Wsrtjtyk

I have the following code which shows a list of quizzes in a UITableView. The problem is that to show the images I call my method prepareImages and I get an index out of range exception when filling the cells in tableView function because it appears that the quizzesImages array is empty (print(self.quizzesImages.count) shows 0), I know it has something to do with how I put the threads to work but I don't see where I am going wrong.

import UIKit

 // Estructura del JSON que devuelve la URL

struct ResponseObject : Codable {

let quizzes : [Quiz]?

let pageno : Int?

    let nextUrl : String?

}



class QuizzesTableViewController: UITableViewController {



// Aquí se guardan los quizzes cargados de la URL

var totalQuizzes = [Quiz]()

var quizzesImages = [UIImage]()



override func viewDidLoad() {

    super.viewDidLoad()

    self.tableView.rowHeight = 90.0

    navigationController?.navigationBar.prefersLargeTitles = true

    downloadQuizzes()

}



override func didReceiveMemoryWarning() {

    super.didReceiveMemoryWarning()

    // Dispose of any resources that can be recreated.

}



func prepareImages(){

    for i in 0...self.totalQuizzes.count-1{



        let imageUrlString = self.totalQuizzes[i].attachment?.url

        let imageUrl:URL = URL(string: imageUrlString!)!

        print(imageUrl)



        // Start background thread so that image loading does not make app unresponsive

        DispatchQueue.global(qos: .userInitiated).async {

            let imageData:NSData = NSData(contentsOf: imageUrl)!



            // When from background thread, UI needs to be updated on main_queue

            DispatchQueue.main.async {

                let image = UIImage(data: imageData as Data)

                print("hola")

                self.quizzesImages.append(image!)

            }

        }

    }



}



func downloadQuizzes(){

    let QUIZZES_URL = "https://quiz2019.herokuapp.com/api/quizzes?token=945d3bf7d4c709d69940"

    if let url = URL(string: QUIZZES_URL){

        let queue = DispatchQueue(label: "download quizzes queue")

        queue.async {

            DispatchQueue.main.async {

                UIApplication.shared.isNetworkActivityIndicatorVisible = true

            }

            defer{

                DispatchQueue.main.async {

                    UIApplication.shared.isNetworkActivityIndicatorVisible = false

                }

            }

            let data = try? Data(contentsOf: url, options: .alwaysMapped)

            let decoder = JSONDecoder()

            do{

                let response = try decoder.decode(ResponseObject.self, from: data!)

                DispatchQueue.main.async {

                    if (response.quizzes!.count != 0){

                        self.totalQuizzes.append(contentsOf: response.quizzes!)

                        self.prepareImages()

                        print(self.totalQuizzes.count)

                        print(self.quizzesImages.count)

                        self.tableView.reloadData()

                    }

                }

            }

            catch {

                print(error)

            }

        }

    }

}

// MARK: - Table view data source



override func numberOfSections(in tableView: UITableView) -> Int {

    // #warning Incomplete implementation, return the number of sections

    return 1

}



override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

    // #warning Incomplete implementation, return the number of rows

    return totalQuizzes.count

}



override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCell(withIdentifier: "Quiz", for: indexPath) as! QuizTableViewCell



    let quiz = totalQuizzes[indexPath.row]

    let images = quizzesImages[indexPath.row]

    cell.authorLabel?.text = quiz.author?.username

    cell.quizLabel?.text = quiz.question

    cell.quizImage?.image = images

    return cell

}}

Thanks in advance for the help!

See this screen shot form my playground

I tried to run your code in playground and managed to get it work with some changes please see the following.

Note

To make it run in playground i have to make some assumptions just replace your core logic with my one.

Happy Coding 😀

import UIKit

// Estructura del JSON que devuelve la URL



struct ResponseObject: Codable {

    let quizzes: [Quiz]?

    let pageno: Int?

    let nextURL: String?



    enum CodingKeys: String, CodingKey {

        case quizzes, pageno

        case nextURL = "nextUrl"

    }

}



struct Quiz: Codable {

    let id: Int?

    let question: String?

    let author: Author?

    let attachment: Attachment?

    let favourite: Bool?

    let tips: [String]?

