extend streaming resource with {status} capability #59712

Which @angular/* package(s) are relevant/related to the feature request?

core

Description

After playing around with the new streaming resource in 19.2.0-next.0 I thought about different usecases where I would like to use this.

First: the resource is only in "Loading" state when a new request is emitted until the promise resolves with the first (signal of) value. Afterwards, when a new value is emitted the resource will just update. This is fine!

But what if the implementation "knows" when it is fetching a value?

Example 1: Polling

Imagine some API which doesn't support streaming, so it has to be polled. The "poller" would then know when it is going to fetch the next value (even on the same request).

Example 2: Retry

Imagine some streaming resource implementation which automatically retries after an error occured. It will then know when it is "Reloading".

Proposed solution

Extend the type of the signal with

{ value: T }
| { error: unknown }
| { status: ResourceStatus.Loading | ResourceStatus.Reloading }

When the signal of the streaming resource emits a "status" the Resource<T> transitions to the given state (I don't think the other ResourceStatus values make sense here).

Alternatives considered

Currently the implementation would have to encode the (re)loading information into its value and that can't be populated to the Resource<T>'s status.