BasicFetchedResultsControllerDelegate.swift

import UIKit
import CoreData

class BasicFetchedResultsControllerDelegate: NSObject, NSFetchedResultsControllerDelegate {
    let tableView: UITableView
    var controllerDidChangeContentCallback: (() -> Void)?
    
    /// The threshold for the number of inserts, deletes, and reloads to peform using animations. If the threshold is exceeded, the table will be reloaded using `reloadData()`. The default value is `50`.
    var numberOfRowOperationsThresholdBeforeFullReloadOccurs = 50
    
    /// The animation used for all inserts, deletes, reloads, of both rows and sections.
    var changeAnimation: UITableViewRowAnimation = .fade
    
    private var rowInserts: [IndexPath] = []
    private var rowDeletes: [IndexPath] = []
    private var rowReloads: [IndexPath] = []
    
    private var sectionInserts: [IndexSet] = []
    private var sectionDeletes: [IndexSet] = []
    
    init(tableView: UITableView, controllerDidChangeContentCallback: (() -> Void)? = nil) {
        self.tableView = tableView
        self.controllerDidChangeContentCallback = controllerDidChangeContentCallback
    }
    
    func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        clearOperationArrays()
    }
    
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
        switch type {
        case .insert:
            sectionInserts.append(IndexSet(integer: sectionIndex))
        case .delete:
            sectionDeletes.append(IndexSet(integer: sectionIndex))
        default: return
        }
    }
    
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
        switch type {
        case .insert:
            if let newIndexPath = newIndexPath {
                rowInserts.append(newIndexPath)
            }
        case .delete:
            if let indexPath = indexPath {
                rowDeletes.append(indexPath)
            }
        case .update:
            if let indexPath = indexPath {
                rowReloads.append(indexPath)
            }
        case .move:
            if let indexPath = indexPath, let newIndexPath = newIndexPath {
                rowDeletes.append(indexPath)
                rowInserts.append(newIndexPath)
            }
        }
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        let totalRowOperations = [rowInserts, rowDeletes, rowReloads].reduce(0) { (total, operations) in
            return total + operations.count
        }
        
        if totalRowOperations > numberOfRowOperationsThresholdBeforeFullReloadOccurs {
            tableView.reloadData()
        } else {
            tableView.beginUpdates()
            sectionInserts.forEach { tableView.insertSections($0, with: changeAnimation) }
            sectionDeletes.forEach { tableView.deleteSections($0, with: changeAnimation) }
            tableView.insertRows(at: rowInserts, with: changeAnimation)
            tableView.deleteRows(at: rowDeletes, with: changeAnimation)
            tableView.reloadRows(at: rowReloads, with: changeAnimation)
            tableView.endUpdates()
        }
        
        controllerDidChangeContentCallback?()
        clearOperationArrays()
    }
    
    private func clearOperationArrays() {
        rowInserts = []
        rowDeletes = []
        rowReloads = []
        
        sectionInserts = []
        sectionDeletes = []
    }
}