feat(Subject): add isObserved() api

api_guard/dist/types/index.d.ts

@@ -392,6 +392,7 @@ export declare class Subject<T> extends Observable<T> implements SubscriptionLik
     closed: boolean;
     hasError: boolean;
     isStopped: boolean;
+    get observed(): boolean;
     observers: Observer<T>[];
     thrownError: any;
     constructor();

spec/Subject-spec.ts

@@ -430,6 +430,30 @@ describe('Subject', () => {
     done();
   });
 
+  it('should expose observed status', () => {
+    const subject = new Subject();
+
+    expect(subject.observed).to.equal(false);
+
+    const sub1 = subject.subscribe(function (x) {
+      //noop
+    });
+
+    expect(subject.observed).to.equal(true);
+
+    const sub2 = subject.subscribe(function (x) {
+      //noop
+    });
+
+    expect(subject.observed).to.equal(true);
+    sub1.unsubscribe();
+    expect(subject.observed).to.equal(true);
+    sub2.unsubscribe();
+    expect(subject.observed).to.equal(false);
+    subject.unsubscribe();
+    expect(subject.observed).to.equal(false);
+  });
+
   it('should have a static create function that works', () => {
     expect(Subject.create).to.be.a('function');
     const source = of(1, 2, 3, 4, 5);

src/internal/Subject.ts

@@ -91,6 +91,10 @@ export class Subject<T> extends Observable<T> implements SubscriptionLike {
     this.observers = null!;
   }
 
+  get observed() {
+    return this.observers?.length > 0;
+  }
+
   /** @internal */
   protected _trySubscribe(subscriber: Subscriber<T>): TeardownLogic {
     this._throwIfClosed();