Add updating logic to text message

[maxkonovalov]

Added support for willBeShown() and wasHidden() functions in TextMessageViewModel. The main purpose of this is to give chance to the client to load the avatar images when needed. Most of the code was copied from the corresponding functions in PhotoMessagePresenter.
Both Text and Photo message models now look pretty similar, so maybe you guys consider moving some parts to the base classes?

Also added the missing observe calls for the avatar image in view models.

And the essential part of this commit is the call to the main queue in updateClosure:

dispatch_async(dispatch_get_main_queue(), {
    self?.updateCurrentCell()
})

It looks strange, but if you try to set the observable value in willBeShown(), although the model itself has been updated, in BaseMessageCollectionViewCell.updateViews() it still gets the old value, so the update has no visible effect. Deferring the update with an async call to the main queue seems to fix the issue here.

[codecov-io]

Current coverage is 63.55% (diff: 42.85%)

Merging #178 into dev will decrease coverage by 0.32%

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- Misses         1239       1258    +19   
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[diegosanchezr]

Hi @maxkonovalov,

Yes, I think we should move the avatar thing to the BaseMessagePresenter and clean the text and photo ones a little bit. We would appreciate if you do this here (not required).

I was tracking down your need for the dispatch_async. Check the root cause here: https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20160711/002580.html. If you don't mind, could you make Observable a class and remove the dispatch_async?

Thank you!

[maxkonovalov]

Hey @diegosanchezr, sorry for the delay, sure, I'll try add these changes as soon as possible.

[diegosanchezr]

Hi @maxkonovalov, are you still able to work on this? We'd definitely like to merge this. Thank you!

[maxkonovalov]

Hey @diegosanchezr, sorry again - very busy with my other projects, but will get back to this issue in a few days!

[diegosanchezr]

Cool, thank you!

[maxkonovalov]

Hey @diegosanchezr! As requested, I updated the Observable - it now seems to work without issues.

For refactoring, I only moved the declarations of the methods willBeShown() and wasHidden() to their common protocol. I think it may still be worth combining TextMessageViewModel and PhotoMessageViewModel to some common superclass instead of inheriting both from a protocol.

Also changed the avatar show/hide logic, because the previous implementation triggered Observable updates resulting in infinite loop.

[diegosanchezr]

@maxkonovalov Thank you!