}



struct Attachment: Codable {

    let filename: String?

    let mime: MIME?

    let url: String?

}



enum MIME: String, Codable {

    case imageJPEG = "image/jpeg"

}



struct Author: Codable {

    let id: Int?

    let isAdmin: Bool?

    let username: String?

}





class QuizzesTableViewController: UITableViewController {



    // Aquí se guardan los quizzes cargados de la URL

    var totalQuizzes = [Quiz]()

    var quizzesImages = [UIImage]()



    override func viewDidLoad() {

        super.viewDidLoad()

        self.tableView.rowHeight = 90.0

        navigationController?.navigationBar.prefersLargeTitles = true

        self.tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Quiz")

        self.tableView.dataSource = self

        self.tableView.delegate = self

        downloadQuizzes()

    }



    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }



    func prepareImages(){

        print("in prepare images")

        // Start background thread so that image loading does not make app unresponsive

        DispatchQueue.global(qos: .userInitiated).async {

            for i in 0...self.totalQuizzes.count-1 {



                let imageUrlString = self.totalQuizzes[i].attachment?.url

                let imageUrl:URL = URL(string: imageUrlString!)!

                print(imageUrl)



            let imageData:NSData = NSData(contentsOf: imageUrl)!

            let image = UIImage(data: imageData as Data)

            print("hola (i)")

            self.quizzesImages.append(image!)

            print(self.quizzesImages.count)

            // When from background thread, UI needs to be updated on main_queue

            }

            DispatchQueue.main.async {

                print(self.quizzesImages.count)

                self.tableView.reloadData()

                UIApplication.shared.isNetworkActivityIndicatorVisible = false

            }

        }

    }



    func downloadQuizzes(){

        let QUIZZES_URL = "https://quiz2019.herokuapp.com/api/quizzes?token=945d3bf7d4c709d69940"

        if let url = URL(string: QUIZZES_URL){

            let queue = DispatchQueue(label: "download quizzes queue")

            queue.async {

                DispatchQueue.main.async {

                    UIApplication.shared.isNetworkActivityIndicatorVisible = true

                }

                let data = try? Data(contentsOf: url, options: .alwaysMapped)

                let decoder = JSONDecoder()

                do{

                    let response = try decoder.decode(ResponseObject.self, from: data!)

                    DispatchQueue.main.async {

                        if (response.quizzes!.count != 0){

                            self.totalQuizzes.append(contentsOf: response.quizzes!)

                            self.prepareImages()

                            print(self.totalQuizzes.count)

                        }

                    }

                }

                catch {

                    print(error)

                }

            }

        }

    }

    // MARK: - Table view data source



    override func numberOfSections(in tableView: UITableView) -> Int {

        // #warning Incomplete implementation, return the number of sections

        return 1

    }



    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

        // #warning Incomplete implementation, return the number of rows

        print("num of rows (totalQuizzes.count)")

        return totalQuizzes.count

    }



    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        print("in cellForRowAt (indexPath.row)")

        let cell = tableView.dequeueReusableCell(withIdentifier: "Quiz")!

        let quiz = totalQuizzes[indexPath.row]

        let images = quizzesImages[indexPath.row]

        cell.textLabel?.text = "(quiz.author?.username) (quiz.question)"

        cell.imageView?.image = images

        return cell

    }}

Since you didn't say what line the exception is happening on, my best guess is that it's happening on this line.

let images = quizzesImages[indexPath.row]

The issue is in downloadQuizzes. In there you call prepareImages then a couple lines later call reloadData. Since prepareImages does an async task, it isn't blocking downloadQuizzes from finishing running. This causes reloadData to be called before the async part of prepareImages is finished.

You'll need to make sure prepareImages is finished running before calling reloadData.

Another option is to use a library like Kingfisher that will only download the images when they need to be displayed in a cell rather then downloading them all and slowing down showing the table.

Additional Note:

You shouldn't be using Data(contentsOf:) to load remote content, URLSession is the correct option for it.