CODEWITHSUNDEEP
swiftcore-data
vib
vib

Reputation: 2274

Core data async fetch ends up on the main thread

I am trying to execute an asynchronous request as part of a search result updater in my app.

I wrote the following code

    func updateSearchResults(for searchController: UISearchController) {
        guard let text = searchController.searchBar.text else {return}
        let threadingContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        threadingContext.parent = self.context
        DispatchQueue.global(qos: .userInitiated).async {
            let fetchRequest = MyObject.fetchRequest() as NSFetchRequest<MyObject>
            fetchRequest.predicate = get_predicate(text)
            do {
                let objects = try threadingContext.fetch(fetchRequest).map({ object in
                    return object.objectID
                })
            }
            catch {return}
            DispatchQueue.main.async {
                // Pass results to the search view controller
            }
        }
    }

but the UI is still slow (even if I don't do any display update), and looking at the Time profiler, I see that my main thread is spending 80% of its time on the following: profiling

So it seems that my request is still being dispatched onto the main thread, which I don't understand. Would anyone see my mistake?

(I tried a few various on the above e.g. using threadingContext.perform but for the same result)

Upvotes: 3

Views: 696

Answers (1)

vib
vib

Reputation: 2274

Ok, I understood it, and I should have read Apple's documentation, but basically

If a context’s parent store is another managed object context, fetch and save operations are mediated by the parent context instead of a coordinator.

This is slightly subtle, but my construction would have been useful if the operations performed on the fetch request, rather than the fetch request itself, had been slow.

The solution is to set threadingContext.persistentStoreCoordinator instead.

Upvotes: 2

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