Why do combined observables do not update template when using Subject or if they emit after ngAfterContentInit

Q1: The 2 observables are Subjects, so my assumption is that subjects are cold observables. Is this correct?

A1: The answer from this question says that the subject itself is hot. This article describes hot, cold and subjects.

Q2: So again, is this because the subscription is made before the observables emit a value?

A2: Yes. Subscription to Subject will receive values AFTER you subscribe. The delay() you introduced should have given time for this to happen.

Why? This is because a BehaviorSubject always hold a value and emits it when subscribed, while a Subject does not hold a value, it just emits values from producer to current subscribers. Details.

Q3: Does this mean BehaviourSubjects are hot observables?

A: See A1.

Apologies if this does not answer your question directly. I am just trying to say that...when dealing with Subject and BehaviorSubject, I do not really care if they are hot or cold.

Instead, I ask: "Do I want the observable to always hold a value when I subscribe?". If yes, I use a BehaviorSubject. If I am ok with no value upon subscription, then Subject is ok. Other than this difference, both of them will receive emitted values after subscription. --- depends on your use case.

Use BehaviorSubjects instead of Subjects

https://stackblitz.com/edit/angular-4gnxto?file=src/app/app.component.ts

The async pipe subscribes after the Subjects have emitted. BehaviorSubjects give the lastest result when you subscribe, Subjects only give you a value as they emit.

When the async pipe subscribes to the result$ observable made with combineLatest it will not emit any value from Subjects that have already emitted, it will only combineLatest for Subjects that emit after the subscription is made.

See how in this StackBtitz

https://stackblitz.com/edit/angular-eb7dge?file=src/app/app.component.ts

If you click emit the values are emitted after the async pipe has made the subscription.