Fetch data from CoreData for iOS 14 widget - swift

Question

I want to display data fetched from Core Data in a widget. But #FetchRequest doesn’t work on widgets.
As I understand, we have to create an app group and make a shared persistent container.
What I want to know is how to read (fetch) data on widgets from that shared persistent container or simply, how to display data fetched from Core Data in widgets.

Answer 1

First you need to create an AppGroup which will be used to create a Core Data Persistent Container (here is a good explanation how to do it)
Then you need to create your own CoreData stack (an example can be found when you create a new empty project with CoreData enabled).
Accessing Core Data Stack in MVVM application
Assuming you have already created your Core Data model (here called DataModel), you now need to set the container url to your custom shared container location:
Share data between main App and Widget in SwiftUI for iOS 14
let containerURL = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: <your_app_group>)!
let storeURL = containerURL.appendingPathComponent("DataModel.sqlite")
let description = NSPersistentStoreDescription(url: storeURL)
let container = NSPersistentContainer(name: "DataModel")
container.persistentStoreDescriptions = [description]
container.loadPersistentStores { ... }
Now you can get the managedObjectContext from your shared Persistent Container:
let moc = CoreDataStack.shared.managedObjectContext
and perform a fetch request with it (more information here)
let predicate = NSPredicate(format: "attribute1 == %#", "test")
let request = NSFetchRequest<SomeItem>(entityName: "SomeItem")
let result = try moc.fetch(request)
Apart from all the links above I recommend you also read this tutorial about Core Data:
Core Data with SwiftUI Tutorial: Getting Started
Here is a GitHub repository with different Widget examples including the Core Data Widget.

Answer 2

For people who did all the work above, and finally can get the connection to your Core Data (e.g. you can get the count of request), but can't fetch the request, is mostly because that the entity you're fetching contains transformable type, and for some reason this error occurred: Cannot decode object of class, try fix this.