Pass context object to link chain when using subscriptions

Makes sure a supplied `context` object is passed through the
link execution chain, when set in `client.subscribe` options.

CHANGELOG.md

@@ -144,6 +144,9 @@
 - Add a default `gc` implementation to `ApolloCache`.  <br/>
   [@justinwaite](https://github.com/justinwaite) in [#5974](https://github.com/apollographql/apollo-client/pull/5974)
 
+- Support passing a `context` object through the link execution chain when using subscriptions.  <br/>
+  [@sgtpepper43](https://github.com/sgtpepper43) in [#4925](https://github.com/apollographql/apollo-client/pull/4925)
+
 ### Bug Fixes
 
 - `useMutation` adjustments to help avoid an infinite loop / too many renders issue, caused by unintentionally modifying the `useState` based mutation result directly.  <br/>

src/__tests__/graphqlSubscriptions.ts

@@ -47,6 +47,9 @@ describe('GraphQL Subscriptions', () => {
       variables: {
         name: 'Changping Chen',
       },
+      context: {
+        someVar: 'Some value'
+      }
     };
 
     defaultSub1 = {
@@ -275,4 +278,23 @@ describe('GraphQL Subscriptions', () => {
       setTimeout(() => link.simulateComplete(), 100);
     });
   });
+
+  it('should pass a context object through the link execution chain', done => {
+    const link = mockObservableLink();
+    const client = new ApolloClient({
+      cache: new InMemoryCache(),
+      link,
+    });
+
+    client.subscribe(options).subscribe({
+      next() {
+        expect(link.operation.getContext().someVar).toEqual(
+          options.context.someVar
+        );
+        done();
+      },
+    });
+
+    link.simulateResult(results[0]);
+  });
 });

src/core/ObservableQuery.ts

@@ -342,6 +342,7 @@ export class ObservableQuery<
       .startGraphQLSubscription({
         query: options.document,
         variables: options.variables,
+        context: options.context,
       })
       .subscribe({
         next: (subscriptionData: { data: TSubscriptionData }) => {

src/core/QueryManager.ts

@@ -844,14 +844,15 @@ export class QueryManager<TStore> {
     query,
     fetchPolicy,
     variables,
+    context = {},
   }: SubscriptionOptions): Observable<FetchResult<T>> {
     query = this.transform(query).document;
     variables = this.getVariables(query, variables);
 
     const makeObservable = (variables: OperationVariables) =>
       this.getObservableFromLink<T>(
         query,
-        {},
+        context,
         variables,
         false,
       ).map(result => {
@@ -883,6 +884,7 @@ export class QueryManager<TStore> {
       const observablePromise = this.localState.addExportedVariables(
         query,
         variables,
+        context,
       ).then(makeObservable);
 
       return new Observable<FetchResult<T>>(observer => {

src/core/watchQueryOptions.ts

@@ -140,6 +140,7 @@ export type SubscribeToMoreOptions<
   variables?: TSubscriptionVariables;
   updateQuery?: UpdateQueryFn<TData, TSubscriptionVariables, TSubscriptionData>;
   onError?: (error: Error) => void;
+  context?: Record<string, any>;
 };
 
 export interface SubscriptionOptions<TVariables = OperationVariables> {
@@ -159,6 +160,11 @@ export interface SubscriptionOptions<TVariables = OperationVariables> {
    * Specifies the {@link FetchPolicy} to be used for this subscription.
    */
   fetchPolicy?: FetchPolicy;
+
+  /**
+   * Context object to be passed through the link execution chain.
+   */
+  context?: Record<string, any>;
 }
 
 export type RefetchQueryDescription = Array<string | PureQueryOptions>;

src/utilities/testing/mocking/mockSubscriptionLink.ts

@@ -15,14 +15,16 @@ export interface MockedSubscriptionResult {
 export class MockSubscriptionLink extends ApolloLink {
   public unsubscribers: any[] = [];
   public setups: any[] = [];
+  public operation: Operation;
 
   private observer: any;
 
   constructor() {
     super();
   }
 
-  public request(_req: any) {
+  public request(operation: Operation) {
+    this.operation = operation;
     return new Observable<FetchResult>(observer => {
       this.setups.forEach(x => x());
       this.observer = observer;