Added new method , which will send the last received message to all n…

…ew subscribers.

src/mqtt.service.ts

@@ -9,7 +9,7 @@ import { UsingObservable } from 'rxjs/observable/UsingObservable';
 import { Subject } from 'rxjs/Subject';
 import { Subscription, AnonymousSubscription } from 'rxjs/Subscription';
 import { merge } from 'rxjs/observable/merge';
-import { filter, publish, refCount } from 'rxjs/operators';
+import { filter, publish, publishReplay, refCount } from 'rxjs/operators';
 
 import {
   IMqttClient,
@@ -126,22 +126,44 @@ export class MqttService {
    * The observable will only emit messages matching the filter.
    * The first one subscribing to the resulting observable executes a mqtt subscribe.
    * The last one unsubscribing this filter executes a mqtt unsubscribe.
-   * @param  {string}                  filter
-   * @return {Observable<IMqttMessage>}        the observable you can subscribe to
+   * Every new subscriber gets the latest message.
+   */
+  public observeRetained(filterString: string): Observable<IMqttMessage> {
+    return this._generalObserve(filterString, () => publishReplay());
+  }
+
+  /**
+   * With this method, you can observe messages for a mqtt topic.
+   * The observable will only emit messages matching the filter.
+   * The first one subscribing to the resulting observable executes a mqtt subscribe.
+   * The last one unsubscribing this filter executes a mqtt unsubscribe.
    */
   public observe(filterString: string): Observable<IMqttMessage> {
+    return this._generalObserve(filterString, () => publish());
+  }
+
+  /**
+   * With this method, you can observe messages for a mqtt topic.
+   * The observable will only emit messages matching the filter.
+   * The first one subscribing to the resulting observable executes a mqtt subscribe.
+   * The last one unsubscribing this filter executes a mqtt unsubscribe.
+   * Depending on the publish function, the messages will either be replayed after new
+   * subscribers subscribe or the messages are just passed through
+   */
+  private _generalObserve(filterString: string, publishFn: Function): Observable<IMqttMessage> {
     if (!this.client) {
       throw new Error('mqtt client not connected');
     }
     if (!this.observables[filterString]) {
-      const rejected = new Subject();
+      const rejected: Subject<IMqttMessage> = new Subject();
       this.observables[filterString] = <Observable<IMqttMessage>>UsingObservable
         .create(
-          // resourceFactory: Do the actual ref-counting MQTT subscription.
-          // refcount is decreased on unsubscribe.
-          () => {
-            const subscription: Subscription = new Subscription();
-            this.client.subscribe(filterString, (err, granted: ISubscriptionGrant[]) => {
+        // resourceFactory: Do the actual ref-counting MQTT subscription.
+        // refcount is decreased on unsubscribe.
+        () => {
+          const subscription: Subscription = new Subscription();
+          this.client.subscribe(filterString, (err, granted: ISubscriptionGrant[]) => {
+            if (granted) { // granted can be undefined when an error occurs when the client is disconnecting
               granted.forEach((granted_: ISubscriptionGrant) => {
                 if (granted_.qos === 128) {
                   delete this.observables[granted_.topic];
@@ -150,22 +172,23 @@ export class MqttService {
                 }
                 this._onSuback.emit({ filter: filterString, granted: granted_.qos !== 128 });
               });
-            });
-            subscription.add(() => {
-              delete this.observables[filterString];
-              this.client.unsubscribe(filterString);
-            });
-            return subscription;
-          },
-          // observableFactory: Create the observable that is consumed from.
-          // This part is not executed until the Observable returned by
-          // `observe` gets actually subscribed.
-          (subscription: AnonymousSubscription) => merge(rejected, this.messages))
+            }
+          });
+          subscription.add(() => {
+            delete this.observables[filterString];
+            this.client.unsubscribe(filterString);
+          });
+          return subscription;
+        },
+        // observableFactory: Create the observable that is consumed from.
+        // This part is not executed until the Observable returned by
+        // `observe` gets actually subscribed.
+        (subscription: AnonymousSubscription) => merge(rejected, this.messages))
         .pipe(
           filter((msg: IMqttMessage) => MqttService.filterMatchesTopic(filterString, msg.topic)),
-          publish(),
+          publishFn(),
           refCount()
-        );
+        ) as Observable<IMqttMessage>;
     }
     return this.observables[filterString];
   }