Fixed issues when using with Angular (#731)

Co-authored-by: Dan Guisinger <dguisinger@aegonam.com>

Author: dguisinger

packages/aws-appsync-subscription-link/package.json

@@ -21,7 +21,8 @@
   "dependencies": {
     "aws-appsync-auth-link": "^3.0.7",
     "debug": "2.6.9",
-    "url": "^0.11.0"
+    "url": "^0.11.0",
+    "zen-observable-ts": "^1.2.5"
   },
   "devDependencies": {
     "@apollo/client": "^3.2.0",

packages/aws-appsync-subscription-link/src/subscription-handshake-link.ts

@@ -3,6 +3,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import { ApolloLink, Observable, Operation, FetchResult, ApolloError } from "@apollo/client/core";
+import * as ZenObservable from 'zen-observable-ts';
 
 import { rootLogger } from "./utils";
 import * as Paho from './vendor/paho-mqtt';
@@ -50,7 +51,7 @@ export class SubscriptionHandshakeLink extends ApolloLink {
         this.subsInfoContextKey = subsInfoContextKey;
     }
 
-    request(operation: Operation) {
+    request(operation: Operation): Observable<FetchResult> | null {
         const {
             [this.subsInfoContextKey]: subsInfo,
             controlMessages: { [CONTROL_EVENTS_KEY]: controlEvents } = { [CONTROL_EVENTS_KEY]: undefined }

packages/aws-appsync-subscription-link/src/types/index.ts

@@ -1,4 +1,5 @@
 import { AuthOptions } from "aws-appsync-auth-link";
+import * as ZenObservable from 'zen-observable-ts';
 
 //#region Subscription link enums