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  • Фото автораAlexey Parkhomenko

Improve UI App Performance in iOS

User interface responsiveness and reducing lag are crucial aspects of iOS app development. Users expect apps to run smoothly and respond quickly to their interactions. If your app is slow or unresponsive, users are likely to become frustrated and move on to a different app. In this blog post, we'll explore some tips and techniques for improving UI responsiveness and reducing lag in iOS apps.

Use Async/Await for Asynchronous Tasks

One of the most common causes of UI lag in iOS apps is performing long-running tasks on the main thread. This can cause the UI to freeze, making the app unresponsive. To avoid this, you should always perform long-running tasks on a background thread. You can use the Grand Central Dispatch (GCD) API to create a background thread and perform the task on that thread. However, the syntax for GCD can be complex, which is why Apple introduced the Async/Await feature in Swift 5.5. Here's an example of how to use Async/Await to perform an asynchronous task on a background thread:

func fetchData() async {
    let data = await url)
    // Process data here

In this example, we use the 'async' keyword to mark the function as asynchronous. Then we use the 'await' keyword to wait for the data to be fetched asynchronously using the 'URLSession' API. I recommend that you learn more about Async/Await in my blog post:

Use Lazy Loading for Images

Loading images can be a time-consuming task, especially if the images are large. To improve UI responsiveness, you should use lazy loading for images. This means that you should only load images when they are needed, rather than loading them all at once.

Here's an example of how to use lazy loading to load images in a 'UITableView':

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath) as! MyTableViewCell
    // Load the image asynchronously .userInitiated).async {
        let imageData = try? Data(contentsOf: imageURL)
        if let imageData = imageData {
            let image = UIImage(data: imageData)
            // Update the cell on the main thread
            DispatchQueue.main.async {
                cell.imageView?.image = image
    return cell

In this example, we load the image asynchronously using a background thread. We then update the imageView property of the cell on the main thread. This ensures that the UI remains responsive while the image is being loaded.

Use UICollectionViewDataSourcePrefetching for Collection Views

If your app uses a 'UICollectionView' to display a large number of images, you should use the 'UICollectionViewDataSourcePrefetching' protocol to prefetch the images. This means that the images are loaded in the background before they are displayed, improving the performance of the app.

Here's an example of how to use 'UICollectionViewDataSourcePrefetching' to prefetch images in a 'UICollectionView':

class MyCollectionViewController: UICollectionViewController, UICollectionViewDataSourcePrefetching {
    // Your array of image URLs to prefetch
    var imageURLs: [URL] = []
    // Your image cache to store prefetched images
    let imageCache = NSCache<NSString, UIImage>()
    override func viewDidLoad() {
        // Register your cell class or nib
        collectionView.register(MyCollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
        // Set the prefetching delegate
        collectionView.prefetchDataSource = self
    // Implement the UICollectionViewDataSourcePrefetching protocol
    func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
        // Prefetch the images asynchronously .userInitiated).async {
            for indexPath in indexPaths {
                let imageURL = self.imageURLs[indexPath.item]
                // Check if the image is already in the cache
                if let image = self.imageCache.object(forKey: imageURL.absoluteString as NSString) {
                // Load the image data asynchronously
                let imageData = try? Data(contentsOf: imageURL)
                // Create the image object
                if let imageData = imageData {
                    let image = UIImage(data: imageData)
                    // Cache the image
                    self.imageCache.setObject(image!, forKey: imageURL.absoluteString as NSString)
    // Implement the UICollectionViewDataSource protocol
    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return imageURLs.count
    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! MyCollectionViewCell
        // Set the cell's image
        let imageURL = imageURLs[indexPath.item]
        if let image = imageCache.object(forKey: imageURL.absoluteString as NSString) {
            cell.imageView.image = image
        } else {
            cell.imageView.image = nil
        return cell

In this example, we first set the 'prefetchDataSource' property of the 'UICollectionView' to the current view controller, which implements the 'UICollectionViewDataSourcePrefetching' protocol.

Then, in the 'collectionView(_:prefetchItemsAt:)' method, we use a background thread to asynchronously prefetch the images at the given 'indexPaths'. We first check if the image is already in our image cache to avoid unnecessary loading, and if not, we load the image data asynchronously and then create and cache the 'UIImage' object.

Finally, in the 'collectionView(_:cellForItemAt:)' method, we check if the image is already in our image cache and set the cell's 'imageView' property accordingly. If the image is not in the cache, we set the `imageView

Use Instruments to Profile App Performance

Xcode includes a powerful profiling tool called Instruments that can help you identify performance issues in your app. You can use Instruments to measure CPU usage, memory usage, and network activity, among other things. This can help you identify areas of your app that are causing lag and optimize them.

To use Instruments, select Product > Profile from the Xcode menu. This will launch the Instruments app. From there, you can choose a profiling template that best suits your needs, such as the Time Profiler template for measuring CPU usage.

Use Autolayout Constraints Efficiently

Autolayout constraints are a powerful tool for designing flexible and responsive UIs. However, if used incorrectly, they can cause performance issues. For example, if you have too many constraints or complex constraints, it can slow down the rendering of your UI.

To use Autolayout constraints efficiently, you should follow these best practices:

  • Use constraints sparingly. Only add constraints that are necessary for the layout.

  • Use simpler constraints whenever possible. For example, use 'leading' and 'trailing' constraints instead of 'left' and 'right' constraints.

  • Avoid ambiguous layouts. Make sure that there is only one valid layout for your UI.

  • Avoid unnecessary constraint updates. Only update constraints when the layout changes.

Here's an example of how to use Autolayout constraints efficiently:

override func viewDidLoad() {
    // Set up the Autolayout constraints
    myView.translatesAutoresizingMaskIntoConstraints = false
        myView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
        myView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
        myView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
        myView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor)

In this example, we use the 'safeAreaLayoutGuide' property of the view to set up the Autolayout constraints. We also set the 'translatesAutoresizingMaskIntoConstraints' property to 'false' to indicate that we are using Autolayout constraints.


Improving UI responsiveness and reducing lag in iOS apps is crucial for providing a good user experience. By following the tips and techniques outlined in this blog post, you can optimize your app's performance and ensure that it runs smoothly and responds quickly to user interactions. Remember to always profile your app's performance using Instruments and test it on a variety of devices to ensure that it performs well for all users.

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