How to detect the end of loading of UITableView

Swift

UITableView

iOS Development

Loading Detection

Mobile App Development

How to detect the end of loading of UITableView

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Introduction

Detecting when a UITableView has finished loading all its cells is needed for tasks like scrolling to a specific row, running animations, taking snapshots, or measuring layout. The challenge is that UITableView loads cells lazily and reloadData() returns immediately before the cells are rendered. There is no built-in delegate method for "loading complete," but several reliable techniques exist.

Method 1: layoutIfNeeded() After reloadData()

Force a synchronous layout after reloading:

swift

1tableView.reloadData()
2tableView.layoutIfNeeded()
3
4// Table is now fully laid out — safe to scroll or measure
5tableView.scrollToRow(at: IndexPath(row: 0, section: 0), at: .top, animated: false)

layoutIfNeeded() forces the table view to lay out all visible cells immediately. This is the simplest approach but blocks the main thread until layout completes.

Method 2: Dispatch to Next Run Loop

Use DispatchQueue.main.async to execute code after the current layout cycle:

swift

1tableView.reloadData()
2
3DispatchQueue.main.async {
4    // Executes after tableView has finished loading visible cells
5    print("Table view finished loading")
6    self.scrollToBottom()
7}

This works because reloadData() schedules layout on the current run loop, and DispatchQueue.main.async executes after that layout pass completes.

Method 3: CATransaction Completion Block

Wrap reloadData() in a CATransaction to get a completion callback:

swift

1CATransaction.begin()
2CATransaction.setCompletionBlock {
3    // Called after all animations and layout from reloadData() complete
4    print("Table view loading complete")
5    self.tableView.scrollToRow(
6        at: IndexPath(row: self.items.count - 1, section: 0),
7        at: .bottom,
8        animated: true
9    )
10}
11
12tableView.reloadData()
13
14CATransaction.commit()

This is the most reliable approach — CATransaction captures all implicit animations triggered by reloadData() and calls the completion block when they finish.

Method 4: willDisplay / didEndDisplaying

Track cell display events to detect when loading completes:

swift

1class ViewController: UIViewController, UITableViewDelegate {
2    private var cellsBeingDisplayed = 0
3
4    func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell,
5                    forRowAt indexPath: IndexPath) {
6        cellsBeingDisplayed += 1
7    }
8
9    func tableView(_ tableView: UITableView, didEndDisplaying cell: UITableViewCell,
10                    forRowAt indexPath: IndexPath) {
11        cellsBeingDisplayed -= 1
12    }
13}

A more practical approach — detect when the last row is displayed:

swift

1func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell,
2                forRowAt indexPath: IndexPath) {
3    let lastSection = tableView.numberOfSections - 1
4    let lastRow = tableView.numberOfRows(inSection: lastSection) - 1
5
6    if indexPath.section == lastSection && indexPath.row == lastRow {
7        print("Last cell is about to be displayed")
8        // Note: this triggers for the last VISIBLE cell, not necessarily the last data item
9    }
10}

Method 5: Custom ReloadData with Completion

Create a UITableView extension that provides a completion handler:

swift

1extension UITableView {
2    func reloadData(completion: @escaping () -> Void) {
3        CATransaction.begin()
4        CATransaction.setCompletionBlock(completion)
5        reloadData()
6        CATransaction.commit()
7    }
8}
9
10// Usage
11tableView.reloadData {
12    print("Loading complete!")
13    self.tableView.scrollToRow(
14        at: IndexPath(row: 0, section: 0),
15        at: .top,
16        animated: true
17    )
18}

This extension is the cleanest API for handling post-reload actions.

Method 6: performBatchUpdates Completion

For targeted updates (insert, delete, reload sections):

swift

1tableView.performBatchUpdates({
2    tableView.insertRows(at: newIndexPaths, with: .automatic)
3    tableView.deleteRows(at: removedIndexPaths, with: .automatic)
4}) { finished in
5    if finished {
6        print("Batch updates complete")
7        self.scrollToBottom()
8    }
9}

performBatchUpdates provides a built-in completion handler — no workarounds needed.

Method 7: KVO on contentSize

Observe the table view's content size changes:

swift

1class ViewController: UIViewController {
2    private var observation: NSKeyValueObservation?
3
4    override func viewDidLoad() {
5        super.viewDidLoad()
6
7        observation = tableView.observe(\.contentSize, options: [.new]) { [weak self] tableView, change in
8            guard let newSize = change.newValue else { return }
9            print("Content size changed: \(newSize)")
10            // Content size stabilizes after loading
11        }
12    }
13
14    deinit {
15        observation?.invalidate()
16    }
17}

This fires multiple times during loading. Debounce or check for stability if you need a single "done" signal.

Detecting End of Pagination Loading

For infinite scroll / load-more patterns:

swift

1func scrollViewDidScroll(_ scrollView: UIScrollView) {
2    let offsetY = scrollView.contentOffset.y
3    let contentHeight = scrollView.contentSize.height
4    let frameHeight = scrollView.frame.height
5
6    // Trigger load when user scrolls near the bottom
7    if offsetY > contentHeight - frameHeight - 100 {
8        loadMoreData()
9    }
10}
11
12func loadMoreData() {
13    guard !isLoading else { return }
14    isLoading = true
15
16    fetchNextPage { [weak self] newItems in
17        guard let self = self else { return }
18        self.items.append(contentsOf: newItems)
19
20        let indexPaths = (self.items.count - newItems.count..<self.items.count)
21            .map { IndexPath(row: $0, section: 0) }
22
23        self.tableView.performBatchUpdates({
24            self.tableView.insertRows(at: indexPaths, with: .automatic)
25        }) { _ in
26            self.isLoading = false
27        }
28    }
29}

Common Pitfalls

reloadData() is asynchronous: It returns immediately but cells are not rendered yet. Never access cell properties or scroll right after reloadData() without using one of the techniques above.
willDisplay fires for recycled cells: willDisplay is called every time a cell becomes visible, not just during initial load. Guard against duplicate logic if using this for one-time setup.
Multiple reloadData() calls: Rapid consecutive reloadData() calls may cause the completion block to fire for an intermediate state. Debounce or cancel previous operations.
Empty table views: If the data source returns 0 rows, willDisplay is never called. Handle the empty state separately.
Background thread: reloadData() and all UIKit calls must happen on the main thread. Calling from a background thread causes undefined behavior or crashes.

Summary

Use CATransaction with a completion block for the most reliable post-reload callback
Use the UITableView extension reloadData(completion:) for a clean API
Use DispatchQueue.main.async after reloadData() for a simple one-liner
Use performBatchUpdates for insert/delete operations — it has a built-in completion handler
Use willDisplay delegate to detect when the last visible cell is rendered
Never access cells or scroll immediately after reloadData() — cells are not rendered yet

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How to detect the end of loading of UITableView

Interview Questions practice on Codemia

Introduction

Method 1: layoutIfNeeded() After reloadData()

Method 2: Dispatch to Next Run Loop

Method 3: CATransaction Completion Block

Method 4: willDisplay / didEndDisplaying

Method 5: Custom ReloadData with Completion

Method 6: performBatchUpdates Completion

Method 7: KVO on contentSize

Detecting End of Pagination Loading

Common Pitfalls

Summary

Related reading

Tackling System Design Interview Problems

Track what you have practised

Interview Questions practice on Codemia