SwiftUI and functions - Cannot find 'viewContext' in scope

I want to separate my functions from SwiftUI Views. I have this function:

import Foundation
import CoreData

func deleteAll() {
    let fetchRequestItems: NSFetchRequest<NSFetchRequestResult> = Item.fetchRequest()
    let deleteRequestItems = NSBatchDeleteRequest(fetchRequest: fetchRequestItems)
    
    do {
        try viewContext.save()
        try viewContext.execute(deleteRequestItems)
        viewContext.reset()
    } catch let error as NSError {
        print(error)
    }
}

I am getting an error on 3 lines with viewContext:

Cannot find 'viewContext' in scope

The same function works when it's inside a SwiftUI View. But the views have viewContext injected like this:

ContentView()
.environment(\.managedObjectContext, persistenceController.container.viewContext)

So the question is: How to inject/use Core Data Context (viewContext) inside functions in separate Swift files?

EDIT: Persistence.swift is from a SwiftUI 2.0 template and looks like this:

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        for _ in 0..<10 {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()
        }
        
        do {
            try viewContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
        return result
    }()

    let container: NSPersistentCloudKitContainer

    init(inMemory: Bool = false) {
        container = NSPersistentCloudKitContainer(name: "Moneto")
        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                /*
                Typical reasons for an error here include:
                * The parent directory does not exist, cannot be created, or disallows writing.
                * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                * The device is out of space.
                * The store could not be migrated to the current model version.
                Check the error message to determine what the actual problem was.
                */
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
    }
}