Table View sections for fetchedResultsController objects - uitableview

Question

I'm trying to index my tableView data, ordering the names of the people listed by its name initials. I've seen lots of tutorials about doing so with genericall non-persistent data, but none with Core Data and making use of fetchedResultsController option (for iCloud auto-sync).
Now my code is as follows (for clarity purposes, I transcribe only question-related parts):
//The two arrays to populate from Core Data, used later to populate the table
var alumnosDictionaryArray = [String:[String]]()
var titulosSeccionArray = [String]()
override func viewDidLoad() {
super.viewDidLoad()
//Obtain data from Core Data
updateFetchedResultsController()
//Populate the arrays with this data
populateArrays()
func updateFetchedResultsController() {
guard let context = container?.viewContext else { return }
context.performAndWait {
let sortDescriptor = NSSortDescriptor(key: "nombre", ascending: true)
request.sortDescriptors = [sortDescriptor]
fetchedResultsController = NSFetchedResultsController<Alumno>(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
do {
try fetchedResultsController?.performFetch()
tableView.reloadData()
} catch {
print("Error obtaining data: \(error)")
}
}
}
func populateArrays() {
if let objects = fetchedResultsController?.fetchedObjects {
for alumno in objects {
let inicial = String(alumno.nombre?.prefix(1) ?? "")
if var values = alumnosDictionaryArray[inicial] {
values.append(alumno.nombre!)
alumnosDictionaryArray[inicial] = values
} else {
alumnosDictionaryArray[inicial] = [alumno.nombre!]
}
}
titulosSeccionArray = [String](alumnosDictionaryArray.keys)
titulosSeccionArray = titulosSeccionArray.sorted(by: {$0 < $1})
}
}
This part seems to work ok, as the arrays are filled correctly, as I have checked with printing statements.
Later, for table data:
override func numberOfSections(in tableView: UITableView) -> Int {
return titulosSeccionArray.count
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
let inicial = titulosSeccionArray[section]
if let values = alumnosDictionaryArray[inicial] {
return values.count
}
return 0
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath) as! AlumnoCell
if self.validateIndexPath(indexPath) {
let object = fetchedResultsController?.object(at: indexPath)
let inicial = titulosSeccionArray[indexPath.section]
if let values = alumnosDictionaryArray[inicial] {
cell.nombreField.text = values[indexPath.section]
cell.especialidadField.text = object?.especialidadRelacionada?.nombre
cell.cursoField.text = object?.cursoRelacionado?.nivel
cell.tutorField.text = object?.tutorRelacionado?.nombre
}
} else {
print("Error from indexPath")
}
return cell
}
//To validate indexPath
func validateIndexPath(_ indexPath: IndexPath) -> Bool {
if let sections = self.fetchedResultsController?.sections,
indexPath.section < sections.count {
if indexPath.row < sections[indexPath.section].numberOfObjects {
return true
}
}
return false
}
override func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
return titulosSeccionArray[section]
}
I get a more-or-less complete table with the correct sections, title of sections (initials), and number of rows for each, but it seems that only firs row works, as the rest of data are empty and throw an error when selecting any: 'no section at index 1'. In fact, I get many "Error from indexPath" messages on the console before exception throwing...
Any suggestion? Thanks.

Answer 1

For Alumno entity add 1 more column of nombreFirstChar.
Then use something like this to save this variable in database:
alumno.nombreFirstChar = nombre.firstCharacter()
Here firstCharacter helper method will be like this:
func firstCharacter() -> String {
var firstCharacter = self.first ?? "#"
if (!(firstCharacter >= "a" && firstCharacter <= "z") && !(firstCharacter >= "A" && firstCharacter <= "Z") ) {
firstCharacter = "#"
}
return String(firstCharacter).capitalized
}
Now in your fetchedResultsController,replace this line
fetchedResultsController = NSFetchedResultsController<Alumno>(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
with
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "nombreFirstChar", ascending: true)]
fetchRequest.propertiesToGroupBy = ["nombreFirstChar"]
fetchedResultsController = NSFetchedResultsController<Alumno>(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nombreFirstChar, cacheName: nil)
Then in table view delegate apis, you can use fetchedResultsController like this:
func numberOfSections(in tableView: UITableView) -> Int {
return fetchedResultsController.sections?.count ?? 0
}
func sectionIndexTitles(for tableView: UITableView) -> [String]? {
guard let fetchedResultsSections: [NSFetchedResultsSectionInfo] = channelFetchedResultsController.sections else {return nil}
return fetchedResultsSections!.map{String($0.name)}
}
func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
guard var fetchedResultsSections: [NSFetchedResultsSectionInfo] = fetchedResultsController.sections else {return nil}
return fetchedResultsSections[section].name
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
guard var sections:[NSFetchedResultsSectionInfo] = fetchedResultsController.sections, let section = sections[section] else {
return 0
}
return section.numberOfObjects
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: cellIdentifier, for: indexPath)
cell.accessoryType = .disclosureIndicator
let alumno = fetchedResultsController.object(at: indexPath)
...
return cell
}