feat(subscriptions): enable passing WebSocket.Server instead… (#2314)

Permit passing a `WebSocket.Server` instance into `installSubscriptionHandlers`.

CHANGELOG.md

@@ -9,7 +9,7 @@ The version headers in this history reflect the versions of Apollo Server itself
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- Allow passing a `WebSocket.Server` to `ApolloServer.installSubscriptionHandlers`. [PR #2314](https://github.com/apollographql/apollo-server/pull/2314)
 
 ### v2.12.0
 

packages/apollo-server-core/src/ApolloServer.ts

@@ -35,6 +35,8 @@ import {
   ExecutionParams,
 } from 'subscriptions-transport-ws';
 
+import WebSocket from 'ws';
+
 import { formatApolloErrors } from 'apollo-server-errors';
 import {
   GraphQLServerOptions,
@@ -667,7 +669,7 @@ export class ApolloServerBase {
     }
   }
 
-  public installSubscriptionHandlers(server: HttpServer) {
+  public installSubscriptionHandlers(server: HttpServer | WebSocket.Server) {
     if (!this.subscriptionServerOptions) {
       if (this.config.gateway) {
         throw Error(
@@ -742,10 +744,12 @@ export class ApolloServerBase {
         },
         keepAlive,
       },
-      {
-        server,
-        path,
-      },
+      server instanceof WebSocket.Server
+        ? server
+        : {
+            server,
+            path,
+          },
     );
   }