[Feature] Persistence (#5)

This PR contains the work done to implement some useful protocols, classes and extensions to use to setup a persistence layer in an application.

To provide further details about the work done:
 - [x] declared the `Persistence` target in the Package file;
 - [x] forgot to declare the `Communications` and `Persistence` target to the `SwiftLibs` library in the `Package` file;
 - [x] defined the `Service` public protocol;
 - [x] implemented the `Fetcher` generic class;
 - [x] implemented the `bitBucket` static property in the `URL+Devices` public extension;
 - [x] updated the `README` file.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #5

Tests/Persistence/Helpers/NSFetchRequest+TestEntity.swift

@@ -0,0 +1,24 @@
+//
+//  NSFetchRequest+TestEntity.swift
+//  PersistenceTests
+//
+//  Created by Javier Cicchelli on 17/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import CoreData
+
+extension NSFetchRequest where ResultType == TestEntity {
+    
+    // MARK: Functions
+    
+    static func allTestEntities() -> NSFetchRequest<TestEntity> {
+        let request = TestEntity.fetchRequest()
+        
+        request.sortDescriptors = []
+        request.resultType = .managedObjectResultType
+        
+        return request
+    }
+
+}

Tests/Persistence/Helpers/TestModel.xcdatamodeld/Model.xcdatamodel/contents

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<model type="com.apple.IDECoreDataModeler.DataModel" documentVersion="1.0" lastSavedToolsVersion="21754" systemVersion="22E261" minimumToolsVersion="Automatic" sourceLanguage="Swift" userDefinedModelVersionIdentifier="">
+    <entity name="TestEntity" representedClassName="TestEntity" syncable="YES" codeGenerationType="class"/>
+</model>

Tests/Persistence/Helpers/TestPersistenceService.swift

@@ -0,0 +1,127 @@
+//
+//  TestPersistenceService.swift
+//  PersistenceTests
+//
+//  Created by Javier Cicchelli on 17/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import CoreData
+import Persistence
+
+struct TestPersistenceService {
+    
+    // MARK: Properties
+    
+    static let shared = TestPersistenceService()
+    
+    private let container: NSPersistentContainer
+    
+    var viewContext: NSManagedObjectContext {
+        container.viewContext
+    }
+    
+    // MARK: Initialisers
+    
+    init() {
+        guard
+            let modelURL = Bundle.module.url(forResource: .Model.name, withExtension: .Model.extension),
+            let managedObjectModel = NSManagedObjectModel(contentsOf: modelURL)
+        else {
+            fatalError("Could not load the Core Data model from the module.")
+        }
+        
+        self.container = .init(
+            name: .Model.name,
+            managedObjectModel: managedObjectModel
+        )
+        
+        setContainer()
+    }
+
+}
+
+// MARK: - Service
+
+extension TestPersistenceService: Service {
+    
+    // MARK: Functions
+    
+    func makeTaskContext() -> NSManagedObjectContext {
+        let taskContext = container.newBackgroundContext()
+        
+        taskContext.automaticallyMergesChangesFromParent = true
+        taskContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
+        
+        return taskContext
+    }
+    
+    func makeChildContext() -> NSManagedObjectContext {
+        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
+        
+        context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
+        context.parent = container.viewContext
+        context.automaticallyMergesChangesFromParent = true
+        
+        return context
+    }
+    
+    func save(context: NSManagedObjectContext) throws {
+        guard context.hasChanges else {
+            return
+        }
+        
+        try context.save()
+    }
+    
+    func save(childContext context: NSManagedObjectContext) throws {
+        guard context.hasChanges else {
+            return
+        }
+        
+        try context.save()
+        
+        guard
+            let parent = context.parent,
+            parent == container.viewContext
+        else {
+            return
+        }
+
+        try parent.performAndWait {
+            try parent.save()
+        }
+    }
+    
+}
+
+// MARK: - Helpers
+
+private extension TestPersistenceService {
+    
+    // MARK: Functions
+    
+    func setContainer() {
+        container.persistentStoreDescriptions = [
+            .init(url: .bitBucket)
+        ]
+        
+        container.loadPersistentStores { _, error in
+            if let error = error as NSError? {
+                fatalError("Unresolved error \(error), \(error.userInfo)")
+            }
+        }
+        
+        container.viewContext.automaticallyMergesChangesFromParent = false
+    }
+
+}
+
+// MARK: - String+Constants
+
+private extension String {
+    enum Model {
+        static let name = "TestModel"
+        static let `extension` = "momd"
+    }
+}