Add shouldNotifyObservers to ObservableState

[rcarver]

This adds shouldNotifyObservers to ObservableState. It's basically copied from Perception, then merged with ObservationRegistrar

TODO

• Fix testCopyMutation_WithPerturbation
• Determine if there are other cases to handle when considering identity

Demo

https://github.com/rcarver/tca-sharing-observation-tests

The TCA case in this app shows that a noop assignment no longer causes a view update. This all started by working on observation for Shared, but I thought it might be informative to first refine this piece within TCA.

[stephencelis]

@rcarver Thanks for looking into this! Are your TODOs still pending or is this ready to review?

[rcarver]

@stephencelis np! Still todo (switched the PR to a draft).

Could you take a look at testCopyMutation_WithPerturbation? I can't quite figure out whether it's not important, or extremely important and points to a whole set of issues to solve around identity. That leads to the second TODO..

Any insights you can provide would be super helpful; I'll take another pass at it.

[rcarver]

@mbrandonw thanks for that. Demo still looks good.

1. do you think there are more cases to look into?
2. do you want to add new integration tests for it?

[stephencelis]

Thanks!