Pass payload into the context function for subscriptions (#1513)

* Pass payload into the context function for subscriptions

The payload can be used for things like auth. You can pass a token on
the payload of the message and use it to fetch the user.

* Update CHANGELOG.md

CHANGELOG.md

@@ -4,6 +4,7 @@ All of the packages in the `apollo-server` repo are released with the same versi
 
 ### vNEXT
 
+- pass payload into context function for subscriptions [#1513](https://github.com/apollographql/apollo-server/pull/1513)
 - Add option to mock the entire schema(i.e. sets preserveResolvers) [PR #1546](https://github.com/apollographql/apollo-server/pull/1546)
 
 ### v2.0.2

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

@@ -291,7 +291,10 @@ export class ApolloServerBase {
           ? onConnect
           : (connectionParams: Object) => ({ ...connectionParams }),
         onDisconnect: onDisconnect,
-        onOperation: async (_: string, connection: ExecutionParams) => {
+        onOperation: async (
+          message: { payload: any },
+          connection: ExecutionParams,
+        ) => {
           connection.formatResponse = (value: ExecutionResult) => ({
             ...value,
             errors:
@@ -306,7 +309,7 @@ export class ApolloServerBase {
           try {
             context =
               typeof this.context === 'function'
-                ? await this.context({ connection })
+                ? await this.context({ connection, payload: message.payload })
                 : context;
           } catch (e) {
             throw formatApolloErrors([e], {