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Type-safe value-oriented collection view data source
//
// 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
> {
/// Clients are responsible for inserting/removing items in the collection view itself.
public var sections: SectionCollection
/// Returns an adapter for this data source that its bridgeable to Objective-C.
public var bridgedDataSource: UICollectionViewDataSource { return bridgedCollectionViewDataSource }
private let cellFactory: Factory
private let bridgedCollectionViewDataSource: BridgeableCollectionViewDataSource! // 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
bridgedCollectionViewDataSource = BridgeableCollectionViewDataSource(
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 {
// This collection's index is Int, which is a RandomAccessIndexType, so this is O(1).
return countElements(sections[section])
}
private func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
return cellFactory.cellForItem(sections[indexPath.section][indexPath.row], inCollectionView: collectionView, atIndexPath: indexPath)
}
private func numberOfSections() -> Int {
// This collection's index is Int, which is a RandomAccessIndexType, so this is O(1).
return countElements(sections)
}
}
/// This separate type is necessary because CollectionViewDataSource itself is necessarily generic, so it can't be bridged to Objective-C.
@objc private class BridgeableCollectionViewDataSource: 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.
}
}
// ...
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.bridgedDataSource
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