Add demo scope provider for Activity

[uber]

Add demo scope provider for Activity

[ZacSweers]

I think we should provide some examples of how to hook one up, but I don't think we should ship something first party. Especially with support library variants out there, kind of makes a mess and most people just implement it in their own base classes I think. Can revisit if Android APIs improve in the future for external observation.

[tadfisher]

Activities have ActivityLifecycleCallbacks.

[ZacSweers]

True, but we can't query the current state of the activity for lifecycle resolution :/. Most we could do would basically be to dispose on destroy, but wouldn't be able to tell if we're already after the lifecycle

[tadfisher]

Do you need to query the current state of the activity? You should get all edges of the state machine in the callbacks instance, and infer state from that. Unless there are edge cases I'm not thinking of.

[ZacSweers]

Say you subscribe after onDestroy, how would you know since no callbacks would ever fire?

[microstudent]

I think some code in Glide do the similar things.
ManagerRetriever.java#319
It attach a Invisible fragment to activity or fragment, as it can receive the lifecycle callback form them.

[ZacSweers]

You can get callbacks, but you can't query the current state right?