slow app entering foreground - ios

Question

My app gets stuck in the icon for about 4-5 seconds when entering foreground, if the networking is bad. In almost all the main loading pages I am fetching images from a server. I have nothing on applicationDidEnterBackground() nor applicationWillEnterForeground().
Here the code in the Main View controller where I have a collection view with images:
override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCellWithReuseIdentifier(homeReuseIdentifier, forIndexPath: indexPath)as! HomeViewCell
// Reload the collection view after insert item at indexPath
cvHome.reloadItemsAtIndexPaths([indexPath])
let entry = dataCell.infoCell[indexPath.row]
cell.backImageView.image = nil
let stringURL = "https://.....\(entry.filename)"
let image = UIImage(named: entry.filename)
cell.backImageView.image = image
if cell.backImageView.image == nil {
if let image = dataCell.cachedImage(stringURL) {
cell.backImageView.image = image
} else {
request = dataCell.getNetworkImage(stringURL) { image in
cell.backImageView.image = image
}
}
}
cell.titleLabel.text = entry.title
cell.authorLabel.text = entry.author
// Fix the collection view cell in size
cell.contentView.frame = cell.bounds
cell.contentView.autoresizingMask = [UIViewAutoresizing.FlexibleWidth, UIViewAutoresizing.FlexibleHeight]
// To edit the cell for deleting
cell.editing = editing
return cell
}
And this is the getNetworkImage function with Alamofire:
func getNetworkImage(urlString: String, completion: (UIImage -> Void)) -> (ImageRequest) {
let queue = decoder.queue.underlyingQueue
let request = Alamofire.request(.GET, urlString)
let imageRequest = ImageRequest(request: request)
imageRequest.request.response(
queue: queue,
responseSerializer: Request.imageResponseSerializer(),
completionHandler: { response in
guard let image = response.result.value else {
return
}
let decodeOperation = self.decodeImage(image) { image in
completion(image)
self.cacheImage(image, urlString: urlString)
}
imageRequest.decodeOperation = decodeOperation
}
)
return imageRequest
}

Answer 1

I'm not sure if you're networking code is bad since you haven't provided any.
My guess is, fetching the data is taking way too long, and it's blocking the UI. So currently, your app is synchronous. So tasks are executed one at a time. the reason your UI is blocked is because it has to wait for the networking to finish.
What you need to do is perform the fetching in the background, and always keep your UI on the main queue.

Answer 2

If you are calling Synchronous you have to wait till the task is finishing.We can start next task once we finish the current task.When you are fetching image from server make sure that whether you are using Synchronous or not.When you don't wait for a second go with Asynchronous.You don't need to wait while fetch the images from server.You can do other tasks when fetching the images from server.
For this GCD is good one to use.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
// Do the back ground process here
......Fetch the image here
dispatch_async(dispatch_get_main_queue(), ^{
// Update the UI here
.....Do your UI design or updation design part here
});
});
Global Queue
Asynchronous task will be performed here.
It does not wait.
Asynchronous function does not block the current thread of execution from proceeding on to the next function.
Main Queue
We must always access UI Kit classes on the main thread.
If you don't want to wait a seconds,go with GCD