What considerations do you need when writing a UITableViewController which shows images downloaded from a remote server?

When writing a UITableViewController that shows images downloaded from a remote server, several key considerations must be taken into account to ensure efficient and smooth performance. Here are the main points to consider:

1. Asynchronous Image Downloading

• Background Thread: Download images asynchronously on a background thread to avoid blocking the main UI thread, which ensures that the user can continue scrolling smoothly while images are being fetched[2][6].

2. Lazy Loading

• On-Demand Download: Only download the image when the cell is scrolled into view, i.e., when cellForRowAtIndexPath is called. This approach prevents unnecessary downloads and reduces memory usage[6][9].

3. Cell Reuse Management

• Check Cell Reuse: When the image has downloaded, check if the cell is still in view or whether it has been re-used by another piece of data. If the cell has been re-used, discard the image; otherwise, switch back to the main thread to update the image on the cell[2][6].

4. Caching

• Image Caching: Implement caching to store downloaded images. This can be done using NSCache or third-party libraries like SDWebImage. Caching helps in avoiding repeated downloads of the same image, thus improving performance and reducing data usage[3][4][5].
  • Memory and Disk Cache: Use both memory and disk cache to store images. Memory cache is faster but limited in size, while disk cache can store larger amounts of data persistently[5].

5. Placeholder Images

• Placeholder Images: Use placeholder images while the actual images are being downloaded. This provides a better user experience by indicating that content is loading[3][4].

6. Main Thread Updates

• UI Updates on Main Thread: Ensure that any UI updates, such as setting the image on the UIImageView, are performed on the main thread. This can be achieved by dispatching the update back to the main thread after the image has been downloaded[2][3].

7. Handling Errors

• Error Handling: Display an error image or a placeholder if the image download fails. This ensures that the UI remains consistent and informative even when there are network issues[3].

8. Avoiding Memory Leaks

• Weak References: Use weak references in closures to avoid retain cycles and potential memory leaks, especially when dealing with asynchronous operations and table view cells[8].

Example Code Snippet

Here is a simplified example of how these considerations can be implemented:

import UIKit
import SDWebImage

class MyTableViewController: UITableViewController {
    var imageURLs: [URL] = [] // Array of image URLs

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -...