Freezes during scroll in UITableView during parsing JSON data - ios

Question

I'm struggling with multithreading in news app. The thing is - my application freezes often when I scroll table view after data was parsed and loaded and its way too often. I think I'm some kind of wrong of reloading data every time.
First part:
final let urlString = "http://api.to.parse"
Here I create array of structs to fill in my data
struct jsonObjects {
var id : Int
var date : String
var title : String
var imageURL : URL
}
var jsonData = [jsonObjects]()
Here's my viewDidLoad of tableView
override func viewDidLoad() {
super.viewDidLoad()
// MARK : - Download JSON info on start
JsonManager.downloadJsonWithURL(urlString: urlString, сompletion: {(jsonArray) -> Void in
guard let data = jsonArray else { print("Empty dude"); return;}
for jsonObject in data {
if let objectsDict = jsonObject as? NSDictionary {
guard
let id = objectsDict.value(forKey: "id") as? Int,
let date = objectsDict.value(forKey: "date") as? String,
let titleUnparsed = objectsDict.value(forKey: "title") as? NSDictionary,
let title = (titleUnparsed as NSDictionary).value(forKey: "rendered") as? String,
let imageString = objectsDict.value(forKey: "featured_image_url") as? String,
let imageURL = NSURL(string: imageString) as URL?
else {
print("Error connecting to server")
return
}
There I go with appending filled structure to array:
self.jsonData.append(jsonObjects(id: id, date: date, title: title,
imageURL: imageURL))
}
}
DispatchQueue.main.async(execute: {
self.tableView.reloadData()
})
})
and downloadJsonWithURL is simply:
class JsonManager {
class func downloadJsonWithURL(urlString: String, сompletion: #escaping (NSArray?) -> Void) {
guard let url = NSURL(string: urlString) else { print("There is no connection to the internet"); return;}
URLSession.shared.dataTask(with: url as URL, completionHandler: { (data, response, error) -> Void in
guard let parseData = data else { print("There is no data"); return;}
if let jsonObj = try? JSONSerialization.jsonObject(with: parseData, options: .allowFragments)
as? NSArray {
сompletion(jsonObj)
}
}).resume()
}
And finally - I input that in my TableViewCell:
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return jsonData.count
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
guard let cell = tableView.dequeueReusableCell(withIdentifier: "newscell") as? NewsTableViewCell else {
fatalError("Could not find cell by identifier")
}
guard let imageData = NSData(contentsOf: jsonData[indexPath.row].imageURL) else {
fatalError("Could not find image")
}
cell.newsTitleLabel.text = self.jsonData[indexPath.row].title
cell.newsTitleLabel.font = UIFont.boldSystemFont(ofSize: 20.0)
cell.newsImageView.image = UIImage(data: imageData as Data)
return cell
}
So there are two questions: how should I distribute my threads and how should I call them so that I have smooth and nice tableview with all downloaded data? and how should I reload data in cell?

Answer 1

Your issue is caused by the imageData its blocking the main thread. The best way to solve this is to download all the images into an image cache. And I would most certainly remove the downloading of images from within the cellForRowAtIndexPath.

Answer 2

Downloading data, parsing in background thread, the updating the UI on main-thread.
Basically if you do correctly like this, everything will be okay.
So you may need to double check one more time if you are rendering UI on main-thread.
On the debugging panel, there's pause/play button.
So whenever your app frozen, try to pause the app immediately:
1) Then check if any of your UI method is running on background-thread.
2) Check if your downloading task or parsing json doing on main-thread.
If it falls under above cases, it needs to be correct.