Type-safe value-oriented collection view data source

CollectionViewDataSource.swift

//
//  CollectionViewDataSource.swift
//  Khan Academy
//
//  Created by Andy Matuschak on 10/14/14.
//  Copyright (c) 2014 Khan Academy. All rights reserved.
//

import UIKit

/// A type which can produce and configure a cell for a given item.
public protocol CollectionViewCellFactoryType {
    typealias Item
    typealias Cell: UICollectionViewCell
    func cellForItem(item: Item, inCollectionView collectionView: UICollectionView, atIndexPath indexPath: NSIndexPath) -> Cell
}

/// A concrete cell factory which makes use of UICollectionView's built-in cell reuse queue.
public struct RegisteredCollectionViewCellFactory<Cell: UICollectionViewCell, Item>: CollectionViewCellFactoryType {
    private let reuseIdentifier: String
    private let cellConfigurator: (Cell, Item) -> ()
    
    /// You must register Cell.Type with your collection view for `reuseIdentifier`.
    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 this reuse identifier for Cell.Type.
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as! Cell
        cellConfigurator(cell, item)
        return cell
    }
}

/// A type-safe collection view data source. Clients can specify its sections and their contents via any CollectionType. Configuration of the cells is delegated to an external factory type.
/// Use `bridgedDataSource` to get a UICollectionViewDataSource instance.
public class CollectionViewDataSource<
    SectionCollection: CollectionType,
    Factory: CollectionViewCellFactoryType,
    Item
    where
    SectionCollection.Index == Int,
    SectionCollection.Generator.Element: CollectionType,
    SectionCollection.Generator.Element.Generator.Element == Item,
    SectionCollection.Generator.Element.Index == Int,
    Factory.Item == Item
>: NSObject, UICollectionViewDataSource {
    
    /// Clients are responsible for inserting/removing items in the collection view itself.
    public var sections: SectionCollection
    private let cellFactory: Factory
    
    public init(sections: SectionCollection, cellFactory: Factory) {
        self.sections = sections
        self.cellFactory = cellFactory

        super.init()
    }
    
    final public func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return sections.count
    }
    
    final public func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return sections[section].count
    }
    
    final public func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        return cellFactory.cellForItem(sections[indexPath.section][indexPath.row], inCollectionView: collectionView, atIndexPath: indexPath)
    }

}

Usage

// ...

let cellFactory = RegisteredCollectionViewCellFactory(reuseIdentifier: "Topic") { (cell: TopicCatalogCell, topic: Topic) in
	cell.topic = topic
}
dataSource = CollectionViewDataSource(sections: [topics], cellFactory: cellFactory)

topicCollectionView = UICollectionView(frame: CGRect(), collectionViewLayout: topicCollectionViewLayout)
topicCollectionView.dataSource = dataSource

Updated for Swift 2