diff --git a/src/repositories/UnoptimizedInMemoryRepository.ts b/src/repositories/UnoptimizedInMemoryRepository.ts
index c5569e9..203dde2 100644
--- a/src/repositories/UnoptimizedInMemoryRepository.ts
+++ b/src/repositories/UnoptimizedInMemoryRepository.ts
@@ -1,5 +1,6 @@
 import { GetterSetterRepository } from "./GetterSetterRepository";
 import { IEntityWithId, IEta, IOrderedStop, IRoute, IShuttle, IStop, ISystem } from "../entities/entities";
+import { TupleKey } from "../types/TupleKey";
 
 /**
  * An unoptimized in memory repository.
@@ -14,6 +15,8 @@ export class UnoptimizedInMemoryRepository implements GetterSetterRepository {
   private etas: IEta[] = [];
   private orderedStops: IOrderedStop[] = [];
 
+  private subscribers: ((eta: IEta) => void)[] = [];
+
   public async getSystems() {
     return this.systems;
   }
@@ -59,9 +62,14 @@ export class UnoptimizedInMemoryRepository implements GetterSetterRepository {
   }
 
   public subscribeToEtaChanges(listener: (eta: IEta) => void) {
+    this.subscribers.push(listener);
   }
 
   public unsubscribeFromEtaChanges(listener: (eta: IEta) => void) {
+    const index = this.subscribers.findIndex((existingListener) => existingListener == listener);
+    if (index >= 0) {
+      this.subscribers.splice(index, 1);
+    }
   }
 
   public async getEtaForShuttleAndStopId(shuttleId: string, stopId: string) {