Swift3: Passing parameters into NSFetchRequest method - swift3

Question

I use a general CoreData query method in my project.
func query(table: String, searchPredicate: NSPredicate) -> [AnyObject]
{
let context = app.managedObjectContext
let fetchRequest = NSFetchRequest(entityName: table)
fetchRequest.predicate = searchPredicate
let results = try! context.fetch(fetchRequest)
return results
}
In Swift 3 this doesn't work. I found this on Apple's web site:
func findAnimals()
{
let request: NSFetchRequest<Animal> = Animal.fetchRequest
do
{
let searchResults = try context.fetch(request)
... use(searchResults) ...
}
catch
{
print("Error with request: \(error)")
}
}
Using the Apple example, how would I pass Animal in to the method as a parameter to make findAnimals more generic?

Answer 1

I haven't tried this but I think something like this would work...
func findCoreDataObjects<T: NSManagedObject>() -> [T] {
let request = T.fetchRequest
do
{
let searchResults = try context.fetch(request)
... use(searchResults) ...
}
catch
{
print("Error with request: \(error)")
}
}
You have to make the entire function generic and so you have to tell it what type T is when calling it.
someObject.findCoreDataObjects<Animal>()
I think that should do the job. Not entirely certain though as I'm new to generics myself :D

Answer 2

How about this.
func query<T: NSManagedObject>(table: String, searchPredicate: NSPredicate) -> [T] {
let context = app.managedObjectContext
let fetchRequest: NSFetchRequest<T> = NSFetchRequest(entityName: table)
fetchRequest.predicate = searchPredicate
let results = try! context.fetch(fetchRequest)
return results
}

Answer 3

Here is the final result that may help someone:
import Foundation
import Cocoa
func addRecord<T: NSManagedObject>(_ type : T.Type) -> T
{
let entityName = T.description()
let context = app.managedObjectContext
let entity = NSEntityDescription.entity(forEntityName: entityName, in: context)
let record = T(entity: entity!, insertInto: context)
return record
}
func recordsInTable<T: NSManagedObject>(_ type : T.Type) -> Int
{
let recs = allRecords(T.self)
return recs.count
}
func allRecords<T: NSManagedObject>(_ type : T.Type, sort: NSSortDescriptor? = nil) -> [T]
{
let context = app.managedObjectContext
let request = T.fetchRequest()
do
{
let results = try context.fetch(request)
return results as! [T]
}
catch
{
print("Error with request: \(error)")
return []
}
}
func query<T: NSManagedObject>(_ type : T.Type, search: NSPredicate?, sort: NSSortDescriptor? = nil, multiSort: [NSSortDescriptor]? = nil) -> [T]
{
let context = app.managedObjectContext
let request = T.fetchRequest()
if let predicate = search
{
request.predicate = predicate
}
if let sortDescriptors = multiSort
{
request.sortDescriptors = sortDescriptors
}
else if let sortDescriptor = sort
{
request.sortDescriptors = [sortDescriptor]
}
do
{
let results = try context.fetch(request)
return results as! [T]
}
catch
{
print("Error with request: \(error)")
return []
}
}
func deleteRecord(_ object: NSManagedObject)
{
let context = app.managedObjectContext
context.delete(object)
}
func deleteRecords<T: NSManagedObject>(_ type : T.Type, search: NSPredicate? = nil)
{
let context = app.managedObjectContext
let results = query(T.self, search: search)
for record in results
{
context.delete(record)
}
}
func saveDatabase()
{
let context = app.managedObjectContext
do
{
try context.save()
}
catch
{
print("Error saving database: \(error)")
}
}
Call it with:
let name = "John Appleseed"
let newContact = addRecord(Contact.self)
newContact.contactNo = 1
newContact.contactName = name
let contacts = query(Contact.self, search: NSPredicate(format: "contactName == %#", name))
for contact in contacts
{
print ("Contact name = \(contact.contactName), no = \(contact.contactNo)")
}
deleteRecords(Contact.self, search: NSPredicate(format: "contactName == %#", name))
recs = recordsInTable(Contact.self)
print ("Contacts table has \(recs) records")
saveDatabase()

Answer 4

I use that way in my projects:
static func retrieveRecords<T: NSManagedObject>(table: String, sortDescriptorKey: NSSortDescriptor? = nil) -> [T] {
do {
let fetchRequest: NSFetchRequest<T> = NSFetchRequest(entityName: table)
fetchRequest.sortDescriptors = [sortDescriptorKey!]
let results = try context.fetch(fetchRequest)
print("\(results)")
return results
} catch let error {
print("Could not fetch \(error.localizedDescription)")
return []
}
}
And to call it:
personen = retrieveRecords(table: "Person", sortDescriptorKey: NSSortDescriptor(key: #keyPath(Person.nachname), ascending: true, selector: #selector(NSString.localizedCompare)))