TableView Cell with Minor Lag (Async Proper Use?) Swift - ios

Question

I have an array full of Data loading images in a tableView Cell. However I am getting minor lag(more of a glitch) when the table view scrolls on image index.
Array Contains Data(Seems to lag more bigger the bytes)
Data in Bytes(624230 bytes)
Data in Bytes(1619677 bytes)
Data in Bytes(2257181 bytes)
Data in Bytes(1120275bytes)
Not Sure How to properly use Async When the data is loaded.
struct messageCellStruct {
let message: String!
let photoData: Data!
}
var messageArray = [messageCellStruct]()
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "cell")
let message2 = cell?.viewWithTag(2) as! UITextView
var photoUploadData = messageArray[indexPath.row].photoData
let main = DispatchQueue.main
let background = DispatchQueue.global()
let helper = DispatchQueue(label: "another_thread")
if(photoUploadData != nil){
print("Data in Bytes\(String(describing: photoUploadData))")
let fullString = NSMutableAttributedString(string: "")
let image1Attachment = NSTextAttachment()
let newImageWidth = (self.message.bounds.size.width - 20 )
let messageDisplayString = self.messageArray[indexPath.row].message
background.async {
image1Attachment.image = UIImage(data: photoUploadData!)
}
image1Attachment.bounds = CGRect.init(x: 0, y: 0, width: newImageWidth, height: 200)
let image1String = NSAttributedString(attachment: image1Attachment)
fullString.append(image1String)
fullString.append(NSAttributedString(string: messageDisplayString!))
message2.attributedText = fullString
message2.textColor = .white
message2.font = UIFont.systemFont(ofSize: 17.0)
}else {
message2.text = self.messageArray[indexPath.row].message
}
return cell!
}
The reason I introduced the Async in the first place was because of the lag. It lags with and without the Async.

Answer 1

You should never perform any UI operations on background thread.
remove background.async from
background.async {
image1Attachment.image = UIImage(data: photoUploadData!)
}
simply use
image1Attachment.image = UIImage(data: photoUploadData!)
EDIT:
The lag is not because of UIImage(data: photoUploadData!) I agree that it is a synchronous call but that wont create a lag the real culprit is let image1String = NSAttributedString(attachment: image1Attachment) NSAttributedString is known to be notorious.
To test the hypothesis you can comment out
let image1String = NSAttributedString(attachment: image1Attachment)
fullString.append(image1String)
fullString.append(NSAttributedString(string: messageDisplayString!))
message2.attributedText = fullString
message2.textColor = .white
message2.font = UIFont.systemFont(ofSize: 17.0)
and you should not see a lag.
Unfortunately I don't know the way to fix the issue with NSAttributedString we had same issue and it would often add up lag on scrolling huge pile of rows. Hence we decided to opt for DTCoreText
Somehow this performs better than NSAttributedString
EDIT:
We concluded that the delay/lag is probably because of conversion of huge data to NSAttributedString. As all that OP wanna do is show image and text below it n he did not know how to handle multiple components in cell I am updating the answer to same.
I am not claiming that this is the only way to do it this way might help the OP is assumption here
Step1:
Create UITableViewCell xib and drag UIImageView to it.
Now imageViews can take implicit size. What does that mean is UIImageView's can grow based on the image shows. If the images you are loading happen to be in your control (Server sending images is yours) and if your backend team can assure that they wont send crazy big images you dont need height constraint to ImageView.
But more often than not, server team claims that its a client team job. Because you would like to show the image best possible way and showing image with aspect fit and let the bigger part of image being chopped off or if image happens to be small leaving the huge space around image ask the backend team to send aspect ratio as a part of the response.
So in that case create a height constraint to imageView
Create an IBOutlet to the height constraint lets assume you call it as imageHeightConstraint
Now when you load the image in cell, you know that imageView's width will be equal to the width of the cell and you know the aspect ratio of the image to be shown so you can calculate the height of supposed image as
either in cellFroRowAtIndexPath
cell.imageHeightConstraint.constant = cell.bounds.size.width * aspectRatioOfImage
or better if you have configure method in a cell where you would expect cell to configure its subviews then
self.imageHeightConstraint.constant = self.bounds.size.width * aspectRatioOfImage
Obviously you might need to convert it to Float from CGFloat am sure u can do it :)
Now if you dont have any control over image and you are downloading it from some random website and hence dont have aspect ratio info then rather than adding height constraint add aspect ratio constraint to imageView and change imageView content mode to .aspectFit this might have side effects I mentioned above but thats the best you get without any support :)
Now add TextView below imageView
add vertical height constraint between imageView and TextView
Select textView and set scroll enabled as false
Select TextView and change vertical content hugging priority and compression resistance to 999
Thats it :)
Now what have you achieved with all these circus is that now you have a TableViewCell which can take implicit size based on content it has without any ambiguity :)
Now that makes the life easy. Implement tableView delegates and dont forget to use
self.tableView.rowHeight = UITableViewAutomaticDimension;
self.tableView.estimatedRowHeight = 44.0;
Thats it :) Now run your code and enjoy self expanding tableView cells based on the content they show :) And you have the layout u wanted
Hope it helps