Advances in diffable data sources

// NSDiffableDataSourceSectionSnapshot

public struct NSDiffableDataSourceSectionSnapshot<ItemIdentifierType> where ItemIdentifierType : Hashable {

    public init()

    public init(_ snapshot: NSDiffableDataSourceSectionSnapshot<ItemIdentifierType>)

    public mutating func append(_ items: [ItemIdentifierType], to parent: ItemIdentifierType? = nil)

    public mutating func insert(_ items: [ItemIdentifierType], before item: ItemIdentifierType)

    public mutating func insert(_ items: [ItemIdentifierType], after item: ItemIdentifierType)

    public mutating func delete(_ items: [ItemIdentifierType])

    public mutating func deleteAll()

    public mutating func expand(_ items: [ItemIdentifierType])

    public mutating func collapse(_ items: [ItemIdentifierType])

    public mutating func replace(childrenOf parent: ItemIdentifierType, using snapshot: NSDiffableDataSourceSectionSnapshot<ItemIdentifierType>)

    public mutating func insert(_ snapshot: NSDiffableDataSourceSectionSnapshot<ItemIdentifierType>, before item: (ItemIdentifierType))

    public mutating func insert(_ snapshot: NSDiffableDataSourceSectionSnapshot<ItemIdentifierType>, after item: (ItemIdentifierType))

    public func isExpanded(_ item: ItemIdentifierType) -> Bool

    public func isVisible(_ item: ItemIdentifierType) -> Bool

    public func contains(_ item: ItemIdentifierType) -> Bool

    public func level(of item: ItemIdentifierType) -> Int

    public func index(of item: ItemIdentifierType) -> Int?

    public func parent(of child: ItemIdentifierType) -> ItemIdentifierType?

    public func snapshot(of parent: ItemIdentifierType, includingParent: Bool = false) -> NSDiffableDataSourceSectionSnapshot<ItemIdentifierType>

    public var items: [ItemIdentifierType] { get }

    public var rootItems: [ItemIdentifierType] { get }

    public var visibleItems: [ItemIdentifierType] { get }
}

// UICollectionViewDiffableDataSource additions for iOS 14

extension UICollectionViewDiffableDataSource<Item, Section> {

    func apply(_ snapshot: NSDiffableDataSourceSectionSnapshot<Item>, 
               to section: Section, 
               animatingDifferences: Bool = true, 
               completion: (() -> Void)? = nil)

    func snapshot(for section: Section) ->   
                  NSDiffableDataSourceSectionSnapshot<Item>
}

// Example of using snapshots and section snapshots together

func update(animated: Bool=true) {

   // Add our sections in a specific order
   let sections: [Section] = [.recent, .top, .suggested]
   var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
   snapshot.appendSections(sections)
   dataSource.apply(snapshot, animatingDifferences: animated)

   // update each section's data via section snapshots in the existing position
   for section in sections {
      let sectionItems = items(for: section)
      var sectionSnapshot = NSDiffableDataSourceSectionSnapshot<Item>()
      sectionSnapshot.append(sectionItems)
      dataSource.apply(sectionSnapshot, to: section, animatingDifferences:animated)
   }
}

// Create hierarchical data for our Outline

var sectionSnapshot = ...

sectionSnapshot.append(["Smileys", "Nature", 
                        "Food", "Activities",
                        "Travel", "Objects", "Symbols"])

sectionSnapshot.append(["🥃", "🍎", "🍑"], to: "Food")

let childSnapshot = sectionSnapshot.snapshot(for: parent, includingParent: false)

struct NSDiffableDataSourceSectionSnapshot<Item: Hashable> {
   func expand(_ items: [Item])
   func collapse(_ items: [Item])
   func isExpanded(_ item: Item) -> Bool
}

// Section Snapshot Handlers: handling user interactions for expand / collapse state changes

extension UICollectionViewDiffableDataSource {

  struct SectionSnapshotHandlers<Item> {
    var shouldExpandItem: ((Item) -> Bool)?
    var willExpandItem: ((Item) -> Void)?
	
    var shouldCollapseItem: ((Item) -> Bool)?
    var willCollapseItem: ((Item) -> Void)?
    
    var snapshotForExpandingParent: ((Item, NSDiffableDataSourceSectionSnapshot<Item>) -> NSDiffableDataSourceSectionSnapshot<Item>)?
  }
  
  var sectionSnapshotHandlers: SectionSnapshotHandlers<Item>
 
}

// Diffable Data Source Reordering Handlers

extension UICollectionViewDiffableDataSource {

  struct ReorderingHandlers {
    var canReorderItem: ((Item) -> Bool)?
    var willReorder: ((NSDiffableDataSourceTransaction<Section, Item>) -> Void)?
    var didReorder: ((NSDiffableDataSourceTransaction<Section, Item>) -> Void)?
  }

  var reorderingHandlers: ReorderingHandlers
}

// NSDiffableDataSourceTransaction

struct NSDiffableDataSourceTransaction<Section, Item> {
   var initialSnapshot: NSDiffableDataSourceSnapshot<Section, Item> { get }
   var finalSnapshot: NSDiffableDataSourceSnapshot<Section, Item> { get }
   var difference: CollectionDifference<Item> { get }
   var sectionTransactions: [NSDiffableDataSourceSectionTransaction<Section, Item>] { get }
}

struct NSDiffableDataSourceSectionTransaction<Section, Item> {
   var sectionIdentifier: Section { get }
   var initialSnapshot: NSDiffableDataSourceSectionSnapshot<Item> { get }
   var finalSnapshot: NSDiffableDataSourceSectionSnapshot<Item> { get }
   var difference: CollectionDifference<Item> { get }
}

dataSource.reorderingHandlers.didReorder = { [weak self] transaction in 
   guard let self = self else { return }

   if let updateBackingStore = self.backingStore.applying(transaction.difference) {
      self.backingStore = updatedBackingStore
   }
}