Implemented the Fetcher class.

Sources/Persistence/Classes/Fetcher.swift

@@ -0,0 +1,154 @@
+//
+//  Fetcher.swift
+//  Persistence
+//
+//  Created by Javier Cicchelli on 16/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import Combine
+import CoreData
+
+/// This class fetches objects from a given managed object context and it notifies of changes in the object fetched if any.
+public class Fetcher<Model: NSManagedObject>: NSObject, NSFetchedResultsControllerDelegate {
+    
+    // MARK: Properties
+    
+    /// The publisher that emits the changes detected to the Location entities in a given object context.
+    public let didChangePublisher = PassthroughSubject<[Change], Never>()
+
+    private let fetchedResultsController: NSFetchedResultsController<Model>
+    
+    /// The number of sections in the data.
+    public var numberOfSections: Int {
+        fetchedResultsController.sections?.count ?? 0
+    }
+    
+    private var inProgressChanges: [Change] = []
+
+    // MARK: Initialisers
+    
+    /// Initialises the fetcher give the given parameters.
+    /// - Parameters:
+    ///   - fetchRequest: The fetch request to use to get the objects.
+    ///   - managedObjectContext: The managed object context against the fetch request is executed.
+    ///   - sectionNameKeyPath: A key path on result objects that returns the section name.
+    ///   - cacheName: The name of the cache file the receiver should use.
+    public init(
+        fetchRequest: NSFetchRequest<Model>,
+        managedObjectContext: NSManagedObjectContext,
+        sectionNameKeyPath: String? = nil,
+        cacheName: String? = nil
+    ) {
+        self.fetchedResultsController = .init(
+            fetchRequest: fetchRequest,
+            managedObjectContext: managedObjectContext,
+            sectionNameKeyPath: sectionNameKeyPath,
+            cacheName: cacheName
+        )
+        
+        super.init()
+        
+        self.fetchedResultsController.delegate = self
+    }
+    
+    // MARK: Functions
+    
+    /// Perform the fetching.
+    public func fetch() throws {
+        try fetchedResultsController.performFetch()
+    }
+    
+    /// Retrieve the number of objects in a given section number.
+    /// - Parameter section: The section number to inquiry about.
+    /// - Returns: A number of objects in the given section number.
+    public func numberOfObjectsInSection(_ section: Int) -> Int {
+        guard
+            let sections = fetchedResultsController.sections,
+            sections.endIndex > section
+        else {
+            return 0
+        }
+        
+        return sections[section].numberOfObjects
+    }
+    
+    /// Retrieve an object out of a given index path.
+    /// - Parameter indexPath: The index path to use to retrieve an object.
+    /// - Returns: A `NSManagedObject` entity positioned in the given index path.
+    public func object(at indexPath: IndexPath) -> Model {
+        return fetchedResultsController.object(at: indexPath)
+    }
+
+    // MARK: NSFetchedResultsControllerDelegate
+    
+    public func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
+        inProgressChanges.removeAll()
+    }
+    
+    public func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
+        didChangePublisher.send(inProgressChanges)
+    }
+    
+    public func controller(
+        _ controller: NSFetchedResultsController<NSFetchRequestResult>,
+        didChange sectionInfo: NSFetchedResultsSectionInfo,
+        atSectionIndex sectionIndex: Int,
+        for type: NSFetchedResultsChangeType
+    ) {
+        if type == .insert {
+            inProgressChanges.append(.section(.inserted(sectionIndex)))
+        } else if type == .delete {
+            inProgressChanges.append(.section(.deleted(sectionIndex)))
+        }
+    }
+    
+    public func controller(
+        _ controller: NSFetchedResultsController<NSFetchRequestResult>,
+        didChange anObject: Any,
+        at indexPath: IndexPath?,
+        for type: NSFetchedResultsChangeType,
+        newIndexPath: IndexPath?
+    ) {
+        switch type {
+        case .insert:
+            guard let newIndexPath else { return }
+            
+            inProgressChanges.append(.object(.inserted(at: newIndexPath)))
+        case .delete:
+            guard let indexPath else { return }
+            
+            inProgressChanges.append(.object(.deleted(from: indexPath)))
+        case .move:
+            guard let indexPath, let newIndexPath else { return }
+            
+            inProgressChanges.append(.object(.moved(from: indexPath, to: newIndexPath)))
+        case .update:
+            guard let indexPath else { return }
+            
+            inProgressChanges.append(.object(.updated(at: indexPath)))
+        default:
+            break
+        }
+    }
+
+}
+
+// MARK: - Enumerations
+
+public enum Change: Hashable {
+    public enum SectionUpdate: Hashable {
+        case inserted(Int)
+        case deleted(Int)
+    }
+    
+    public enum ObjectUpdate: Hashable {
+        case inserted(at: IndexPath)
+        case deleted(from: IndexPath)
+        case updated(at: IndexPath)
+        case moved(from: IndexPath, to: IndexPath)
+    }
+    
+    case section(SectionUpdate)
+    case object(ObjectUpdate)
+}