cell use random image - ios

Question

I have two UICollectionView:
1 UICollectionView - Post
2 UICollectionView - Categories
Logic:
When user tap on any categories, that we send new api request and then update post collection view
In my UICollectionViewCell have two conditions.
1 Image Item
2 Video Item
Below my code:
class PopularPostsCell: UICollectionViewCell {
#IBOutlet weak var myImage : UIImageView!
func setupItems(childPostsItem: ChildPostsModel, index: Int) {
if childPostsItem.media_type != "video" {
myImage.image = nil
myImage.sd_setImage(with: URL(string: childPostsItem.media), placeholderImage: UIImage(named: ""))
} else {
DispatchQueue.global(qos: .background).async {
let locImage = Helpers.videoSnapshot(filePathLocal: childPostsItem.media)
DispatchQueue.main.async {
self.myImage.image = nil
self.myImage.image = locImage
}
}
}
}
override func prepareForReuse() {
super.prepareForReuse()
myImage.image = nil
}
}
if video, I need get image from Video URL below my code:
static func videoSnapshot(filePathLocal: String) -> UIImage? {
do {
let asset = AVURLAsset(url: URL(string: filePathLocal)!, options: nil)
let imgGenerator = AVAssetImageGenerator(asset: asset)
imgGenerator.appliesPreferredTrackTransform = true
let cgImage = try imgGenerator.copyCGImage(at: CMTimeMake(0, 1), actualTime: nil)
let thumbnail = UIImage(cgImage: cgImage)
return thumbnail
} catch let error {
print("*** Error generating thumbnail: \(error.localizedDescription)")
return nil
}
}
Problem: If I choose several item in category very fast, my cell use random image. If I choose category slowly, that my cell use image ok.
My screen first: if I choose category fast
second: if I choose category slowly, in the screen everything is good.
If I don't use background async thread, all ok, but everything starts to slow down
Please, any help.

Answer 1

You are using a background thread and the main thread in a sequential manner while by definition they are parallel and don't wait for one another. Therefore in :
DispatchQueue.global(qos: .background).async {
let locImage = Helpers.videoSnapshot(filePathLocal: childPostsItem.media)
DispatchQueue.main.async {
self.myImage.image = nil
self.myImage.image = locImage
}
}
The two threads are completely asynchronous. You can't expect the background thread to be finished before the main thread starts.
There are ways to communicate thread events with signals and semaphores but in your case I think using asynchronous functions with completion handlers would be way more appropriate.
Check out how it works : https://stackoverflow.com/a/50531255/5922920