NSFRCChangeConsolidator.swift

//
//  NSFRCChangeConsolidator.swift
//
//  Created by Jaanus Kase on 14/05/2019.
//  Copyright © 2019 Jaanus Kase. All rights reserved.
//

import Foundation
import CoreData

/**
 This class consolidates change notifications from NSFetchedResultsController, and returns them in the right order,
 appropriate for consumption by NSTableView/UITableView.
 
 Currently, only row/item changes are implemented, since I didn’t need section change processing myself.
 If needed, section changes can be added with a similar approach.
 
 How to use this in your NSFetchedResultsController:
 - in controllerWillChangeContent, initialize an instance of this
 - in controller:didChange, call the ingestItemChange method for each changed item
 - in controllerDidChangeContent, obtain the sorted rows in the correct order and update the tableView accordingly,
    then deallocate this
 
 Based on the idea here: https://stackoverflow.com/a/56106309/49951
 */
class NSFRCChangeConsolidator {
    
    private var rowDeletes: [IndexPath] = []
    private var rowInserts: [IndexPath] = []
    private var rowUpdates: [IndexPath] = []
    
    func ingestItemChange(ofType changeType: NSFetchedResultsChangeType, oldIndexPath: IndexPath?, newIndexPath: IndexPath?) {
        switch changeType {
        case .insert:
            self.rowInserts.append(newIndexPath!)
        case .delete:
            self.rowDeletes.append(oldIndexPath!)
        case .move:
            self.rowDeletes.append(oldIndexPath!)
            self.rowInserts.append(newIndexPath!)
        case .update:
            self.rowUpdates.append(newIndexPath!)
        @unknown default:
            fatalError("Unknown change type")
        }
    }
    
    /// Reverse-sorted row deletes, suitable for feeding into table views.
    func sortedRowDeletes() -> [IndexPath] {
        return rowDeletes.sorted { $0.item > $1.item }
    }
    
    /// Sorted row inserts, suitable for feeding into table views.
    func sortedRowInserts() -> [IndexPath] {
        return rowInserts.sorted { $0.item < $1.item }
    }

    /// Sorted row updates, suitable for feeding into table views.
    func sortedRowUpdates() -> [IndexPath] {
        return rowUpdates.sorted { $0.item < $1.item }
    }
}