WidgetPlantProvider.swift

final class WidgetPlantProvider {
    private var cancellables = [AnyCancellable]()
    private let plantContainer: PlantContainer // NSPersistentContainer subclass
    
    init(plantContainer: PlantContainer, notificationCenter: NotificationCenter = .default) {
        self.plantContainer = plantContainer
        
        let notificationCancellable = notificationCenter.publisher(for: .NSManagedObjectContextDidSave, object: plantContainer.viewContext).sink { [weak self] _ in
            self?.reloadData()
        }
        cancellables.append(notificationCancellable)
    }
    
    // Called when NSPersistentContainer is first loaded
    func reloadData() {
        plantContainer.performBackgroundTask { context in
            let descriptors = [NSSortDescriptor(keyPath: \Plant.nextWateringDate, ascending: true)]
            // fetchDictionaries is convenience method which creates and executes NSFetchRequest<NSDictionary> and sets resultType = .dictionaryResultType
            let dictionaries = Plant.fetchDictionaries(context, sortDescriptors: descriptors, fetchLimit: 3) as! [[String: Any]]
                                              
            // NSDate -> double conversion
            let converted = dictionaries.map { (plantDictionary) -> [String: Any] in
                return plantDictionary.mapValues { (value) -> Any in
                    guard let date = value as? Date else { return value }
                    return date.timeIntervalSince1970
                }
            }
            do {
                let needsFileReload: Bool = {
                    guard let storedData = try? Data(contentsOf: Self.sharedDataFileURL) else { return true }
                    guard let storedDictionaries = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(storedData) as? [NSDictionary] else { return true }
                    return storedDictionaries != converted as [NSDictionary]
                }()
                if !needsFileReload {
                    os_log(.debug, log: .widget, "Plants already up to date for widget")
                    return
                }
                let data = try NSKeyedArchiver.archivedData(withRootObject: converted, requiringSecureCoding: true)
                try data.write(to: Self.sharedDataFileURL)
                os_log(.debug, log: .widget, "Reloading widget because plants changed")
                WidgetCenter.shared.reloadAllTimelines()
            }
            catch {
                os_log(.debug, log: .widget, "Failed updating plants for widget with error %s", error.localizedDescription)
            }
        }
    }
}