ReactiveCollectionsKit

ReactiveCollectionsKit uses a uni-directional data flow pattern based on two core components:

1. CellViewModel: The "atomic" component. It encapsulates all data, configuration, interaction, and registration for a single cell. It is responsible for defining how a cell is configured and how it handles interaction events.
2. CollectionViewModel: The immutable representation of the entire collection structure. It acts as the "driver" for the collection.

The Workflow:

1. Fetch or update your underlying data models.
2. Generate CellViewModel objects and a complete CollectionViewModel from that data.
3. Set the CollectionViewModel on the CollectionViewDriver.
4. The driver performs a diff against the previously set model and updates the UICollectionView automatically.

# Example workflow concept
# 1. Data -> 2. CellViewModels + CollectionViewModel -> 3. Driver -> 4. UI

To perform efficient diffing, the library distinguishes between identity (who an object is) and equality (what an object looks like). This is implemented via the DiffableViewModel protocol.

• Identity (Identifiable): A permanent and unique identifier (the id property). It answers "who is this?" and is used to determine if an item is present, moved, added, or deleted.
• Equality (Hashable/Equatable): Ephemeral traits or properties that change over time. It answers "is this the most up-to-date version?" and is used to determine if a specific item needs to be reloaded or refreshed.

Note on UIKit Compatibility: UICollectionViewDiffableDataSource only handles identity (structure changes). ReactiveCollectionsKit handles both identity and equality (content changes), ensuring items are reloaded when their properties change.

typealias UniqueIdentifier = AnyHashable

protocol DiffableViewModel: Identifiable, Hashable {
    var id: UniqueIdentifier { get }
}

To build a collection view, you define your data using a hierarchy of view models: CollectionViewModel contains SectionViewModels, which in turn contain cell view models. You then initialize a CollectionViewDriver with your UICollectionView and these view models. The driver handles all diffing, registration, and updates automatically. When your data changes, simply call driver.update(viewModel:) with a new view model instance.

let models = [/* array of some data models */]

// create cell view models from the data models
let cellViewModels = models.map {
    MyCellViewModel($0)
}

// create the sections with cells
let section = SectionViewModel(id: "my_section", cells: cellViewModels)

// create the collection with sections
let collectionViewModel = CollectionViewModel(sections: [section])

// initialize the driver with the view model and other components
let driver = CollectionViewDriver(
    view: collectionView,
    viewModel: collectionViewModel,
    emptyViewProvider: provider,
    cellEventCoordinator: coordinator
)

// the collection view is updated and animated automatically

// when the models change, generate a new view model (like above)
let updated = CollectionViewModel(sections: [/* updated items and sections */])
driver.update(viewModel: updated)