Sorting a Realm Results object in the background thread - swift

Question

I am trying to sort a Realm Results instance in a background thread. But I am getting 'Realm accessed from incorrect thread.' exception. What am I doing wrong here?.
I'm using this function to filter and update the table with the result as the text in the search bar text field changes.
Thanks in advance.
var previousSearchWork?
func getInvoicesFor(searchedTerm: String, completion: #escaping ([Invoice]) -> Void) {
previousSearchWork?.cancel()
let newSearchWork = DispatchWorkItem {
guard let realm = try? Realm() else { return }
var filteredInvoices = [Invoice]()
if searchedTerm.first!.isLetter { // searching by customer name
let predicate = NSPredicate(format: "name BEGINSWITH[cd] %# || name CONTAINS[cd] %#", searchedTerm, searchedTerm)
let invoices = realm.objects(Invoice.self).filter(predicate)
filteredInvoices = invoices.sorted {
$0.name!.levenshteinDistance(searchedTerm) < $1.name!.levenshteinDistance(searchedTerm)
}
} else { // searching by id
// ...
}
completion(filteredInvoices)
}
previousSearchWork = newSearchWork
DispatchQueue.global(qos: .userInitiated).asyncAfter(deadline: .now() + .milliseconds(30), execute: newSearchWork)
}

Answer 1

As #Jay has mentioned in a reply to the original question:
... that Realm is on a background thread so the objects are on that thread; what happens with [Invoice] upon completion?
Yep, it turns out I've been fetching Realm persisted objects on a background thread and send it to the caller via completion closure and then the caller tries to read them on main thread. That's what triggered the 'Realm accessed from incorrect thread'
First of all, I couldn't find a way to sort the objects without transforming it to an array of realm objects since I needed to use a custom sorting method.
All I did to fix the above function was instead of returning an array of Objects that are fetched inside a background thread, I am returning references to those objects so I can refer to them in main thread
According to my poor research, I've found two ways to pass those objects from background thread to main thread. (I went for the second way cause as to what've read, it's faster for this case.)
let backgroundQueue = DispatchQueue.global()
let mainThread = DispatchQueue.main
// Passing as ThreadSafeReferences to objects
backgroundQueue.async {
let bgRealm = try! Realm()
let myObjects = bgRealm.objects(MyObject.self)
// ......
let myObjectsArray = .....
let references: [ThreadSafeReference<MyObject>] = myObjectsArray.map { ThreadSafeReference(to: $0) }
mainThread.async {
let mainRealm = try! Realm()
let myObjectsArray: [MyObject?] = references.map { mainRealm.resolve($0) }
}
}
// Passing primaryKeys of objects
backgroundQueue.async {
let bgRealm = try! Realm()
let myObjects = bgRealm.objects(MyObject.self)
// ......
let myObjectsArray = .....
// MyObject has a property called 'id' which is the primary key
let keys: [String] = itemsArray.map { $0.id }
mainThread.async {
let mainRealm = try! Realm()
let myObjectsArray: [MyObject?] = keys.map { mainRealm.object(ofType: MyObject.self, forPrimaryKey: $0) }
}
}
After adjusting the function (and completing it for my need):
var previousSearchWork: DispatchWorkItem?
func getInvoicesFor(searchedTerm: String, completion: #escaping ([String]) -> Void) {
previousSearchWork?.cancel()
let newSearchWork = DispatchWorkItem {
autoreleasepool {
var filteredIDs = [String]()
guard let realm = try? Realm() else { return }
let allInvoices = realm.objects(Invoice.self).filter(NSPredicate(format: "dateDeleted == nil"))
if searchedTerm.first!.isLetter {
let predicate = NSPredicate(format: "name BEGINSWITH[cd] %# || name CONTAINS[cd] %#", searchedTerm, searchedTerm)
let invoices = allInvoices.filter(predicate)
filteredIDs = invoices.sorted {
$0.name!.levenshtein(searchedTerm) < $1.name!.levenshtein(searchedTerm)
}.map {$0.id}
} else {
var predicates = [NSPredicate(format: "%# IN ticket.pattern.sequences", searchedTerm)]
if searchedTerm.count > 3 {
let regex = searchedTerm.charactersSorted().reduce("*") {$0 + "\($1)*"}
let predicate = NSPredicate(format: "ticket.pattern.id LIKE %#", regex)
predicates.append(predicate)
}
let invoices = allInvoices.filter(NSCompoundPredicate(orPredicateWithSubpredicates: predicates)).sorted(byKeyPath: "dateCreated", ascending: false)
filteredIDs = Array(invoices.map {$0.id})
}
DispatchQueue.main.async {
completion(filteredIDs)
}
}
}
previousSearchWork = newSearchWork
DispatchQueue.global(qos: .userInitiated).asyncAfter(deadline: .now() + .milliseconds(30), execute: newSearchWork)
}