Stop admin server listening for unused events

This now subscribes to only the Mockttp events that the remote client is
interested in, which allows the server to skip some processing of
irrelevant events, in the same way it works for local-only client usage.

Author: pimterry

src/admin/mockttp-admin-model.ts

@@ -111,15 +111,27 @@ export function buildAdminServerModel(
         ruleParams
     };
 
-    for (let [gqlName, eventName] of graphqlSubscriptionPairs) {
-        mockServer.on(eventName as any, (evt) => {
-            pubsub.publish(eventName, { [gqlName]: evt });
-        });
-    }
+    // Build a set of event publishing callbacks (but don't subscribe them yet - we only
+    // want to subscribe on demand, to allow the server to opt-out unused event processing).
+    const eventListeners = graphqlSubscriptionPairs.reduce((acc, [, eventName]) => {
+        acc[eventName] = (evt: any) => {
+            pubsub.publish(eventName, { [eventName]: evt });
+        };
+        return acc;
+    }, {} as { [eventName: string]: (...args: any[]) => void });
 
     const subscriptionResolvers = Object.fromEntries(graphqlSubscriptionPairs.map(([gqlName, eventName]) => ([
         gqlName, {
-            subscribe: () => pubsub.asyncIterator(eventName)
+            subscribe: () => {
+                // Subscribe to the underlying server event, if we haven't already. Needs to actively check
+                // currently listeners because reset() clears all listeners, so they may disappear any time.
+                if (mockServer.listenerCount(eventName, eventListeners[eventName]) === 0) {
+                    mockServer.on(eventName as any, (evt) => {
+                        pubsub.publish(eventName, { [gqlName]: evt });
+                    });
+                }
+                return pubsub.asyncIterator(eventName);
+            }
         }
     ])));
 

src/server/mockttp-server.ts

@@ -363,6 +363,10 @@ export class MockttpServer extends AbstractMockttp implements Mockttp {
         return Promise.resolve();
     }
 
+    public listenerCount(event: string, listener?: Function): number {
+        return this.eventEmitter.listenerCount(event, listener);
+    }
+
     private announceBodyDataAsync(
         type: 'request' | 'response',
         id: string,