}
 
  ngOnInit() {
    this.todos = this.apollo
      .watchQuery({
        query: GET_TODOS,
      })
      .valueChanges.pipe(
        map(({ data }) => this.getVisibleTodos(data.todos, data.visibilityFilter)),
      );
  }
 
  private getVisibleTodos(todos, filter) {
    switch (filter) {
      case 'SHOW_ALL':
        return todos;
      case 'SHOW_COMPLETED':
        return todos.filter(t => t.completed);
      case 'SHOW_ACTIVE':
        return todos.filter(t => !t.completed);
      default:
        throw new Error('Unknown filter: ' + filter);
    }
  }
}

First, we create our GraphQL query and add @client directives to todos and visibilityFilter. Then, we pass the query to Apollo.watchQuery and assign it to a component’s property. Reading from the Apollo cache is synchronous, so you won’t have to worry about tracking loading state.

Client-Side Schema

You can optionally pass a client-side schema to the typeDefs config property. This schema is not used for validation like it is on the server because the graphql-js modules for schema validation would dramatically increase your bundle size. Instead, your client-side schema is used for introspection in Apollo DevTools, where you can explore your schema in GraphiQL.

Your schema should be written in Schema Definition Language. Let’s view our schema for our todo app:

const typeDefs = `
  type Todo {
    id: Int!
    text: String!
    completed: Boolean!
  }
 
  type Mutation {
    addTodo(text: String!): Todo
    toggleTodo(id: Int!): Todo
  }
 
  type Query {
    visibilityFilter: String
    todos: [Todo]
  }
`;

If you open up Apollo DevTools and click on the GraphiQL tab, you’ll be able to explore your client schema in the “Docs” section. This app doesn’t have a remote schema, but if it did, you would be able to see your local queries and mutations alongside your remote ones. That’s the cool part about Apollo Client - it enables you to use GraphQL as a single, unified interface for all of your app’s data.

Combining Local and Remote Data

What’s really cool about using a @client directive to specify client-side only fields is that you can actually combine local and remote data in one query.

Migrating from apollo-link-state

The apollo-link-state project was the first to bring local state handling into the Apollo ecosystem. Handling local resolvers through the addition of an ApolloLink was a great starting point, and proved that @client based queries make sense, and work really well for local state management.

While apollo-link-state achieved some of the goals of local state handling, the information available when using any ApolloLink is limited by the modularity of the link system. We consider local state management a core part of the Apollo ecosystem, and as Apollo Client progresses, we want to make sure local resolvers are integrated as tightly as possible into core. This integration opens up new possibilities (like @export handling) and ties nicely into the future planned adjustments to cache data retention, invalidation, garbage collection, and other planned features that impact both local and remote data.

Updating your application to use Apollo Client’s local state management features, instead of apollo-link-state, is fairly straightforward. The necessary steps are outlined below.

1. Including apollo-link-state as a dependency, and importing it to use withClientState, is no longer necessary. You can remove the apollo-link-state dependency since local state management is included with apollo-client >= 2.5.0.

2. Using withClientState is no longer supported. The following

const cache = new InMemoryCache();
const stateLink = withClientState({ cache, resolvers: { ... } });
const link = ApolloLink.from([stateLink, new HttpLink({ uri: '...' })]);
const client = new ApolloClient({
  cache,
  link,
});

becomes

const client = new ApolloClient({
  cache: new InMemoryCache(),
  link: new HttpLink({ uri: '...' }),
  resolvers: { ... },
});

3. defaults are no longer supported. To prep the cache, use cache.writeData directly instead. So

const cache = new InMemoryCache();
const stateLink = withClientState({
  cache,
  resolvers: { ... },
  defaults: {
    someField: 'some value',
  },
});
const link = ApolloLink.from([stateLink, new HttpLink({ uri: '...' })]);
const client = new ApolloClient({
  cache,
  link,
});

becomes:

const cache = new InMemoryCache();
const client = new ApolloClient({
  cache,
  link: new HttpLink({ uri: '...' }),
  resolvers: { ... },
});
cache.writeData({
  data: {
    someField: 'some value',
  },
});

4. If you’re using Apollo Boost, you shouldn’t have to change anything. Apollo Boost has been updated to use Apollo Client’s integrated local state handling, which means it is no longer using apollo-link-state. Behind the scenes, the Apollo Boost clientState constructor parameter now feeds the necessary local state initialization directly into Apollo Client.

5. Test thoroughly! 🙂

Next Steps

Managing your local data with Apollo Client can simplify your state management code since the Apollo cache is your single source of truth for all data in your application. If you’d like to learn more about Apollo Angular, check out:

• Local state docs: Dive deeper into the concepts we just learned, such as resolvers and mixed queries, by taking a look at the Apollo Tutorial docs.