Hot and Cold Observables: Understanding the Flow of Data

Are All Rx Observables Cold by Default?

By default, all Rx observables are cold, except for subjects. This means they only emit values when they have at least one observer subscribed to them.

Rx Operators to Turn Cold Observables into Hot Observables

There are two main operators that can convert a cold observable into a hot observable:

• publish: Returns a connectable observable, which will only start emitting values when it is connected.
• share: Similar to publish, but it automatically connects the observable when the first observer subscribes.

withLatestFrom Operator and Cold Observables

withLatestFrom does not change the coldness or hotness of an observable. In your example:

• cold$.withLatestFrom(sth$,...) will still be a cold observable.
• If multiple observables are subscribed to cold$ using withLatestFrom, they will each consume the same sequence of values, regardless of their subscription times.

Rx.fromEvent and Hot/Cold Behavior

The discrepancy you observed in the CodePen example is due to the fact that the event emits only when an element is clicked, not when the Rx.fromEvent observable is subscribed to. Because of this, each subscription to the observable receives a different event.

Simplified Flow Diagram for Cold Observables

To illustrate the simplified flow of data for cold observables:

Source -> Observer1 -> Observer2

Simplified Flow Diagram for Hot Observables

For hot observables, the flow is:

Source -> Subject -> Observer1 -> Observer2

The subject acts as a central hub, multicast incoming data to all subscribed observers.

Multicasting Operators (publish/share)

Multicasting operators create a subject internally and return a connectable observable. When the observable is connected, the subject subscribes to the upstream observable and multicasts data to all subscribed observers.

Consider the Data Flow When Using Operators

Understanding the data flow and the behavior of operators is crucial. Even if an observable is hot, it's important to consider how subsequent operators may affect its hot or cold behavior.