Support for macOS?

[jessesquires]

⚠️ This is a very large task. Please coordinate here before doing any work.

ℹ️ I don't have a need for this yet, but might in the future.

ToDo

Here's a rough, high-level outline of what needs to be done.

• Figure out the main differences between the NSCollectionView infra and UICollectionView infra
• What can we reuse? What cannot be reused?
• Should macOS just get its own library? Does it make sense to include it here?
• Tests for everything
• Add macOS example app to demonstrate, write UI Tests

Resources

• Implementing Modern Collection Views

[jessesquires]

Actually, macOS might need all of its own infra to make this work... I'm not sure how much we can reuse.

Especially with the new design here that uses associatedTypes for CellViewModel, etc.

But, "cells" on macOS are actually NSViewControllers, which is... weird.
https://developer.apple.com/documentation/appkit/nscollectionviewitem

Maybe something like this:

public protocol CellViewModel: DiffableViewModel, ViewRegistrationProvider {

    #if os(iOS)
        associatedtype CellType: UICollectionViewCell
    #elseif os(macOS)
        associatedtype CellType: NSCollectionViewItem
    #endif

    // ....

}

However... I think this could get ugly very quickly.

In any case, this is a significant undertaking.