Foundation set up for compositional layout

SectionKit/Sources/CollectionViewAdapter/ListCollectionViewAdapter/ListCollectionViewAdapter+Convenience.swift

@@ -58,12 +58,20 @@ extension ListCollectionViewAdapter {
 extension ListCollectionViewAdapter {
     @inlinable
     public func flowDelegate(at indexPath: IndexPath) -> SectionFlowDelegate? {
-        controller(at: indexPath)?.flowDelegate
+        if #available(iOS 13.0, *) {
+            return controller(at: indexPath)?.layoutProvider?.flowLayoutProvider
+        } else {
+            return controller(at: indexPath)?.flowDelegate
+        }
     }
 
     @inlinable
     public func flowDelegate(at index: Int) -> SectionFlowDelegate? {
-        controller(at: index)?.flowDelegate
+        if #available(iOS 13.0, *) {
+            return controller(at: index)?.layoutProvider?.flowLayoutProvider
+        } else {
+            return controller(at: index)?.flowDelegate
+        }
     }
 }
 

SectionKit/Sources/CollectionViewAdapter/ListCollectionViewAdapter/ListCollectionViewAdapter.swift

@@ -49,6 +49,12 @@ open class ListCollectionViewAdapter: NSObject, CollectionViewAdapter {
             collectionView.dragDelegate = self
             collectionView.dropDelegate = self
         }
+        if #available(iOS 13.0, *),
+           let layout = collectionView.collectionViewLayout as? SectionKitCompositionalLayout {
+            layout.sections = { [weak self] in
+                self?.collectionViewSections ?? []
+            }
+        }
     }
 
     /**
@@ -90,6 +96,12 @@ open class ListCollectionViewAdapter: NSObject, CollectionViewAdapter {
             collectionView.dragDelegate = self
             collectionView.dropDelegate = self
         }
+        if #available(iOS 13.0, *),
+           let layout = collectionView.collectionViewLayout as? SectionKitCompositionalLayout {
+            layout.sections = { [weak self] in
+                self?.collectionViewSections ?? []
+            }
+        }
     }
 
     public let context: CollectionViewContext

SectionKit/Sources/CollectionViewAdapter/SingleSectionCollectionViewAdapter/SingleSectionCollectionViewAdapter+Convenience.swift

@@ -58,12 +58,20 @@ extension SingleSectionCollectionViewAdapter {
 extension SingleSectionCollectionViewAdapter {
     @inlinable
     public func flowDelegate(at indexPath: IndexPath) -> SectionFlowDelegate? {
-        controller(at: indexPath)?.flowDelegate
+        if #available(iOS 13.0, *) {
+            return controller(at: indexPath)?.layoutProvider?.flowLayoutProvider
+        } else {
+            return controller(at: indexPath)?.flowDelegate
+        }
     }
 
     @inlinable
     public func flowDelegate(at index: Int) -> SectionFlowDelegate? {
-        controller(at: index)?.flowDelegate
+        if #available(iOS 13.0, *) {
+            return controller(at: index)?.layoutProvider?.flowLayoutProvider
+        } else {
+            return controller(at: index)?.flowDelegate
+        }
     }
 }
 

SectionKit/Sources/SectionController/Generic/BaseSectionController.swift

@@ -2,7 +2,7 @@ import UIKit
 
 /**
  A base implementation of all `SectionController` datasource and delegate protocols.
- 
+
  Every declaration is marked `open` and can be overridden.
  */
 @MainActor
@@ -36,6 +36,9 @@ open class BaseSectionController: SectionController,
     @available(iOS 11.0, *)
     open var dropDelegate: SectionDropDelegate? { self }
 
+    @available(iOS 13.0, *)
+    open var layoutProvider: SectionLayoutProvider? { .flowLayout(self) }
+
     open func didUpdate(model: Any) { }
 
     // MARK: - SectionDataSource
@@ -266,40 +269,77 @@ open class BaseSectionController: SectionController,
 
     // MARK: - SectionFlowDelegate
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func sizeForItem(
         at indexPath: SectionIndexPath,
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.itemSize ?? CGSize(width: 50, height: 50)
+        layout.flowLayout?.itemSize ?? FlowLayoutConstants.defaultItemSize
     }
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func inset(using layout: UICollectionViewLayout, in context: CollectionViewContext) -> UIEdgeInsets {
-        (layout as? UICollectionViewFlowLayout)?.sectionInset ?? .zero
+        layout.flowLayout?.sectionInset ?? FlowLayoutConstants.defaultInset
     }
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func minimumLineSpacing(using layout: UICollectionViewLayout, in context: CollectionViewContext) -> CGFloat {
-        (layout as? UICollectionViewFlowLayout)?.minimumLineSpacing ?? 10
+        layout.flowLayout?.minimumLineSpacing ?? FlowLayoutConstants.defaultMinimumLineSpacing
     }
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func minimumInteritemSpacing(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGFloat {
-        (layout as? UICollectionViewFlowLayout)?.minimumInteritemSpacing ?? 10
+        layout.flowLayout?.minimumInteritemSpacing ?? FlowLayoutConstants.defaultMinimumInteritemSpacing
     }
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func referenceSizeForHeader(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.headerReferenceSize ?? .zero
+        layout.flowLayout?.headerReferenceSize ?? FlowLayoutConstants.defaultHeaderSize
     }
 
+    @available(
+        iOS,
+        introduced: 6.0,
+        deprecated: 13.0,
+        message: "Please use the layoutProvider with the flowLayout type"
+    )
     open func referenceSizeForFooter(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.footerReferenceSize ?? .zero
+        layout.flowLayout?.footerReferenceSize ?? FlowLayoutConstants.defaultFooterSize
     }
+
 }

SectionKit/Sources/SectionController/SectionController.swift

@@ -27,6 +27,9 @@ public protocol SectionController: AnyObject {
     @available(iOS 11.0, *)
     var dropDelegate: SectionDropDelegate? { get }
 
+    @available(iOS 13.0, *)
+    var layoutProvider: SectionLayoutProvider? { get }
+
     /// The model of this section controller changed.
     func didUpdate(model: Any)
 }
@@ -44,4 +47,48 @@ extension SectionController {
 
     @available(iOS 11.0, *)
     public var dropDelegate: SectionDropDelegate? { nil }
+
+    @available(iOS 13.0, *)
+    public var layoutProvider: SectionLayoutProvider? { nil }
+}
+
+@available(iOS 13.0, *)
+public enum SectionLayoutProvider {
+    case flowLayout(FlowLayoutProvider)
+    case compositionalLayout(CompositionalLayoutProvider)
+}
+
+public typealias FlowLayoutProvider = SectionFlowDelegate
+
+@MainActor
+@available(iOS 13.0, *)
+public struct CompositionalLayoutProvider {
+    /// Provide the layout section for the Compositional Layout
+    /// - Parameters:
+    ///   - layoutEnvironment: the environment value for the layout
+    /// - Returns: The layout for the section
+    var layoutSection: (_ layoutEnvironment: NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection
+
+    public init(
+        layoutSection: @escaping (any NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection
+    ) {
+        self.layoutSection = layoutSection
+    }
+}
+
+@available(iOS 13.0, *)
+public extension SectionLayoutProvider {
+    var flowLayoutProvider: FlowLayoutProvider? {
+        guard case .flowLayout(let flowLayoutProvider) = self else {
+            return nil
+        }
+        return flowLayoutProvider
+    }
+
+    var compositionalLayoutProvider: CompositionalLayoutProvider? {
+        guard case .compositionalLayout(let compositionalLayoutProvider) = self else {
+            return nil
+        }
+        return compositionalLayoutProvider
+    }
 }

SectionKit/Sources/SectionController/SectionFlowDelegate.swift

@@ -76,47 +76,62 @@ public protocol SectionFlowDelegate: AnyObject {
     func referenceSizeForFooter(using layout: UICollectionViewLayout, in context: CollectionViewContext) -> CGSize
 }
 
