Trying to implement GCD and concurrency on a table view - ios

Question

Currently using Alamofire synchronously within cellForRowAtIndexPath that queries a JSON array from Heroku, and within a for loop, creates a struct from each JSON object within the JSON array with image and text properties and then appends each struct in an array property within the table view controller. Not surprising that this is really slow. On app launch, the initial VC is a container VC that either shows a navigation controller or page VC based on if the user is "logged in." The initial VC in the page VC is a container VC that holds the table VC in question.
I'm totally new to GCD and the concept of concurrency. Was wondering how I can populate my array that serves as the foundational data for each of the table view cells.
Here's my current code - changing some variable names because I signed an NDA for this project:
import UIKit
import Alamofire
import Alamofire_Synchronous
final class PopularPearsTableViewController: UITableViewController {
let screenSize: CGRect = UIScreen.main.bounds
var pears: [Pear] = []
override func viewDidLoad() {
super.viewDidLoad()
tableView.register(PopularPearTableViewCell.self, forCellReuseIdentifier: "popularPear")
tableView.rowHeight = (screenSize.height) * 0.3
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
// MARK: - Table View Data Source
extension PopularShopsTableViewController {
override func numberOfSections(in tableView: UITableView) -> Int {
return 1
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
// just a stub, will be replaced with dynamic code later on
return 5
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
print(#function)
let cell = tableView.dequeueReusableCell(withIdentifier: "popularPear", for: indexPath) as! PopularPearTableViewCell
let userDefaults = UserDefaults.standard
guard let pearUUID = userDefaults.string(forKey: "pearUUID"),
let pearToken = userDefaults.string(forKey: "pearToken")
else {
return cell
}
if indexPath.row == 0 {
let header = createAuthenticatedHeader(user: pearUUID, password: pearToken)
let pearResponse = Alamofire.request("url", headers: header).responseJSON()
if let pearsFromResponse = (pearResponse.result.value! as! JSON)["data"] as? [JSON] {
for pear in pearsFromResponse {
let name = pear["name"] as! String
let pictureURL = pear["picture_url"] as! String
let imageURL = URL(string: pictureURL)
let imageData = NSData(contentsOf: imageURL!)
let image = UIImage(data: imageData as! Data)!
let newPear = Pear(name: name, image: image)
self.pears.append(newPear)
}
}
}
cell.giveCell(pearImage: pears[indexPath.row].image, pearName: pears[indexPath.row].name)
return cell
}
}