Delegate is not invoked from DispatchQueue.global() thread - ios

Question

I have a scenario where I need to load data from a JSON object, I've created an helper class that does that and it looks like this
protocol JSONDumpHelperDelegate {
func helper(_: JSONDumpHelper, didFinishFetching: [Link])
func helper(_: JSONDumpHelper, completionProcess: Double)
}
struct JSONDumpHelper {
static let pointsOfInterest = OSLog(subsystem: "com.mattrighetti.Ulry", category: .pointsOfInterest)
var delegate: JSONDumpHelperDelegate?
func loadFromFile(
with filemanager: FileManager = .default,
from url: URL,
decoder: JSONDecoder = JSONDecoder(),
context: NSManagedObjectContext = CoreDataStack.shared.managedContext,
dataFetcher: DataFetcher = DataFetcher()
) {
os_signpost(.begin, log: JSONDumpHelper.pointsOfInterest, name: "loadFromFile")
let data = try! Data(contentsOf: url)
let dump = try! decoder.decode(Dump.self, from: data)
var tagsHash: [UUID:Tag] = [:]
if let tagsCodable = dump.tags {
for tagCodable in tagsCodable {
let tag = Tag(context: context)
tag.id = tagCodable.id
tag.name = tagCodable.name
tag.colorHex = tagCodable.colorHex
tagsHash[tag.id] = tag
}
}
var groupHash: [UUID:Group] = [:]
if let groupsCodable = dump.groups {
for groupCodable in groupsCodable {
let group = Group(context: context)
group.id = groupCodable.id
group.name = groupCodable.name
group.colorHex = groupCodable.colorHex
group.iconName = groupCodable.iconName
groupHash[group.id] = group
}
}
var links: [Link] = []
if let linksCodable = dump.links {
let total = linksCodable.count
var completed = 0.0
delegate?.helper(self, completionProcess: 0.0)
for linkCodable in linksCodable {
let link = Link(context: context)
link.id = linkCodable.id
link.url = linkCodable.url
link.createdAt = linkCodable.createdAt
link.updatedAt = linkCodable.updatedAt
link.colorHex = linkCodable.colorHex
link.note = linkCodable.note
link.starred = linkCodable.starred
link.unread = linkCodable.unread
if let uuidGroup = linkCodable.group?.id {
link.group = groupHash[uuidGroup]
}
if let tags = linkCodable.tags {
link.tags = Set<Tag>()
for tagUUID in tags.map({ $0.id }) {
link.tags?.insert(tagsHash[tagUUID]!)
}
}
links.append(link)
completed += 1
delegate?.helper(self, completionProcess: completed / Double(total))
}
}
os_signpost(.end, log: JSONDumpHelper.pointsOfInterest, name: "loadFromFile")
}
}
This could potentially be a very long running task, just imagine an array with 1k records that also need to fetch data from the internet (not shown in implementation, error still exist with posted code) and you can easily end up with 10s in execution time.
What I'm trying to achieve is to show the user an alert that will show him the progress of the import process, by updating the values with the delegate protocols below.
extension BackupViewController: JSONDumpHelperDelegate {
func helper(_: JSONDumpHelper, didFinishFetching: [Link]) {
DispatchQueue.main.async {
self.completionViewController.remove()
}
}
func helper(_: JSONDumpHelper, completionProcess: Double) {
DispatchQueue.main.async {
self.completionViewController.descriptionLabel.text = "Completed \(Int(completionProcess * 100))%"
}
}
}
The import method is fired from a UITableView, immediately after the user choses a file from a DocumentPickerViewController
extension BackupViewController: UIDocumentPickerDelegate {
func documentPickerWasCancelled(_ controller: UIDocumentPickerViewController) {
dismiss(animated: true, completion: nil)
}
func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) {
self.initImport(for: urls)
}
}
private func initImport(for urls: [URL]) {
if let url = urls.first {
completionViewController.add(self, frame: self.view.bounds)
completionViewController.descriptionLabel.text = "Fetching"
DispatchQueue.global(qos: .userInteractive).async {
self.dumpHelper.loadFromFile(from: url)
}
}
}
The problem I am facing is that when the user initiates the import process, the UI is not updated until the process itself is finished.
If I place breakpoints both at the protocol implementations and at the delegate calls in the helper class I can see that the delegate is not called immediately but they all get fired when the process ended (but the alert controller does not update its values).
Just to place some more info I'm going to replicate an import of N elements from JSON:
User clicks import process
initImport is executed (nothing is shown on UI even if I add the custom vc to view)
JSONDumpHelper.loadFromFile is executed entirely, calling delegate N times, nothing called in the delegate implementation
loadFromFile finishes execution
delegate implementation of helper(_:, completionProcess) is executed N times, UI always shows "Completed 0%"
delegate implementation of helper(_:, didFinishFetching) is executed, controller is removed from view
Can anybody point out what is wrong with this implementation? It seems like the loadFromFile function is not working in a separate Queue and UI is stuck and can't update as expected.