# GraphQL

We use [Apollo] and [Vue Apollo][vue-apollo] for working with GraphQL
on the frontend.

In order to use GraphQL, you need to enable the `graphql` feature flag,
read more about [Feature Flags][feature-flags].

## Apollo Client

To save duplicated clients getting created in different apps, we have a
[default client][default-client] that should be used. This setups the
Apollo client with the correct URL and also sets the CSRF headers.

## GraphQL Queries

To save query compilation at runtime, webpack can directly import `.graphql`
files. This allows webpack to preprocess the query at compile time instead
of the client doing compilation of queries.

## Usage in Vue

To use Vue Apollo, import the [Vue Apollo][vue-apollo] plugin as well
as the default client. This should be created at the same point
the Vue application is mounted.

```javascript
import Vue from 'vue';
import VueApollo from 'vue-apollo';
import createDefaultClient from '~/lib/graphql';
Vue.use(VueApollo);

const apolloProvider = new VueApollo({
  defaultClient: createDefaultClient(),
});

new Vue({
  ...,
  apolloProvider,
  ...
});
```

Read more about [Vue Apollo][vue-apollo] in the [Vue Apollo documentation][vue-apollo-docs].

### Local state with Apollo

It is possible to manage an application state with Apollo by passing
in a resolvers object when creating the default client. The default state can be set by writing
to the cache after setting up the default client.


```javascript
import Vue from 'vue';
import VueApollo from 'vue-apollo';
import createDefaultClient from '~/lib/graphql';
Vue.use(VueApollo);

const defaultClient = createDefaultClient({
  Query: {
    ...
  },
  Mutations: {
    ...
  },
});

defaultClient.cache.writeData({
  data: {
    isLoading: true,
  },
});

const apolloProvider = new VueApollo({
  defaultClient,
});
```

Read more about local state management with Apollo in the [Vue Apollo documentation](https://vue-apollo.netlify.com/guide/local-state.html#local-state).

### Testing

With [Vue test utils][vue-test-utils] it is easy to quickly test components that
fetch GraphQL queries. The simplest way is to use `shallowMount` and then set
the data on the component

```javascript
it('tests apollo component', () => {
  const vm = shallowMount(App);

  vm.setData({
    ...mock data
  });
});
```

Another possible way is testing queries with mocked GraphQL schema. Read more about this way in [Vue Apollo testing documentation](https://vue-apollo.netlify.com/guide/testing.html#tests-with-mocked-graqhql-schema)

## Usage outside of Vue

It is also possible to use GraphQL outside of Vue by directly importing
and using the default client with queries.

```javascript
import defaultClient from '~/lib/graphql';
import query from './query.graphql';

defaultClient.query(query)
  .then(result => console.log(result));
```

Read more about the [Apollo] client in the [Apollo documentation][apollo-client-docs].

[Apollo]: https://www.apollographql.com/
[vue-apollo]: https://github.com/Akryum/vue-apollo/
[vue-apollo-docs]: https://akryum.github.io/vue-apollo/
[feature-flags]: ../feature_flags.md
[default-client]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/lib/graphql.js
[apollo-client-docs]: https://www.apollographql.com/docs/tutorial/client.html
[vue-test-utils]: https://vue-test-utils.vuejs.org/
[apollo-link-state]: https://www.apollographql.com/docs/link/links/state.html
