Optimizing Prototype Cells' Performance on iOS: A Solution Using Dispatch Queues

Understanding the Issue with Prototype Cells Shaking or Shivering on iOS

When building applications for iOS, developers often encounter issues related to optimizing performance and managing resources efficiently. In this article, we will delve into a specific problem related to scrolling cells in a UITableView instance, where prototype cells are shaking or shivering as they scroll through the table view.

What Causes Prototype Cells to Shake or Shiver?

To understand why prototype cells might be experiencing this behavior, let’s first explore what causes it. When building custom UITableViewCell instances for use within a UITableView, there are several factors that can contribute to this issue. In our case, we have identified the primary cause as the incorrect usage of UIKit methods.

In particular, when setting the image for an UIImageView within a prototype cell, if not done properly, it may result in the cell shaking or shivering. This behavior is often due to the fact that the table view is still loading and updating its content at the time the cell’s properties are being accessed, which can cause inconsistencies and issues with the cell’s appearance.

The Problem with Using `UIImage(named:)`

In our example code snippet, we noticed that using UIImage(named:) directly to set the image for the UIImageView might be causing the issue. This method is used to load an image from a bundle or other location, but it does not guarantee that the image has been fully loaded.

var imageName = UIImage(named: transportItems[indexPath.row])
cell.imageView?.image = imageName

By using this method directly, we are essentially trying to set the UIImageView’s image without waiting for the image to be fully loaded. This can cause issues with the cell’s appearance, especially when scrolling.

A Better Approach Using Dispatch Queues

To resolve this issue, we need to use dispatch queues to ensure that the table view is updated on the main thread before trying to access its properties. One way to achieve this is by using dispatch_async to execute a block of code on a separate queue, and then dispatching it back to the main queue once it has completed.

let qos = Int(QOS_CLASS_USER_INITIATED.rawValue)
dispatch_async(dispatch_get_global_queue(qos, 0)) { () -> Void in
    if let imageName = UIImage(named: transportItems[indexPath.row]) {
        dispatch_async(dispatch_get_main_queue()) {
            self.cell.imageView?.image = imageName
        }
    }
}

Understanding QoS and Dispatch Queues

Before we dive deeper into the code, it’s essential to understand what QoS (Quality of Service) and dispatch queues are.

QoS is a mechanism used by Apple to prioritize certain types of tasks or processes within an application. This allows developers to optimize their app’s performance and resource usage, especially when dealing with critical or time-sensitive operations.

Dispatch queues, on the other hand, are used to execute code blocks asynchronously, allowing for more efficient use of system resources and better performance in applications that rely heavily on multiple threads or concurrent execution.

The Benefits of Using Dispatch Queues

By using dispatch queues to set the image for our UIImageView, we can ensure that:

The table view is updated on the main thread before trying to access its properties.
We avoid unnecessary computations and optimize resource usage.
Our application remains responsive, even when dealing with complex or time-consuming operations.

Example Code

Here’s an example code snippet that incorporates the suggested changes:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCellWithIdentifier("transportCell") as! UITableViewCell

    // Set the text label's text
    cell.textLabel?.text = transportItems[indexPath.row]

    // Use dispatch queues to load and display the image
    let qos = Int(QOS_CLASS_USER_INITIATED.rawValue)
    dispatch_async(dispatch_get_global_queue(qos, 0)) { () -> Void in
        if let imageName = UIImage(named: transportItems[indexPath.row]) {
            dispatch_async(dispatch_get_main_queue()) {
                // Set the image view's image
                self.cell.imageView?.image = imageName
            }
        }
    }

    return cell
}

Best Practices for Optimizing Performance

In addition to using dispatch queues effectively, there are several other best practices that can help optimize performance in our application:

Avoid excessive use of tableView.dequeueReusableCellWithIdentifier: If you’re reusing a significant number of cells, consider creating a custom table view cell class or reusing a single instance.
Use caching mechanisms when possible: Cache frequently accessed data or resources to reduce the number of requests made to external sources.
Optimize your code’s structure and organization: Break down complex logic into smaller, more manageable functions, and use clear variable names to improve readability.

Conclusion

In this article, we explored a common issue related to scrolling cells in UITableView instances and provided a solution using dispatch queues. By optimizing our code’s performance and resource usage, we can create more responsive and efficient applications that provide the best possible user experience.

Last modified on 2024-02-09