Lifetime.observeEnded and take(during:) fix.

[andersio]

1. New methods and operator: attach, observeEnded and +=.

   ended provides a full Signal interface that is overblown for the general use cases of Lifetime.

2. [Bugfix?] Safe observation & deprecating Lifetime.ended.

   ended.observeCompleted does not handle the observations to a terminated ended Signal, and the action is simply ignored.

   It should have been invoked regardless, like how adding a Disposable to disposed CompositeDisposable would dispose of the disposable being added.

   Composition of Lifetime is suggested to be done through lifetime operators instead, e.g. and and or, should the demand arise.

[sharplet]

I think I'm 👍 on attach and observeEnded, although I think attach could use a more descriptive name. But I think adding += would be a step backwards in terms of clarity.

[sharplet]

It's a nitpick, but do you think we can make this change without reindenting the file? It makes the diff a little hard to follow, and there's no shared setup/teardown code.

[mdiep]

I really like the simplicity of exposing ended: Signal<(), NoError>. Clear, unambiguous, and does very little to expand the API. I also think we should prefer Signals to Disposables.

If we're concerned about the safety issue you mentioned, adding the terminated events to Observer seems like a fine approach.

[inamiy]

I'm +1 for not exposing ended: Signal<(), NoError>.
It's hard to grasp from the type that only ended.observeCompleted { ... } will work.

But I don't get why disposable is needed as an argument of func attach.
I think just plain () -> () is sufficient.
(lifetime.observeEnded seems enough to take () -> (), and maybe attach is overkill)

Also, I don't think Observer(terminated:) is necessary because end only does sendCompleted.
(Is there any chance of sendInterrupted or others?)

P.S. I think Signal<Never, NoError> describes a bit better than Signal<(), NoError>.

[andersio]

@inamiy When the lifetime has already terminated.

[andersio]

@mdiep #143 might have a stake in this, if it is still on the table. #143 also touches on where += might make sense, though it is purely sugar.

[inamiy]

@andersio
Ah, you are right. Lifetime may sendInterrupted.
And with #143, I got the idea of func attach.

[mdiep]

I still prefer ended: Signal<(), NoError>.

I can be 👍 to observeEnded if people find ended confusing.

But I'd prefer not to add attach or +=. I think that muddles the job of Lifetime—which should be just to notify observers when the lifetime has ended. I don't think we should give Lifetime any knowledge of Disposable.

[andersio]

I don't quite see the problem here. We can just treat a fire-exactly-once Disposable as a termination observer. They are all a () -> Void sink anyway...

[mdiep]

Yeah, so I think writing lifetime.observeEnded(disposable.dispose) is fine. I don't think it requires extra sugar that couples the two concepts.

[andersio]

The deprecation of ended is reverted, while observeEnded is documented as the preferred API unless composition is needed.

[sharplet]

👍

[sharplet]

Does this still need to be @testable?