Combining two NSPredicates with independent NSSortDescriptors in Swift - arrays

Question

I am using CoreData and Swift 3, but I think this is more of a general question about NSPredicates, NSSortDescriptors, and NSFetchedResultsController...
I have an entity with a integer attribute that I want to sort on, but I need to split the results based on a passed in integer and sort the two halves independently in descending order and then combine them. All the entities with the attribute <= the splitting value go at the front (descending), and all the entities that are > the splitting value go at the end (descending).
Here's an example:
Normal fetched results sorted by the integer attribute in descending order:
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
Given a splitting value of "5", the final result should be:
[5, 4, 3, 2, 1, 10, 9, 8, 7, 6]
Or with a splitting value of 8, the final result should be:
[8, 7, 6, 5, 4, 3, 2, 1, 10, 9]
What I am doing is creating a roll-over point, the way days a month roll-over at 28, 29, 30, or 31. In my case the roll-over point is dynamic, and I am not working with days.
I cannot figure out how to do this with predicates and sort descriptors. Is this even possible?
I have read about doing multiple fetches, using a "sortOrder" attribute in the entity. Basically, fetching once, the putting the results in an array, sorting the array the way I want, then setting the "sortOrder" attribute on every entity, saving the context, then fetching again, sorting by "sortOrder". I can do that, but I was hoping for something more elegant.
Here's some code that does the proper sorting on an array:
func rolloverSort(withArray: [Int], andIndex index: Int) -> [Int] {
let inputArray = withArray.sorted()
var newArray = [Int]()
var splitIndex: Int = 0
for item in inputArray {
if item <= index {
newArray.insert(item, at: 0)
splitIndex += 1
}
if item > index {
newArray.insert(item, at: splitIndex)
}
}
return newArray
}
The result will be driving a UITableView, so I need to do this within the context of an NSFetchedResultContoller, which is why I was thinking of predicates and sort descriptors.
Here is my NSFetchedResultsController and some surrounding context:
// sortMethod (used in sortDescriptor) is set via an action sheet button.
// Currently it's just a string for the entity attribute to sort on.
// The closure on each button sets self._fetchedRequestController = nil,
// sets sortMethod to the desired sorting string, then calls
// tableView.reloadData().
var fetchedResultsController: NSFetchedResultsController<Entity> {
if _fetchedResultsController != nil {
return _fetchedResultsController!
}
let fetchRequest: NSFetchRequest<Entity> = Entity.fetchRequest()
fetchRequest.fetchBatchSize = 20
let sortDescriptor = NSSortDescriptor(key: sortMethod, ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
let aFetchedResultsController = NSFetchedResultsController(
fetchRequest: fetchRequest,
managedObjectContext: managedObjectContext!,
sectionNameKeyPath: nil,
cacheName: "Master")
aFetchedResultsController.delegate = self
_fetchedResultsController = aFetchedResultsController
do {
try _fetchedResultsController!.performFetch()
} catch {
// FIXME: Replace error handling stub.
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
return _fetchedResultsController!
}
var _fetchedResultsController: NSFetchedResultsController<Card>? = nil
That works with simple sorting, but I am struggling to figure out how to slot in the sorting function from A.Jam.

Answer 1

Lets say you have a data store with 10 entities called Number already inserted. The model entity has one attribute called value of type Int32 which you want to sort on (the values in your data store range from 1 to 10).
Use this function to split your data based on your key value and sort them using an instance of NSSortDescriptor:
func fetchSortEntites(basedOn integer: Int32) -> ([Int32], [Number]){
var sortedNumberArray: [Int32] = []
var sortedManagedObjects: [Number] = []
// Phase 1
let predicate1 = NSCompoundPredicate(format: "value <= %ld", integer)
let sortDescriptor1 = NSSortDescriptor(key: "value", ascending: false)
let fetchRequest1 = NSFetchRequest<Number>(entityName: "Number")
fetchRequest1.predicate = predicate1
fetchRequest1.sortDescriptors = [sortDescriptor1]
do{
let managedObjects = try managedObjectContext.fetch(fetchRequest1)
for managedObject in managedObjects{
sortedNumberArray.append(managedObject.value)
sortedManagedObjects.append(managedObject)
}
} catch let error as NSError {
print(error.debugDescription)
fatalError("*** Failed to fetch managed objects from the context!")
}
// Phase 2
let predicate2 = NSCompoundPredicate(format: "value > %ld", integer)
let sortDescriptor2 = NSSortDescriptor(key: "value", ascending: false)
let fetchRequest2 = NSFetchRequest<Number>(entityName: "Number")
fetchRequest2.predicate = predicate2
fetchRequest2.sortDescriptors = [sortDescriptor2]
do{
let managedObjects = try managedObjectContext.fetch(fetchRequest2)
for managedObject in managedObjects{
sortedNumberArray.append(managedObject.value)
sortedManagedObjects.append(managedObject)
}
} catch let error as NSError {
print(error.debugDescription)
fatalError("*** Failed to fetch managed objects from the context!")
}
return (sortedNumberArray, sortedManagedObjects)
}
}
Result:
let myTuple = fetchSortEntites(basedOn: 5)
for number in myTuple.0{
print (number)
}
let myTuple = fetchSortEntites(basedOn: 8)
for number in myTuple.0{
print (number)
}
Hope this helped!

Answer 2

You can create wrapper around two NSFetchedResultsController (next resultsController).
First resultsController should fetch values before (<=) the threshold and second resultsController should fetch values after (>) the threshold.
After that wrapper can return required data as union of datas from the requestControllers.
For example:
//Get count cells
int countCells = resultsController1.sections.first.numberOfObjects + resultsController2.sections.first.numberOfObjects;
<...>
//Get value for cell
int value = 0;
if (cellIndex >= <counts_in_first_resultsController>)
{
value = <Get_from_the_second>;
}
else
{
value = <Get_from_the_first>;
}