CoreData iOS macOS Swift WidgetKit

Sharing data from CoreData storage with a Widget on iOS

WWDC’20 introduced WidgetKit which is a new framework for building widgets on iOS, iPadOS, and macOS. Widgets provide a quick way for displaying content from your app either on the home screen on iOS or on the notification center on macOS. As I have an iOS app which stores data with CoreData then let’s see what it takes to share it with a widget. Note that we’ll only concentrate on sharing data between the app and the widget. For adding a widget to an existing project I would recommend taking a look at Apple’s excellent article: “Creating a Widget Extension”.

Configuring the project for sharing data with the widget

The project I have is an iOS app which keeps track of plants. Therefore, we’ll look into providing plants to a simple widget which just displays one of the plants which needs to be watered next. CoreData store contains all the plants with previous and next watering date. As widgets are meant to be lightweight extensions to your app we’ll aim at passing the minimum amount of data to the widget. WidgetKit does not provide a connectivity framework like WatchOS does because widgets are not running all the time. Therefore we will store data in a file and write the file to a shared container which the app and the widget can access. This can be done by adding app groups capability to both targets. The group name could be something like When this is set, then the url to a shared container can be created using FileManager like shown below.

extension WidgetPlantProvider {
static let sharedDataFileURL: URL = {
let appGroupIdentifier = ""
if let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: appGroupIdentifier) {
return url.appendingPathComponent("Plants.plist")
else {
preconditionFailure("Expected a valid app group container")
Creating an URL for shared file.

Updating the shared file

The iOS app has a class named WidgetPlantProvider which is responsible of updating the shared file and letting WidgetKit know when the data has changed. This class uses NSPersistentContainer for accessing CoreData storage and fetches dictionary representations of Plant entities. As those dictionaries contain NSDate objects then we’ll need to convert dates to double values which represent dates as seconds from the year of 1970. This enables us to archive the list of dictionaries to a data object with NSKeyedArchiver and writing the data object into the shared container. Last step is letting WidgetKit to know that timelines should be reloaded because data has changed. The implementation of the class is available below including observing managed object save notification.

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
// 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 = { (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")
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")
catch {
os_log(.debug, log: .widget, "Failed updating plants for widget with error %s", error.localizedDescription)
WidgetPlantProvider which stores Plant entities as dictionaries in the shared file.

Reading the shared file in the widget

Reading the file in the widget requires us to create an URL pointing at the shared container, reading the data, and converting the data to a list of plants. As the data contains a list of dictionary objects then we can take advantage of JSONDecoder and convert dictionaries to PlantRepresentation value type. PlantRepresentation struct conforms to Codable protocol which enables converting dictionary object to a JSON data representation and then decoding the JSON data to a value type. Date properties are represented as seconds from the year of 1970, then JSONDecoder’s dateDecodingStrategy must be set to DateDecodingStrategy.secondsSince1970. This approach of converting dictionary to a value type is discussed in detail in “Storing struct in UserDefaults”. An example provider type with described approach is available below.

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)
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 {
return []
private static var sharedDataFileURL: URL {
let identifier = ""
if let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: identifier) {
return url.appendingPathComponent("Plants.plist")
else {
preconditionFailure("Expected a valid app group container")
// DictionaryDecodable:
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]
view raw Provider.swift hosted with ❤ by GitHub
Timeline provider for a Widget which reads the data from the shared file.


We went through the steps of setting up app groups and sharing data in CoreData store with a widget. Next steps would be to use the timeline and polishing the Widget’s appearance.

If this was helpful, please let me know on Mastodon@toomasvahter or Twitter @toomasvahter. Feel free to subscribe to RSS feed. Thank you for reading.

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