How to get DateSectionTitles using FetchedResultsController in Swift 4

Question

I am using FRC and I want to create sections that groups data according to date (DD MMMM)! Each task has a date and i am using that date and formatting it for the section header titles.

I am using Apple's sample code that they have provided in Objective C and converted it in to swift. Here is the code:

import UIKit
import CoreData

class completedNotes: NSManagedObject
{
    @NSManaged var cNote: String
    @NSManaged var cPriorityColor: UIColor
    @NSManaged var cDate: Date

}


extension completedNotes {
var sectionIdentifier : String? {
    // Create and cache the section identifier on demand.

    self.willAccessValue(forKey: "sectionIdentifier")
    var tmp = self.primitiveValue(forKey: "sectionIdentifier") as? String
    self.didAccessValue(forKey: "sectionIdentifier")

    if tmp == nil {
        if let timeStamp = self.value(forKey: "cDate") as? NSDate {
            /*
             Sections are organized by month and year. Create the section
             identifier as a string representing the number (year * 1000) + month;
             this way they will be correctly ordered chronologically regardless
             of the actual name of the month.
             */
            let calendar  = NSCalendar.current

            let components = calendar.dateComponents([.year, .month,], from: timeStamp as Date)
            tmp = String(format: "%ld", components.year! * 1000 + components.month!)
            self.setPrimitiveValue(tmp, forKey: "sectionIdentifier")
        }
    }
    return tmp
}
}

The above code is in NSManagedObject where I am creating a transient property sectionIdentifier. It takes value of cDate which is NSManagedObject.

Then in titlesforheaderinSection

 func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    struct Formatter {
        static let formatter : DateFormatter = {
            let fmt = DateFormatter()
            let dateFormat = DateFormatter.dateFormat(fromTemplate: "MMMM yyyy", options: 0,
                                                      locale: NSLocale.current)
            fmt.dateFormat = dateFormat
            return fmt
        }()
    }

    if let theSection = fetchedResultsController1.sections?[section] as? NSFetchedResultsSectionInfo,
        let numericSection = Int(theSection.name) {
        var components = DateComponents()
        components.year = numericSection / 1000
        components.month = numericSection % 1000
        if let date = Calendar.current.date(from: components) {
            let titleString = Formatter.formatter.string(from: date)
            return titleString
        }
    }
    return nil

}

This is the code I am using for other tableview functions:

func numberOfSections(in tableView: UITableView) -> Int {
    return (fetchedResultsController1.sections?.count)!
}

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return fetchedResultsController1.sections![section].numberOfObjects
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: cellid, for: indexPath) as! TasksTableViewCell
    cell.selectionStyle = .none

    cell.textview.text = fetchedResultsController1.object(at: indexPath).cNote
    cell.priorityline.backgroundColor = fetchedResultsController1.object(at: indexPath).cPriorityColor

    return cell
}

This is NSFetchedResultsController:

let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.moContext1, sectionNameKeyPath: "sectionIdentifier", cacheName: nil)
    aFetchedResultsController.delegate = self
    _fetchedResultsController = aFetchedResultsController as? NSFetchedResultsController<completedNotes>

Why am I not able to get any sections to show up in the tableview and why when I remove the last tableview cell, the app crashes?

Thanks!

Answer 1

The key path passed to the sectionNameKeyPath: argument of the fetched results controller must be visible to the Objective-C runtime. In Swift 4 that requires an explicit annotation with @objc (compare also How can I deal with @objc inference deprecation with #selector() in Swift 4?):

extension completedNotes {
    @objc var sectionIdentifier : String? { 
        // ...
    }
}

Without that, the section name key path is silently ignored.