visible thumbnail tracker

gistfile1.txt

//
//  Blehhhh.swift
//  Khan Academy
//
//  Created by Mike Parker on 11/2/15.
//  Copyright © 2015 Khan Academy. All rights reserved.
//

import UIKit
import KHACore


public class Blehhhh: NSObject {
	/// Emits thumbnails as they become available for content item identifiers which have been designated as visible with setVisibleContentItems(:). Always emits an initial value, but values may trail behind the cells that are currently visible (as thumbnails are fetched and rendered).
	public var thumbnailMappingSignal: RACSignal/*<Box<[ContentItemIdentifier: UIImage]>>*/ { return visibleContentItemIdentifierToThumbnailsSubject }

	private let thumbnailStore: ThumbnailStore
	private let preferredThumbnailSize: CGSize

	private var visibleContentItems: Set<ContentItem> = []
	private let visibleContentItemIdentifierToThumbnailsSubject = RACReplaySubject/*<Box<[ContentItemIdentifier: UIImage]>>*/(capacity: 1)
	private var visibleContentItemIdentifierToThumbnails = [ContentItemIdentifier: UIImage]()

	private var contentItemIdentifierToThumbnailSubscriptions = [ContentItemIdentifier: RACDisposable]()

	init(thumbnailStore: ThumbnailStore, preferredThumbnailSize: CGSize) {
		self.thumbnailStore = thumbnailStore
		self.preferredThumbnailSize = preferredThumbnailSize
	}

	deinit {
		visibleContentItemIdentifierToThumbnailsSubject.sendCompleted()
	}

	public func setVisibleContentItems(visibleContentItems: Set<ContentItem>, contentItemsAndTopicTreePaths: [(ContentItem, TopicTreePath)]) {
		// TODO(andy): I'd like to use aggregateWithStart, but RAC chokes because Box isn't an NSObject?! Revisit with RAC 3.
		let previousVisibleContentItems = self.visibleContentItems
		let addedContentItems = visibleContentItems.subtract(previousVisibleContentItems)
		let removedContentItems = previousVisibleContentItems.subtract(visibleContentItems)
		self.visibleContentItems = visibleContentItems

		let contentItemsToTopicTreePaths = Dictionary(elements: contentItemsAndTopicTreePaths)

		for addedContentItem in addedContentItems {
			let addedContentItemID = addedContentItem.contentItemID

			let subscription = self.thumbnailStore.thumbnailSignalForContentItem(addedContentItem, topicTreePath: contentItemsToTopicTreePaths[addedContentItem].flatMap { $0 }, size: preferredThumbnailSize)
				.deliverOn(RACScheduler.mainThreadScheduler())
				.takeUntil(rac_willDeallocSignal())
				.subscribeNext { [unowned self] in
					let image = $0 as! UIImage
					self.visibleContentItemIdentifierToThumbnails[addedContentItemID] = image
					self.visibleContentItemIdentifierToThumbnailsSubject.sendNext(Box(self.visibleContentItemIdentifierToThumbnails))
			}
			contentItemIdentifierToThumbnailSubscriptions[addedContentItemID] = subscription
		}

		for removedContentItem in removedContentItems {
			let removedContentItemID = removedContentItem.contentItemID
			visibleContentItemIdentifierToThumbnails[removedContentItemID] = nil
			contentItemIdentifierToThumbnailSubscriptions[removedContentItemID]?.dispose()
			contentItemIdentifierToThumbnailSubscriptions[removedContentItemID] = nil
		}
	}
}