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Advanced Swift Sample Code
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| import Foundation | |
| struct Episode: Identifiable, Codable { | |
| var id: String | |
| var poster_url: URL | |
| var collection: String | |
| // ... | |
| static let url = URL(string: "https://talk.objc.io/episodes.json")! | |
| } | |
| struct Collection: Identifiable, Codable { | |
| var id: String | |
| var title: String | |
| static let url = URL(string: "https://talk.objc.io/collections.json")! | |
| } | |
| struct UnknownError: Error { } | |
| // Load the first poster image (for which you need to load the ST data first), with cancellation support | |
| /// A thread-safe cancellation token. | |
| final class Cancellable { | |
| private var _isCancelled: Bool | |
| private var onCancel: (() -> Void)? | |
| private let queue = DispatchQueue(label: "Cancellable") | |
| init(onCancel: @escaping () -> Void) { | |
| self._isCancelled = false | |
| self.onCancel = onCancel | |
| } | |
| var isCancelled: Bool { | |
| queue.sync { _isCancelled } | |
| } | |
| func cancel() { | |
| queue.async { [self] in | |
| _isCancelled = true | |
| onCancel?() | |
| onCancel = nil | |
| } | |
| } | |
| } | |
| func loadFirstPosterContCancellable(_ completion: @escaping (Result<Data, Error>) -> ()) -> Cancellable { | |
| let session = URLSession.shared | |
| var outerTask: URLSessionDataTask? = nil | |
| var innerTask: URLSessionDataTask? = nil | |
| let cancellable = Cancellable { | |
| outerTask?.cancel() | |
| innerTask?.cancel() | |
| } | |
| outerTask = session.dataTask(with: Episode.url) { data, _, err in | |
| do { | |
| guard let d = data else { | |
| throw (err ?? UnknownError()) | |
| } | |
| let episodes = try JSONDecoder().decode([Episode].self, from: d) | |
| // Check for cancellation again after JSON decoding. | |
| // This is necessary because innerTask == nil at this point. | |
| guard !cancellable.isCancelled else { | |
| throw CancellationError() | |
| } | |
| innerTask = session.dataTask(with: episodes[0].poster_url, completionHandler: { data, _, err in | |
| completion(Result { | |
| guard let d = data else { | |
| throw (err ?? UnknownError()) | |
| } | |
| return d | |
| }) | |
| }) | |
| innerTask!.resume() | |
| } catch { | |
| completion(.failure(error)) | |
| } | |
| } | |
| outerTask!.resume() | |
| return cancellable | |
| } |
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| // Sample code for Advanced Swift | |
| import Foundation | |
| struct Episode: Identifiable, Codable { | |
| var id: String | |
| var poster_url: URL | |
| var collection: String | |
| // ... | |
| static let url = URL(string: "https://talk.objc.io/episodes.json")! | |
| } | |
| struct Collection: Identifiable, Codable { | |
| var id: String | |
| var title: String | |
| static let url = URL(string: "https://talk.objc.io/collections.json")! | |
| } | |
| struct UnknownError: Error { } | |
| func loadPosterImagesCont(for episodes: [Episode], _ completion: @escaping (Result<[Episode.ID: Data], Error>) -> ()) { | |
| DispatchQueue.global().async { | |
| // Access to shared state from multiple threads must be synchronized. | |
| var childResults: [Result<(Episode.ID, Data), Error>] = [] | |
| let resultQueue = DispatchQueue(label: "protect state") | |
| // DispatchGroup allows the "parent task" to wait for the "child tasks". | |
| let group = DispatchGroup() | |
| let session = URLSession.shared | |
| for episode in episodes { | |
| group.enter() | |
| let task = session.dataTask(with: episode.poster_url) { imageData, _, error in | |
| resultQueue.sync { | |
| defer { group.leave() } | |
| if let imageData = imageData { | |
| childResults.append(.success((episode.id, imageData))) | |
| } else { | |
| childResults.append(.failure(error ?? UnknownError())) | |
| } | |
| } | |
| } | |
| task.resume() | |
| } | |
| // Wait for "child tasks" to complete. | |
| group.notify(queue: resultQueue) { | |
| completion(Result { | |
| try childResults.reduce(into: [:]) { (dict, pair) in | |
| let (episodeID, imageData) = try pair.get() | |
| dict[episodeID] = imageData | |
| } | |
| }) | |
| } | |
| } | |
| } |
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