问为什么URLSession.DataTask.shared不能正常工作？

EN

import Foundation
import UIKit

class QuizService {
    let baseUrl = "https://iosquiz.herokuapp.com/api/quizzes"

    func fetchQuizzes(completion: @escaping (([Quiz]?) -> Void)) -> Void {
        if let url = URL(string: baseUrl) {
            let request = URLRequest(url: url)

            let dataTask = URLSession.shared.dataTask(with: request) { (data, response, error) in
                if let data = data {
                    do {
                        let json = try JSONSerialization.jsonObject(with: data, options: [])
                        if let resultsList = json as? [String: Any], let results = resultsList["quizzes"] as? [[String: Any]] {
                            let quizzes = results.map({ json -> Quiz? in
                                print(json)
                                if
                                    let title = json["title"] as? String,
                                    let id = json["id"] as? Int,
                                    let level = json["level"] as? Int,
                                    let description = json["description"] as? String,
                                    let category = json["category"] as? String,
                                    let questions = json["questions"] as? [String: Any],
                                    let imageUrl = json["image"] as? String {
                                    let  quiz=Quiz(id:id,title:title,descript:description,category:category,level:level,imageUrl:imageUrl,questions:questions)
                                    return quiz
                                } else {
                                    return nil
                                }
                            }).filter { $0 != nil } .map { $0! }
                            completion(quizzes)
                        } else {
                            completion(nil)
                        }
                    } catch {
                        completion(nil)
                    }
                } else {
                    completion(nil)
                }
            }
            dataTask.resume()
        } else {
            completion(nil)
        }

    }
}