Lazy image downloading on UITableViewCell using defer - ios

Question

Is it safe and does it make sense to defer an asynchronous image download for a cell? The idea behind this is that I want the callback function from URLSession.shared.image(...) to be executed after creating the cell and only once calling cellForRow(at: indexPath) is valid, since I think that without deferring this method at this point should return nil.
URLSession.shared.image is a private extension that runs a data task and gives a escaping callback only if the url provided in the argument is valid and contains an image.
setImage(image:animated) is a private extension that allows you to set an image in a UIImageView using a simple animation.
If defer is not the way to go, please indicate an alternative.
Any feedback is appreciate, thanks!
override func tableView(_ tableView: UITableView, cellForRowAt
indexPath: IndexPath) -> UITableViewCell {
let cell = baseCell as! MyCell
let datum = data[indexPath.row]
cell.imageView.setImage(placeholderImage, for: .normal)
defer {
URLSession.shared.image(with: datum.previewURL) { image, isCached in
if let cell = tableView.cellForRow(at: indexPath) as? MyCell {
cell.imageView.setImage(image, animated: !isCached)
}
}
}
return cell
}

Answer 1

NSHipster have a good article on how / when to use defer, here.
I wouldn't use defer in such a way. The intention for defer is to clean up / deallocate memory in one block of code, rather than scattering it across many exit points.
Think about having multiple guard statements throughout a function and having to deallocate memory in every one of them.
You shouldn't use this to simply add additional code after the fact.
As mentioned by #jagveer there are many third party libraries that do this already, such as SDWebImage cache. AFNetworking and AlamoFire also have the same built in. No need to re-invent the wheel when its already been done.

Answer 2

In this case, the defer isn't being effective. defer sets up a block to be called when the scope exits, which you are doing immediately.
I think what you want to schedule the block to run in a different thread using Dispatch. You need to get back onto the main thread to update the UI.
As this can happen later, you need to make sure the cell is still being used for the same entry and has not been reused as the user has scrolled further. Fetching the cell again isn't a good idea if it has been reused as you'd end up triggering the initial call again. I usually add some identifier to a custom UITableViewCell class to check against.
Also, you're not creating the cell, just fetching it from some other variable. This is likely to be a problem, if there is more than one cell.
override func tableView(_ tableView: UITableView, cellForRowAt
indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "base") as! MyCell
cell.row = indexPath.row
let datum = data[indexPath.row]
cell.imageView.setImage(placeholderImage, for: .normal)
DispatchQueue.global().async {
// Runs in a background thread
URLSession.shared.image(with: datum.previewURL) { image, isCached in
DispatchQueue.main.async {
// Runs in the main thread; safe for updating the UI
// but check this cell is still being used for the same index path first!
if cell.row == indexPath.row {
cell.imageView.setImage(image, animated: !isCached)
}
}
}
}
return cell
}