+enum FlowLayoutConstants {
+    static let defaultItemSize = CGSize(width: 50, height: 50)
+    static let defaultInset: UIEdgeInsets = .zero
+    static let defaultMinimumLineSpacing: CGFloat = 10
+    static let defaultMinimumInteritemSpacing: CGFloat = 10
+    static let defaultHeaderSize: CGSize = .zero
+    static let defaultFooterSize: CGSize = .zero
+}
+
 extension SectionFlowDelegate {
     public func sizeForItem(
         at indexPath: SectionIndexPath,
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.itemSize ?? CGSize(width: 50, height: 50)
+        layout.flowLayout?.itemSize ?? FlowLayoutConstants.defaultItemSize
     }
 
     public func inset(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> UIEdgeInsets {
-        (layout as? UICollectionViewFlowLayout)?.sectionInset ?? .zero
+        layout.flowLayout?.sectionInset ?? FlowLayoutConstants.defaultInset
     }
 
     public func minimumLineSpacing(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGFloat {
-        (layout as? UICollectionViewFlowLayout)?.minimumLineSpacing ?? 10
+        layout.flowLayout?.minimumLineSpacing ?? FlowLayoutConstants.defaultMinimumLineSpacing
     }
 
     public func minimumInteritemSpacing(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGFloat {
-        (layout as? UICollectionViewFlowLayout)?.minimumInteritemSpacing ?? 10
+        layout.flowLayout?.minimumInteritemSpacing ?? FlowLayoutConstants.defaultMinimumInteritemSpacing
     }
 
     public func referenceSizeForHeader(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.headerReferenceSize ?? .zero
+        layout.flowLayout?.headerReferenceSize ?? FlowLayoutConstants.defaultHeaderSize
     }
 
     public func referenceSizeForFooter(
         using layout: UICollectionViewLayout,
         in context: CollectionViewContext
     ) -> CGSize {
-        (layout as? UICollectionViewFlowLayout)?.footerReferenceSize ?? .zero
+        layout.flowLayout?.footerReferenceSize ?? FlowLayoutConstants.defaultFooterSize
+    }
+}
+
+extension UICollectionViewLayout {
+    var flowLayout: UICollectionViewFlowLayout? {
+        self as? UICollectionViewFlowLayout
     }
 }

SectionKit/Sources/Utility/CompositionalLayout/NSCollectionLayoutSection+Empty.swift

@@ -0,0 +1,18 @@
+import UIKit
+
+@available(iOS 13.0, *)
+extension NSCollectionLayoutSection {
+    static let empty: NSCollectionLayoutSection = {
+        let layoutSize = NSCollectionLayoutSize(
+            widthDimension: .absolute(0),
+            heightDimension: .absolute(0)
+        )
+        return NSCollectionLayoutSection(
+            group: .vertical(
+                layoutSize: layoutSize,
+                subitem: .init(layoutSize: layoutSize),
+                count: 1
+            )
+        )
+    }()
+}

SectionKit/Sources/Utility/CompositionalLayout/SectionKitCompositionalLayout.swift

@@ -0,0 +1,50 @@
+import UIKit
+
+/// This compositional layout is designed for the section kit.
+/// It ensures that the layout provider utilizes the layout section provided by the compositional layout section controller.
+@MainActor
+@available(iOS 13.0, *)
+final public class SectionKitCompositionalLayout: UICollectionViewCompositionalLayout {
+    // Use internally to bridge the sections from the adapter to the section provider.
+    var sections: (() -> [Section])?
+
+    public init() {
+        var sections: (() -> [Section])?
+        super.init { index, environment in
+            guard let sections = sections?() else {
+                assertionFailure("The section update closure doesn't set up correctly, please use the `CollectionViewAdapter`")
+                return .empty
+            }
+            guard sections.count > index else {
+                assertionFailure("Section index out of bound")
+                return .empty
+            }
+            guard case .compositionalLayout(let provider) = sections[index].controller.layoutProvider else {
+                assertionFailure("Please set the layout provider with `CompositionalLayoutProvider`")
+                return .empty
+            }
+            return provider.layoutSection(environment)
+        }
+        sections = { [weak self] in
+            self?.sections?() ?? []
+        }
+    }
+
+    @available(*, unavailable)
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+
+    @available(*, unavailable)
+    override init(
+        sectionProvider: @escaping UICollectionViewCompositionalLayoutSectionProvider,
+        configuration: UICollectionViewCompositionalLayoutConfiguration
+    ) {
+        fatalError("init(sectionProvider:configuration:) has not been implemented")
+    }
+
+    @available(*, unavailable)
+    override init(sectionProvider: @escaping UICollectionViewCompositionalLayoutSectionProvider) {
+        fatalError("init(sectionProvider:) has not been implemented")
+    }
+}