feat: return function to delete observer

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `WakuRelay.addObserver` now returns a function to delete the observer.
+
 ### Changed
 
 - `queryCallbackOnPromise`'s return value has been simplified to `Promise<void>`.

src/lib/waku_relay/index.node.spec.ts

@@ -248,8 +248,10 @@ describe("Waku Relay [node only]", () => {
       // The promise **fails** if we receive a message on this observer.
       const receivedMsgPromise: Promise<WakuMessage> = new Promise(
         (resolve, reject) => {
-          waku2.relay.addObserver(reject, ["added-then-deleted-observer"]);
+          const deleteObserver = waku2.relay.addObserver(reject, [
-          waku2.relay.deleteObserver(reject, ["added-then-deleted-observer"]);
+            "added-then-deleted-observer",
+          ]);
+          deleteObserver();
           setTimeout(resolve, 500);
         }
       );

src/lib/waku_relay/index.ts

@@ -131,12 +131,12 @@ export class WakuRelay extends GossipSub {
    * @param callback called when a new message is received via waku relay
    * @param contentTopics Content Topics for which the callback with be called,
    * all of them if undefined, [] or ["",..] is passed.
-   * @returns {void}
+   * @returns Function to delete the observer
    */
   addObserver(
     callback: (message: WakuMessage) => void,
     contentTopics: string[] = []
-  ): void {
+  ): () => void {
     if (contentTopics.length === 0) {
       if (!this.observers[""]) {
         this.observers[""] = new Set();
@@ -150,6 +150,20 @@ export class WakuRelay extends GossipSub {
         this.observers[contentTopic].add(callback);
       });
     }
+
+    return () => {
+      if (contentTopics.length === 0) {
+        if (this.observers[""]) {
+          this.observers[""].delete(callback);
+        }
+      } else {
+        contentTopics.forEach((contentTopic) => {
+          if (this.observers[contentTopic]) {
+            this.observers[contentTopic].delete(callback);
+          }
+        });
+      }
+    };
   }
 
   /**