A class written to provide data for my implementation of the Friends app, for the Days 60-61 challenge in #100DaysOfSwiftUI

DataProvider.swift

//
//  DataProvider.swift
//  Friends
//
//  Created by Russell Gordon on 2020-12-26.
//

import CoreData
import Foundation
import SwiftUI

// BASED ON: https://developer.apple.com/documentation/coredata/loading_and_displaying_a_large_data_feed
class DataProvider {
    
    init() {
        
        // Count records on a background thread
        let backgroundManagedObjectContext = PersistenceController.shared.container.newBackgroundContext()
        backgroundManagedObjectContext.performAndWait {
            
            // Access User entity
            let fetchRequest : NSFetchRequest<User> = User.fetchRequest()
            do {
                let itemCount = try backgroundManagedObjectContext.count(for: fetchRequest)
                if itemCount == 0 {
                    
                    // No user data exists, so go get it
                    print("No user data exists, going out to get it...")
                    fetchAndSaveUserData()
                    
                } else {
                    print("User data already exists.")
                }
                
            }
            catch let error as NSError {
                
                print("Error counting users: \(error.localizedDescription)")
                
            }

        }

    }
    
    /// Get the basic user data from Hacking with Swift site
    func fetchAndSaveUserData() {

        // 1. Prepare a URLRequest to get user data
        let url = URL(string: "https://www.hackingwithswift.com/samples/friendface.json")
        var request = URLRequest(url: url!)
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "GET"
        
        // 2. Run the request and process the response
        URLSession.shared.dataTask(with: request) { data, response, error in
            
            // handle the result here – attempt to unwrap optional data provided by task
            guard let unwrappedData = data else {
                
                // Show the error message
                print("No data in response: \(error?.localizedDescription ?? "Unknown error")")
                
                return
            }
            
            // DEBUG: Inspect contents of JSON file retrieved from the server
            print(String(data: unwrappedData, encoding: .utf8)!)
            
            // Now decode from JSON directly into Core Data managed objects
            // Do this on a background thread to avoid concurrency issues
            // SEE: https://stackoverflow.com/questions/49454965/how-to-save-to-managed-object-context-in-a-background-thread-in-core-data
            // SEE: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CoreData/Concurrency.html
            // SEE: https://www.donnywals.com/using-codable-with-core-data-and-nsmanagedobject/
            let backgroundManagedObjectContext = PersistenceController.shared.container.newBackgroundContext()
            
            // Set unused undoManager to nil for macOS (it is nil by default on iOS)
            // to reduce resource requirements.
            backgroundManagedObjectContext.undoManager = nil
            
            // Set the merge policy for duplicate entries – ensure that duplicate entries get merged into a single unique record
            // "To help resolve this, Core Data gives us constraints: we can make one attribute constrained so that it must always be unique. We can then go ahead and make as many objects as we want, unique or otherwise, but as soon as we ask Core Data to save those objects it will resolve duplicates so that only one piece of data gets written."
            backgroundManagedObjectContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
            
            // Decode on a background thread
            backgroundManagedObjectContext.performAndWait {
                
                // Decode into Core Data objects
                let decoder = JSONDecoder(context: backgroundManagedObjectContext)
                if let decodedData = try? decoder.decode([User].self, from: unwrappedData) {
                    
                    print("There were \(decodedData.count) users placed into Core Data")
                                        
                } else {
                    print("Could not decode from JSON into Core Data")
                }
                                                
                // Required to actually write the changes to Core Data
                do {
                    
                    // Save Core Data objects
                    try backgroundManagedObjectContext.save()
                    
//                    // DEBUG: Count records
//                    let fetchRequest : NSFetchRequest<User> = User.fetchRequest()
//                    do {
//                        let itemCount = try backgroundManagedObjectContext.count(for: fetchRequest)
//                        print("There are \(itemCount) users now.")
//                    }
//                    catch let error as NSError {
//                        print("Error counting users: \(error.localizedDescription)")
//                    }
                    
                    // Now actually try to load the avatars
                    self.fetchAndSaveAvatars()
                    
                } catch {
                    fatalError("Failure to save context: \(error)")
                }
                                
            }
            
        }.resume()

    }
    
    /// Get avatars for all the users just loaded
    func fetchAndSaveAvatars() {
        
        // We know there should be exactly 100 users, so...
        for _ in 1...100 {

            // 1. Prepare a URL to fetch the image from
            let someNumber = Int.random(in: 0...99)
            let avatarAddress = "https://randomuser.me/api/portraits/women/\(someNumber).jpg"
            let url = URL(string: avatarAddress)!
            
            // 2. Run the request and process the response
            URLSession.shared.dataTask(with: url) { data, response, error in
                
                // DEBUG: Did avatar data come back?
//                print("retrieved avatar data")
                
                // Handle the result here – attempt to unwrap optional data provided by task
                guard let imageData = data else {
                    
                    // Show the error message
                    print("No data in response: \(error?.localizedDescription ?? "Unknown error")")
                    
                    return
                }
                
                // Attempt to create an instance of UIImage using the data from the server
                if let loadedImage = UIImage(data: imageData) {
                    
                    // Set up a managed object context to run on a background thread
                    let backgroundManagedObjectContext = PersistenceController.shared.container.newBackgroundContext()
                    backgroundManagedObjectContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
                    backgroundManagedObjectContext.undoManager = nil

                    // Run this block on a background thread
                    backgroundManagedObjectContext.performAndWait {
                        
                        // Build a fetch request that will get the next single user without an avatar image
                        let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
                        fetchRequest.predicate = NSPredicate(format: "avatar == nil")
                        fetchRequest.sortDescriptors = [
                            NSSortDescriptor(keyPath: \User.name, ascending: true)
                        ]
                        fetchRequest.fetchLimit = 1
                        
                        // Get the user
                        var results: [User] = []
                        do {
                            results = try backgroundManagedObjectContext.fetch(fetchRequest)
                        }
                        catch let error as NSError {
                            print("Error getting results from Users entity: \(error.localizedDescription)")
                        }
                        
                        // Get a reference to the user returned
                        // There should only be one object in the result set
                        if let user = results.first {
                            
                            // Save data to object encoded as a JPEG
                            user.avatar = loadedImage.jpegData(compressionQuality: 1.0)
                            
                            // DEBUG
//                            print("successfully retrieved and saved avatar image to core data")
                            
                            // Required to actually write the changes to Core Data
                            do {
                                
                                // Save Core Data objects
                                try backgroundManagedObjectContext.save()
                                                                                
                            } catch {
                                fatalError("Failure to save context: \(error)")
                            }
                        }
                        
                    }
                    
                } else {
                    
                    print("Couldn't load avatar image from server.")
                    
                }
                
            }.resume()
            
        }
        
    }
    
}