make ring$ a behavior and add code comments to justify/explain the change.

Signed-off-by: Timo K <toger5@hotmail.de>

src/state/CallViewModel.ts

@@ -880,23 +880,28 @@ export class CallViewModel extends ViewModel {
     ? this.allOthersLeft$
     : NEVER;
 
-  /**
+  private readonly didSendCallNotification$ = fromEvent(
-   * Whenever the RTC session tells us that it intends to ring the remote
-   * participant's devices, this emits an Observable tracking the current state of
-   * that ringing process.
-   */
-  private readonly ring$: Observable<
-    Observable<"ringing" | "timeout" | "decline">
-  > = (
-    fromEvent(
     this.matrixRTCSession,
     MatrixRTCSessionEvent.DidSendCallNotification,
   ) as Observable<
     Parameters<
       MatrixRTCSessionEventHandlerMap[MatrixRTCSessionEvent.DidSendCallNotification]
     >
-    >
+  >;
-  ).pipe(
+  /**
+   * Whenever the RTC session tells us that it intends to ring the remote
+   * participant's devices, this emits an Observable tracking the current state of
+   * that ringing process.
+   */
+  // This is a behavior since we need to store the latest state for when we subscribe to this after `didSendCallNotification$`
+  // has already emitted but we still need the latest observable with a timeout timer that only gets created on after receiving `notificationEvent`.
+  // A behavior will emit the latest observable with the running timer to new subscribers.
+  // see also: callPickupState$ and in particular the line: `return this.ring$.pipe(mergeAll());` here we otherwise might get an EMPTY observable if
+  // `ring$` would not be a behavior.
+  private readonly ring$: Behavior<
+    Observable<"ringing" | "timeout" | "decline"> | Observable<never>
+  > = this.scope.behavior(
+    this.didSendCallNotification$.pipe(
       filter(
         ([notificationEvent]) => notificationEvent.notification_type === "ring",
       ),
@@ -926,7 +931,8 @@ export class CallViewModel extends ViewModel {
                 ([event]) =>
                   event.getType() === EventType.RTCDecline &&
                   event.getRelation()?.rel_type === "m.reference" &&
-                event.getRelation()?.event_id === notificationEvent.event_id &&
+                  event.getRelation()?.event_id ===
+                    notificationEvent.event_id &&
                   event.getSender() !== this.userId,
               ),
             ),
@@ -934,6 +940,8 @@ export class CallViewModel extends ViewModel {
           endWith("decline" as const),
         );
       }),
+    ),
+    EMPTY,
   );
 
   /**
@@ -972,6 +980,8 @@ export class CallViewModel extends ViewModel {
               return of("success" as const);
             }
             // Show the ringing state of the most recent ringing attempt.
+            // ring$ is a behavior so it will emit the latest observable which very well might already have a running timer.
+            // this is important in case livekitConnectionState$ and someoneElseJoined$ emit after didSendCallNotification$ has already emitted.
             return this.ring$.pipe(switchAll());
           }),
           // The state starts as 'unknown' because we don't know if the RTC