Sketch of type safe, non-AnyObject-supporting data source

gistfile1.swift

public protocol CountableCollectionType: CollectionType {
	var count: Int { get }
}

extension Array: CountableCollectionType {}

public protocol CollectionViewCellFactoryType {
	typealias Item
	typealias Cell: UICollectionViewCell
	func cellForItem(item: Item, inCollectionView collectionView: UICollectionView, atIndexPath indexPath: NSIndexPath) -> Cell
}

public struct RegisteredCollectionViewCellFactory<Cell: UICollectionViewCell, Item>: CollectionViewCellFactoryType {
	private let reuseIdentifier: String
	private let cellConfigurator: (Cell, Item) -> ()

	public init(reuseIdentifier: String, cellConfigurator: (Cell, Item) -> ()) {
		self.reuseIdentifier = reuseIdentifier
		self.cellConfigurator = cellConfigurator
	}

	public func cellForItem(item: Item, inCollectionView collectionView: UICollectionView, atIndexPath indexPath: NSIndexPath) -> Cell {
		// Will abort if you haven't already registered a cell of type Cell with the collection view under reuseIdentifier.
		let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as Cell
		cellConfigurator(cell, item)
		return cell
	}
}

public class StaticCollectionViewDataSource<
	SectionCollection: CountableCollectionType,
	Factory: CollectionViewCellFactoryType,
	ItemType
	where
		SectionCollection.Index == Int,
		SectionCollection.Generator.Element: CountableCollectionType,
		SectionCollection.Generator.Element.Generator.Element == ItemType,
		SectionCollection.Generator.Element.Index == Int,
		Factory.Item == ItemType> {

	public var sections: SectionCollection
	private let cellFactory: Factory
	private let staticCollectionViewDataSourceBridge: StaticCollectionViewDataSourceBridge! // The bridge is initialized with a reference to self, so Swift thinks (correctly) that this could conceivably be used uninitialized.

	public init(sections: SectionCollection, cellFactory: Factory) {
		self.sections = sections
		self.cellFactory = cellFactory
		staticCollectionViewDataSourceBridge = StaticCollectionViewDataSourceBridge(
			numberOfItemsInSectionHandler: { [weak self] in self?.numberOfItemsInSection($0) ?? 0 },
			cellForItemAtIndexPathHandler: { [weak self] in self?.collectionView($0, cellForItemAtIndexPath: $1) },
			numberOfSectionsHandler: { [weak self] in self?.numberOfSections() ?? 0 })
	}

	private func numberOfItemsInSection(section: Int) -> Int {
		return sections[section].count
	}

	private func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
		let section = sections[indexPath.section]
		let item: ItemType = section[indexPath.row]
		return cellFactory.cellForItem(item, inCollectionView: collectionView, atIndexPath: indexPath)
	}

	private func numberOfSections() -> Int {
		return sections.count
	}

	public var bridgedDataSource: UICollectionViewDataSource { return staticCollectionViewDataSourceBridge }
}


@objc private class StaticCollectionViewDataSourceBridge: NSObject, UICollectionViewDataSource {
	
	private let numberOfItemsInSectionHandler: Int -> Int
	private let cellForItemAtIndexPathHandler: (UICollectionView, NSIndexPath) -> UICollectionViewCell?
	private let numberOfSectionsHandler: () -> Int

	init(numberOfItemsInSectionHandler: Int -> Int, cellForItemAtIndexPathHandler: (UICollectionView, NSIndexPath) -> UICollectionViewCell?, numberOfSectionsHandler: () -> Int) {
		self.numberOfItemsInSectionHandler = numberOfItemsInSectionHandler
		self.cellForItemAtIndexPathHandler = cellForItemAtIndexPathHandler
		self.numberOfSectionsHandler = numberOfSectionsHandler
	}

	func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
		return numberOfSectionsHandler()
	}

	func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
		return numberOfItemsInSectionHandler(section)
	}

	// The cell that is returned must be retrieved from a call to -dequeueReusableCellWithReuseIdentifier:forIndexPath:
	func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
		return cellForItemAtIndexPathHandler(collectionView, indexPath)! // Better not have the data source bridge outlive the data source itself.
	}

}