An example of how to easily use previews with SwiftUI views that query core data.

CoreDataPreviewProvider.swift

#if DEBUG
import CoreData
import SwiftUI

/// A protocol for allowing easy use of CoreData
/// with preview provider.
///
/// NOTE: you must still
/// make preview comply with PreviewProvider:
/// ```
/// struct OurPreview: CoreDataPreviewProvider, PreviewProvider {
///     static var body: some View {
///         // your view code
///     }
///
///     static func setupDb(_ context: NSManagedObjectContext) {
///         // setup db
///     }
/// }
/// ```
///
/// This helper is also unnecessary if you are passing in core data objects manually
/// instead of trying to query them using a fetched results controller.
protocol CoreDataPreviewProvider {
    associatedtype Body: View

    /// CoreDataPreviewProvider calls body to generate
    /// previews method and automatically attach core data context to
    /// environment
    static var body: Body { get }

    /// A clean contextObject will be sent to setupDB and can be
    /// used to seed records for sake of previewing
    ///
    /// Example usage:
    /// ```
    ///     func setupDb(_ context: NSManagedObjectContext) {
    ///         let user = User(context: Context)
    ///         user.name = "Joe"
    ///
    ///         try! context.save()
    ///     }
    ///
    static func setupDb(_ context: NSManagedObjectContext)
}

extension CoreDataPreviewProvider {
    /// Don't override this method. Instead set body static method and
    /// it will be called and setup with core data.
    static var previews: some View {
        let context = buildContextWithSetupData()
        return body.environment(\.managedObjectContext, context)
    }

    private static func buildContextWithSetupData() -> NSManagedObjectContext {
        let context = cleanPreviewContext()

        setupDb(context)

        return context
    }

    /// Iterates over all entities and destroys records, as well as resetting context and returning it
    private static func cleanPreviewContext() -> NSManagedObjectContext {
        let container = (UIApplication.shared.delegate as! AppDelegate).persistentContainer
        let context = container.viewContext
        context.reset()
        try! container.managedObjectModel.entities.forEach { entity in
            let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entity.name!)
            let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
            context.reset()
            try context.execute(deleteRequest)
        }

        return context
    }
}
#endif