Nested observable models trigger redundant updates in parent

[maximkrouk]

Currently I'm facing 2 issues with current observation API:

• Redundant updates

  • When child model changes parent component sets it to child component
  • When child model is set, child component subscribes to it
  • It happens on the same call stack inside of observe function
  • So observation of child fields is observed by the parent what makes parent subscribe to child local updates

• If I use @testable import to access internal observe function

  • I'm able to create my own observe function with derived tracking and updates
  • It fixes redundant updates
  • But observe function that applies transaction keys (including animations) is private and applies stuff only in tracking context (so derived updates doesn't get wrapped into transactionKeys.perform)
  • I probably can acheive similar fix for redundant updates with DispatchQueue.main.async, but as I'm writing it I'm starting to think that it won't fix animations since it will escape not only tracking context, but transactionKeys.perform as well

Here is a project that helps to reporduce the issue (didn't update the readme, so it mentions possible DispatchQueue.main.async fix, but seems like it won't work):
https://github.com/maximkrouk/swift-navigation-test-repo/tree/main

I decided to start a discussion before opening a PR to figure out if I'm missing smth and maybe additions to the lib are not needed in that case c:

[mbrandonw]

Hi @maximkrouk, I'm having a hard time understanding what exactly is wrong with the app you provided. Can you try creating a simpler reproduction of the problem with as few moving parts as possible? Ideally it would even just be a unit test that we could run to see a failure that you expect to pass.

[maximkrouk]

Hey 👋 Sorry for the delay, I just updated the repo with a simpler example. I kept the old one as well, if you play with it you'll notice glitches (if the middle star is filled there should be no glitches since it enables my workaround), they are happening because of redundant updates and those updates can be debugged in the new simple example (see console after pressing "Run logical example" button with/without "Experimental observation" toggle enabled)

[maximkrouk]

I also raised a PR with my vision of the fix and some tests showcasing the difference

[maximkrouk]

The final API I'd like to achieve is somewhat like

func bind(
  _ model: Model,
  into cancellables: inout Set<AnyCancellable>
) {
  model
    .observe(\.modelProperty, assign(to: \.viewProperty))
    .store(in: &cancellables)
  
  model
    .observe(\.childModel, bind(to: \.childView))
    .store(in: &cancellables)

  // ...
}

(I do basically have this implemented, but it requires split "tracking" and "onChange" closures for observe function)

observe(_ apply:) in current public API causes problems when you have some nesting of UIView-based UIComponents, so

- topComponent
  - middleComponent
    - bottomComponent

structure for a topComponent.setModel(topModel) call will be an equivalent of the following call

topComponent.model = topModel
topComponent.observe {
  middleComponent.model = middleModel // this will be triggered
  middleComponent.observe {
    bottomComponent.model = bottomModel
    bottomComponent.observe {
      bottomComponent.label.text = bottomModel.value.description // on this change, since all blocks are nested
    }
  }
}