Enable observers to filter by contentTopic

examples/cli-chat/src/chat.ts

@@ -42,12 +42,15 @@ export default async function startChat(): Promise<void> {
 
   console.log(`Hi, ${nick}!`);
 
-  waku.relay.addObserver((message) => {
+  waku.relay.addObserver(
+    (message) => {
       if (message.payload) {
         const chatMsg = ChatMessage.decode(message.payload);
         console.log(formatMessage(chatMsg));
       }
-  });
+    },
+    [ChatContentTopic]
+  );
 
   if (opts.staticNode) {
     console.log(`Dialing ${opts.staticNode}`);

examples/web-chat/src/App.tsx

@@ -97,7 +97,7 @@ export default function App() {
         .then(() => console.log('Waku init done'))
         .catch((e) => console.log('Waku init failed ', e));
     } else {
-      stateWaku.relay.addObserver(handleRelayMessage);
+      stateWaku.relay.addObserver(handleRelayMessage, [ChatContentTopic]);
 
       stateWaku.libp2p.peerStore.on(
         'change:protocols',

src/lib/waku_relay/index.spec.ts

@@ -87,6 +87,42 @@ describe('Waku Relay', () => {
     expect(receivedMsg.payloadAsUtf8).to.eq(messageText);
   });
 
+  it('Filter on content topics', async function () {
+    this.timeout(10000);
+
+    const fooMessageText = 'Published on content topic foo';
+    const barMessageText = 'Published on content topic bar';
+    const fooMessage = WakuMessage.fromUtf8String(fooMessageText, 'foo');
+    const barMessage = WakuMessage.fromUtf8String(barMessageText, 'bar');
+
+    const receivedBarMsgPromise: Promise<WakuMessage> = new Promise(
+      (resolve) => {
+        waku2.relay.addObserver(resolve, ['bar']);
+      }
+    );
+
+    const allMessages: WakuMessage[] = [];
+    waku2.relay.addObserver((wakuMsg) => {
+      allMessages.push(wakuMsg);
+    });
+
+    await waku1.relay.send(fooMessage);
+    await waku1.relay.send(barMessage);
+
+    const receivedBarMsg = await receivedBarMsgPromise;
+
+    expect(receivedBarMsg.contentTopic).to.eq(barMessage.contentTopic);
+    expect(receivedBarMsg.version).to.eq(barMessage.version);
+    expect(receivedBarMsg.payloadAsUtf8).to.eq(barMessageText);
+    expect(allMessages.length).to.eq(2);
+    expect(allMessages[0].contentTopic).to.eq(fooMessage.contentTopic);
+    expect(allMessages[0].version).to.eq(fooMessage.version);
+    expect(allMessages[0].payloadAsUtf8).to.eq(fooMessageText);
+    expect(allMessages[1].contentTopic).to.eq(barMessage.contentTopic);
+    expect(allMessages[1].version).to.eq(barMessage.version);
+    expect(allMessages[1].payloadAsUtf8).to.eq(barMessageText);
+  });
+
   describe('Interop: Nim', function () {
     describe('Nim connects to js', function () {
       let waku: Waku;

src/lib/waku_relay/index.ts

@@ -56,7 +56,13 @@ interface GossipOptions {
  */
 export class WakuRelay extends Gossipsub implements Pubsub {
   heartbeat: RelayHeartbeat;
-  public observers: Array<(message: WakuMessage) => void>;
+  /**
+   * observers called when receiving new message.
+   * Observers under key "" are always called.
+   */
+  public observers: {
+    [contentTopic: string]: Array<(message: WakuMessage) => void>;
+  };
 
   /**
    *
@@ -73,7 +79,7 @@ export class WakuRelay extends Gossipsub implements Pubsub {
     );
 
     this.heartbeat = new RelayHeartbeat(this);
-    this.observers = [];
+    this.observers = {};
 
     const multicodecs = [constants.RelayCodec];
 
@@ -90,9 +96,18 @@ export class WakuRelay extends Gossipsub implements Pubsub {
   public start(): void {
     this.on(constants.RelayDefaultTopic, (event) => {
       const wakuMsg = WakuMessage.decode(event.data);
-      this.observers.forEach((callbackFn) => {
+      if (this.observers['']) {
+        this.observers[''].forEach((callbackFn) => {
           callbackFn(wakuMsg);
         });
+      }
+      if (wakuMsg.contentTopic) {
+        if (this.observers[wakuMsg.contentTopic]) {
+          this.observers[wakuMsg.contentTopic].forEach((callbackFn) => {
+            callbackFn(wakuMsg);
+          });
+        }
+      }
     });
 
     super.start();
@@ -114,10 +129,27 @@ export class WakuRelay extends Gossipsub implements Pubsub {
    * Register an observer of new messages received via waku relay
    *
    * @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}
    */
-  addObserver(callback: (message: WakuMessage) => void): void {
+  addObserver(
-    this.observers.push(callback);
+    callback: (message: WakuMessage) => void,
+    contentTopics: string[] = []
+  ): void {
+    if (contentTopics.length === 0) {
+      if (!this.observers['']) {
+        this.observers[''] = [];
+      }
+      this.observers[''].push(callback);
+    } else {
+      contentTopics.forEach((contentTopic) => {
+        if (!this.observers[contentTopic]) {
+          this.observers[contentTopic] = [];
+        }
+        this.observers[contentTopic].push(callback);
+      });
+    }
   }
 
   /**