From f47425d349475231c0f3542bb6ecef16a63e933a Mon Sep 17 00:00:00 2001
From: Bill Barry <after.fallout@gmail.com>
Date: Fri, 21 May 2021 13:37:23 -0400
Subject: [PATCH] feat: All subjects now have an `observed` property. This will
 allow users to check whether a subject has current subscribers without us
 allowing access to the `observers` array, which is going to be made private
 in future versions. (#6405)

* feat(Subject): add isObserved() api

* fix(Subject): typo, remove done from sync test

* feat(subject): change observed method to property

* chore: update api_guardian files

Co-authored-by: Ben Lesh <ben@benlesh.com>
---
 api_guard/dist/types/index.d.ts |  1 +
 spec/Subject-spec.ts            | 24 ++++++++++++++++++++++++
 src/internal/Subject.ts         |  4 ++++
 3 files changed, 29 insertions(+)

diff --git a/api_guard/dist/types/index.d.ts b/api_guard/dist/types/index.d.ts
index 24ba35fb29..4c34cc78a3 100644
--- a/api_guard/dist/types/index.d.ts
+++ b/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();
diff --git a/spec/Subject-spec.ts b/spec/Subject-spec.ts
index 2691c6421c..1f82ee6a87 100644
--- a/spec/Subject-spec.ts
+++ b/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);
diff --git a/src/internal/Subject.ts b/src/internal/Subject.ts
index 57f5d9d42a..0fef20a6d7 100644
--- a/src/internal/Subject.ts
+++ b/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();