ios - TableView 的 JSON 数据太慢

Question

我在 TableView 中显示 JSON 数据，但应用程序速度很慢。它在显示前等待 5-10 秒。我在 Xcode 输出中收到此错误:

2017-06-05 12:49:47.505 Ekstraders[1167:137558] This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.

---

 Stack:(
    0   CoreFoundation                      0x000000010ad78b0b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x0000000108020141 objc_exception_throw + 48
    2   CoreFoundation                      0x000000010ade1625 +[NSException raise:format:] + 197
    3   Foundation                          0x0000000107d1917b _AssertAutolayoutOnAllowedThreadsOnly + 105
    4   Foundation                          0x0000000107d18f0f -[NSISEngine _optimizeWithoutRebuilding] + 61
    5   Foundation                          0x0000000107b487e6 -[NSISEngine optimize] + 108
    6   Foundation                          0x0000000107d16ef4 -[NSISEngine performPendingChangeNotifications] + 84
    7   UIKit                               0x00000001085b5412 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1787
    8   QuartzCore                          0x000000010dce6904 -[CALayer layoutSublayers] + 146
    9   QuartzCore                          0x000000010dcda526 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 370
    10  QuartzCore                          0x000000010dcda3a0 _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
    11  QuartzCore                          0x000000010dc69e92 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 294
    12  QuartzCore                          0x000000010dc96130 _ZN2CA11Transaction6commitEv + 468
    13  QuartzCore                          0x000000010dc9642c _ZN2CA11Transaction14release_threadEPv + 214
    14  libsystem_pthread.dylib             0x000000010c065413 _pthread_tsd_cleanup + 544
    15  libsystem_pthread.dylib             0x000000010c06514d _pthread_exit + 152
    16  libsystem_pthread.dylib             0x000000010c0636d1 pthread_attr_getschedpolicy + 0
    17  libsystem_pthread.dylib             0x000000010c0630f1 start_wqthread + 13
)

ViewController.swift:

import UIKit


class FirstViewController: UIViewController,UITableViewDataSource,UITableViewDelegate {

    @IBOutlet weak var tableView: UITableView!

    var tbCount = 0

    var myMp3s = [Mp3]()


    override func viewDidLoad() {
        super.viewDidLoad()


        tableView.dataSource = self
        tableView.delegate = self


        let url = URL(string: "example.php")

         DispatchQueue.main.async() { //JSON to Mp3 object.


        URLSession.shared.dataTask(with:url!, completionHandler: {(data, response, error) in
            guard let data = data, error == nil else { return }


            do {
                let json = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as! [String:Any]
                let posts = json["server_response"] as? [[String: Any]] ?? []

                for i in posts {

                    print(i["id"]!)

                    let mp3 = Mp3(id: i["id"] as! String, category: i["kategori"] as! String)

                    self.myMp3s.append(mp3)


                }

                self.tableView.reloadData()
                self.tableView.tableFooterView = UIView(frame: .zero)

                print("counte", self.myMp3s.count)


            } catch let error as NSError {
                print(error)
            }
        }).resume()

        }

    }



    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return myMp3s.count

    }
    func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {

        var height:CGFloat = CGFloat()

        height = 100
        return height
    }


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



        let cell  = UITableViewCell()
        cell.backgroundColor =  UIColor.darkGray



        //Design 
        cell.textLabel?.text = self.myMp3s[indexPath.row].category 

        cell.textLabel?.backgroundColor = .red
        cell.textLabel?.layer.borderWidth = 3
        cell.textLabel?.layer.borderColor = UIColor.lightGray.cgColor
        cell.textLabel?.heightAnchor.constraint(equalToConstant: 50).isActive = true
        cell.textLabel?.widthAnchor.constraint(equalToConstant: 200).isActive = true


        cell.textLabel?.layer.cornerRadius = 10
        cell.textLabel?.clipsToBounds = true
        cell.textLabel?.centerYAnchor.constraint(equalTo: cell.centerYAnchor).isActive = true
        cell.textLabel?.centerXAnchor.constraint(equalTo: cell.centerXAnchor).isActive = true

        cell.textLabel?.textAlignment = .center
        cell.textLabel?.textColor = .white
        cell.textLabel?.font = UIFont.boldSystemFont(ofSize: 16.0)

        return cell


    }


    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        print(myMp3s[indexPath.row].category)
    }


    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}

我可以在 tableview 中显示 JSON 数据，没问题，但问题是它太慢了。它正在等待显示。我怎么解决这个问题？我哪里错了？

Answer 1

URLSession.shared.dataTask 正在后台线程中执行完成处理程序。这会导致意外问题，因为影响 UI 的操作必须由主线程完成。

因此，您应该使用 DispatchQueue.main.async {} 包装这些部分:

DispatchQueue.main.async {
  self.tableView.reloadData()
  self.tableView.tableFooterView = UIView(frame: .zero)
}