Issue 60 stack structure

[Liquidsoul]

Pull request checklist

• All tests pass. Demo project builds and runs.
• I have resolved any merge conflicts.
• I have followed the coding style, and reviewed the contributing guidelines.

This fixes issue #60

What's in this pull request?

• change stack structure so that both background and main contexts are connected to the persistent store coordinator
• sync mechanism between contexts now use mergeChangesFromContextDidSaveNotification(_:)

What's not in this pull request?

• child contexts creation with background context as parent.
  childContext use the main context as parent. This may not be something we want with this new structure (at least not all the time).
• some of the tests may be simplified or changed to reflect the new stack structure and update mechanism.

[jessesquires]

let's change this to:

Data between the main and background contexts is perpetually kept in sync.

[jessesquires]

Whoa @Liquidsoul ! This is awesome! 👍 😄 Great work. Perfect pull request. ✨

Only one small comment to address, then I'll merge. Thanks so much for working on this!

I'll be releasing this with 4.0.0 soon.

[jessesquires]

What's not in this pull request?

• child contexts creation with background context as parent. childContext use the main context as parent. This may not be something we want with this new structure (at least not all the time).

Yep, agreed. I created #73 to follow up here. 😄

• some of the tests may be simplified or changed to reflect the new stack structure and update mechanism.

I think what you've done with the tests is great! 👍 Is there something specific you think we should change?

[Liquidsoul]

Thanks @jessesquires ! 😊

I've amended my last commit to change the documentation and rebased the branch.

I think what you've done with the tests is great! 👍 Is there something specific you think we should change?

I was wondering if the sibling context updates checks in SaveTests.swift could be removed.
We test that saving the background and the main context will merge the changes into the other, it may not be necessary to do this kind of check in all the tests.
Maybe this could reduce the size of some of those tests without reducing the coverage.

[jessesquires]

@Liquidsoul ahhh, I see. I actually like this. Feels very thorough. 😄