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 group.com.company.appname.widget. When this is set, then the url to a shared container can be created using FileManager like shown below.
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.
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.
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.