Added sectionIdentifier(for:) method for data sources

Sources/AppKit/CocoaCollectionViewDiffableDataSource.swift

@@ -54,6 +54,16 @@ open class CocoaCollectionViewDiffableDataSource<SectionIdentifierType: Hashable
  return core.snapshot()
  }
 
+ /// Returns an section identifier for given section index.
+ ///
+ /// - Parameters:
+ /// - section: An section index for the section identifier.
+ ///
+ /// - Returns: An section identifier for given section index.
+ public func sectionIdentifier(for section: Int) -> SectionIdentifierType? {
+ return core.sectionIdentifier(for: section)
+ }
+
  /// Returns an item identifier for given index path.
  ///
  /// - Parameters:

Sources/Internal/DiffableDataSourceCore.swift

@@ -57,6 +57,14 @@ final class DiffableDataSourceCore<SectionIdentifierType: Hashable, ItemIdentifi
  return snapshot
  }
 
+ func sectionIdentifier(for section: Int) -> SectionIdentifierType? {
+ guard 0..<sections.endIndex ~= section else {
+ return nil
+ }
+
+ return sections[section].differenceIdentifier
+ }
+
  func itemIdentifier(for indexPath: IndexPath) -> ItemIdentifierType? {
  guard 0..<sections.endIndex ~= indexPath.section else {
  return nil

Sources/UIKit/CollectionViewDiffableDataSource.swift

@@ -60,6 +60,16 @@ open class CollectionViewDiffableDataSource<SectionIdentifierType: Hashable, Ite
  return core.snapshot()
  }
 
+ /// Returns an section identifier for given section index.
+ ///
+ /// - Parameters:
+ /// - section: An section index for the section identifier.
+ ///
+ /// - Returns: An section identifier for given section index.
+ public func sectionIdentifier(for section: Int) -> SectionIdentifierType? {
+ return core.sectionIdentifier(for: section)
+ }
+
  /// Returns an item identifier for given index path.
  ///
  /// - Parameters:

Sources/UIKit/TableViewDiffableDataSource.swift

@@ -57,6 +57,16 @@ open class TableViewDiffableDataSource<SectionIdentifierType: Hashable, ItemIden
  return core.snapshot()
  }
 
+ /// Returns an section identifier for given section index.
+ ///
+ /// - Parameters:
+ /// - section: An section index for the section identifier.
+ ///
+ /// - Returns: An section identifier for given section index.
+ public func sectionIdentifier(for section: Int) -> SectionIdentifierType? {
+ return core.sectionIdentifier(for: section)
+ }
+
  /// Returns an item identifier for given index path.
  ///
  /// - Parameters:

Tests/CocoaCollectionViewDiffableDataSourceTests.swift

@@ -90,6 +90,20 @@ final class CocoaCollectionViewDiffableDataSourceTests: XCTestCase {
  XCTAssertEqual(snapshot7.itemIdentifiers, [0, 1, 2, 3, 4, 5])
  }
 
+ func testSectionIdentifier() {
+ let collectionView = MockCollectionView()
+ let dataSource = CocoaCollectionViewDiffableDataSource<Int, Int>(collectionView: collectionView) { _, _, _ in
+ NSCollectionViewItem()
+ }
+
+ var snapshot = DiffableDataSourceSnapshot<Int, Int>()
+ snapshot.appendSections([10, 20, 30])
+ dataSource.apply(snapshot)
+
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 1), 20)
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 100), nil)
+ }
+
  func testItemIdentifier() {
  let collectionView = MockCollectionView()
  let dataSource = CocoaCollectionViewDiffableDataSource<Int, Int>(collectionView: collectionView) { _, _, _ in

Tests/CollectionViewDiffableDataSourceTests.swift

@@ -90,6 +90,20 @@ final class CollectionViewDiffableDataSourceTests: XCTestCase {
  XCTAssertEqual(snapshot7.itemIdentifiers, [0, 1, 2, 3, 4, 5])
  }
 
+ func testSectionIdentifier() {
+ let collectionView = MockCollectionView()
+ let dataSource = CollectionViewDiffableDataSource<Int, Int>(collectionView: collectionView) { _, _, _ in
+ UICollectionViewCell()
+ }
+
+ var snapshot = DiffableDataSourceSnapshot<Int, Int>()
+ snapshot.appendSections([10, 20, 30])
+ dataSource.apply(snapshot)
+
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 1), 20)
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 100), nil)
+ }
+
  func testItemIdentifier() {
  let collectionView = MockCollectionView()
  let dataSource = CollectionViewDiffableDataSource<Int, Int>(collectionView: collectionView) { _, _, _ in

Tests/TableViewDiffableDataSourceTests.swift

@@ -90,6 +90,20 @@ final class TableViewDiffableDataSourceTests: XCTestCase {
  XCTAssertEqual(snapshot7.itemIdentifiers, [0, 1, 2, 3, 4, 5])
  }
 
+ func testSectionIdentifier() {
+ let tableView = MockTableView()
+ let dataSource = TableViewDiffableDataSource<Int, Int>(tableView: tableView) { _, _, _ in
+ UITableViewCell()
+ }
+
+ var snapshot = DiffableDataSourceSnapshot<Int, Int>()
+ snapshot.appendSections([10, 20, 30])
+ dataSource.apply(snapshot)
+
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 1), 20)
+ XCTAssertEqual(dataSource.sectionIdentifier(for: 100), nil)
+ }
+
  func testItemIdentifier() {
  let tableView = MockTableView()
  let dataSource = TableViewDiffableDataSource<Int, Int>(tableView: tableView) { _, _, _ in