Swift UIApplication.delegate must be used from main thread only - swift

Question

Xcode is complaining when I try to set the appDelegate & context variables, required in order to use CoreData.
Essentially, I would like to store the results of my Vision / CoreML image classification request into a Core Data database for offline analysis.
Seen related threads to this, and tried a bunch. Problem does not go away, and now (unknown to me!) the CoreData save errors after few hundred records. I am hoping removing this issue altogether will solve the error problem or I can troubleshoot it later...
This is specifically for debugging, and won't likely need CoreData when data analyses has finished.
Tried putting the variable declarations right at the top of the ViewController class, with appending "!" as I know I will be setting them later. Tried putting the 2 lines in a DispatchQueue.main.async closure.
Tried wrapping these 2 lines inside a "DispatchQueue.main.async({ })" line, but then I can no longer reference the context on the "newItem" lines. Wrapping the whole section does not work either, probably as the CoreData cannot see / access the data within the image request(?)
The code:
func processCameraBuffer(sampleBuffer: CMSampleBuffer) {
let coreMLModel = Inceptionv3()
if let model = try? VNCoreMLModel(for: coreMLModel.model) {
let request = VNCoreMLRequest(model: model, completionHandler: { (request, error) in
if let results = request.results as? [VNClassificationObservation] {
var counter = 1
for classification in results {
let timestamp = NSDate().timeIntervalSince1970
// Purple Error is here
let appDelegate = UIApplication.shared.delegate as! AppDelegate
let context = appDelegate.persistentContainer.viewContext
let newItem = NSEntityDescription.insertNewObject(forEntityName: "Predictions", into: context)
newItem.setValue(classification.confidence, forKey: "confidence")
newItem.setValue(classification.identifier, forKey: "identifier")
newItem.setValue(counter, forKey: "index")
newItem.setValue(timestamp, forKey: "timestamp")
newItem.setValue("inceptionv3", forKey: "mlmodel")
print("counter: \(counter) \(classification.identifier)")
do {
try context.save()
} catch {
print("CoreData Save error")
print(error.localizedDescription)
}
counter += 1
}
}
})
if let pixelBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) {
let handler = VNImageRequestHandler(cvPixelBuffer: pixelBuffer, options: [:])
do {
try handler.perform([request])
} catch {
print(error.localizedDescription)
}
}
}
}

Answer 1

This is an annoying warning; however, it seems to be there for a reason as iOS wants to avoid deadlocks(???).
Using Swift 3, here's what I've found is a safe way to manage Core Data + AppDelegate (your mileage may vary):
DispatchQueue.main.async {
if let app = UIApplication.shared.delegate as? AppDelegate {
app.managedObjectContext.performAndWait {
// .. Core Data work here
// .. context.save()
} // app.managedObjectContext.performAndWait
} // if let app = UIApplication.shared.delegate as? AppDelegate
} // DispatchQueue.main.async
Hope this helps!