Why does UITableView cache cells when reloading data? - ios

Question

If you were to call tableView.reloadData() on a table with custom UITableViewCell, then it will cache those cells when you reload data. (It is discussed here: https://github.com/nicklockwood/FXForms/issues/92)
If you continue reloading data, after a while, there could be 30/40 cells that are cached and retained. My question is: why is that?
In particular:
1) What possible usage could there be to retain cells that have since been lost after reloading the table?
2) What could be done to prevent/reduce the memory cache storing the cells?
3) Most importantly, why does this happen? Why would cells be retained when reloadData() is called?
Example
To give you a basic example in code, click on one of the cells, then, when you see the objects in memory, there are more retained cells:
ViewController.Swift
import UIKit
class ViewController: UIViewController {
let tableViewController = QuizTableViewController(style: .grouped)
override func viewDidLoad() {
super.viewDidLoad()
self.setupTableViewController()
}
private func setupTableViewController() {
self.view.addSubview(tableViewController.view)
let topAnchor = self.tableViewController.view.topAnchor.constraint(equalTo: self.view.topAnchor)
let leadingAnchor = self.tableViewController.view.leadingAnchor.constraint(equalTo: self.view.leadingAnchor)
let trailingAnchor = self.tableViewController.view.trailingAnchor.constraint(equalTo: self.view.trailingAnchor)
let bottomAnchor = self.tableViewController.view.bottomAnchor.constraint(equalTo: self.view.bottomAnchor)
NSLayoutConstraint.activate([topAnchor, leadingAnchor, trailingAnchor, bottomAnchor])
}
}
QuizTableViewController.Swift
import UIKit
class QuizTableViewController: UITableViewController {
override init(style: UITableViewStyle) {
super.init(style: style)
self.view.translatesAutoresizingMaskIntoConstraints = false
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 5
}
override func numberOfSections(in tableView: UITableView) -> Int {
return 1
}
override func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
return 60
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = QuizCell(subText: "HELLO WORLD")
cell.textLabel?.text = "THIS IS WHAT IS DISPLAYED"
return cell
}
override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
tableView.reloadData()
}
}
QuizCell.Swift
import UIKit
class QuizCell: UITableViewCell {
weak var subText: UILabel?
init(subText: String) {
self.subText = UILabel()
self.subText?.text = subText
super.init(style: .default, reuseIdentifier: "QuizCell")
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
deinit {
print("This is called after the cache is full.")
}
}

Answer 1

The problem is that you are not using the proper approach to creating table cells. You are explicitly creating a new instance of QuizCell in the cellForRowAt. What you should be doing is dequeueing a reusable cell using dequeueReusableCell. Then your issue goes away because the table will only keep the minimum number of cells needed.
Please find any one of the countless tutorials on using table views for proper examples.

Answer 2

Like #rmaddy mentioned, the problem is how you are dequeuing the cell in cellForRowAt function. Instead, change it to something similar to:
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "QuizCell", for: indexPath) as! QuizCell
cell.textLabel?.text = "THIS IS WHAT IS DISPLAYED"
return cell
}