Provider.swift

struct Provider: TimelineProvider {
    // …
    
    func getTimeline(in context: Context, completion: @escaping (Timeline<Entry>) -> ()) {
        let plants = loadPlants()
        let entry = PlantEntry(date: Date(), plants: plants)
        let timeline = Timeline(entries: [entry], policy: .atEnd)
        completion(timeline)
    }
    
    private func loadPlants() -> [PlantRepresentation] {
        do {
            let data = try Data(contentsOf: Self.sharedDataFileURL)
            let plantDictionaries = try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data) as? [[String: Any]]
            let decoder = JSONDecoder()
            decoder.dateDecodingStrategy = .secondsSince1970
            return plantDictionaries?.compactMap({ PlantRepresentation(dictionary: $0, decoder: decoder) }) ?? []
        }
        catch {
            print(error.localizedDescription)
            return []
        }
    }
    
    private static var sharedDataFileURL: URL {
        let identifier = "group.com.company.appname.widget"
        if let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: identifier) {
            return url.appendingPathComponent("Plants.plist")
        }
        else {
            preconditionFailure("Expected a valid app group container")
        }
    }
}

// DictionaryDecodable: https://augmentedcode.io/2019/05/12/storing-struct-in-userdefaults/
struct PlantRepresentation: Identifiable, Decodable, DictionaryDecodable {
    let id: String
    let name: String
    let lastWateringDate: Date
    let nextWateringDate: Date
}

struct PlantEntry: TimelineEntry {
    let date: Date
    let plants: [PlantRepresentation]
}