NSManagedObjectContext extension methods to delete all managed objects, or all objects of a given type in the context.

DeleteManagedObjectsExtension.swift

//
//  Created by Collin Donnell on 7/22/15.
//  Copyright (c) 2015 Collin Donnell. All rights reserved.
//

import CoreData

extension NSManagedObjectContext {

    convenience init(parentContext parent: NSManagedObjectContext, concurrencyType: NSManagedObjectContextConcurrencyType) {
        self.init(concurrencyType: concurrencyType)
        parentContext = parent
    }
    
    func deleteAllObjects(error: NSErrorPointer) {
        
        if let entitesByName = persistentStoreCoordinator?.managedObjectModel.entitiesByName as? [String: NSEntityDescription] {
            
            for (name, entityDescription) in entitesByName {
                deleteAllObjectsForEntity(entityDescription, error: error)
                
                // If there's a problem, bail on the whole operation.
                if error.memory != nil {
                    return
                }
            }
        }
    }
    
    func deleteAllObjectsForEntity(entity: NSEntityDescription, error: NSErrorPointer) {
        let fetchRequest = NSFetchRequest()
        fetchRequest.entity = entity
        fetchRequest.fetchBatchSize = 50
        
        let fetchResults = executeFetchRequest(fetchRequest, error: error)

        if error.memory != nil {
            return
        }
        
        if let managedObjects = fetchResults as? [NSManagedObject] {
            for object in managedObjects {
                deleteObject(object)
            }
        }
    }
    
}

Didn't work with Swift 2

an updated version that works as of Swift 5.3, thanks for the scaffolding:

extension NSManagedObjectContext {

    func deleteAllObjects() throws {
        guard let entitesByName = persistentStoreCoordinator?.managedObjectModel.entitiesByName else { return }
    
        for (_, entityDescription) in entitesByName {
            try deleteAllObjects(for: entityDescription)
        }
    }

    func deleteAllObjects(for entity: NSEntityDescription) throws {
        let fetchRequest = NSFetchRequest<NSFetchRequestResult>()
        fetchRequest.entity = entity
    
        let fetchResults = try fetch(fetchRequest)
    
        if let managedObjects = fetchResults as? [NSManagedObject] {
            for object in managedObjects {
                delete(object)
            }
        }
    }
}