Following WWDC24 video "Discover Swift enhancements in the Vision framework" recommendations (cfr video at 10'41"), I used the following code to perform multiple new iOS 18 `RecognizedTextRequest' in parallel.
Problem: if more than 2 request are run in parallel, the request will hang, leaving the app in a state where no more requests can be started. -> deadlock
I tried other ways to run the requests, but no matter the method employed, or what device I use: no more than 2 requests can ever be run in parallel.
func triggerDeadlock() {}
try await withThrowingTaskGroup(of: Void.self) { group in
// See: WWDC 2024 Discover Siwft enhancements in the Vision framework at 10:41
// ############## THIS IS KEY
let maxOCRTasks = 5 // On a real-device, if more than 2 RecognizeTextRequest are launched in parallel using tasks, the request hangs
// ############## THIS IS KEY
for idx in 0..<maxOCRTasks {
let url = ... // URL to some image
group.addTask {
// Perform OCR
let _ = await performOCRRequest(on: url: url)
}
}
var nextIndex = maxOCRTasks
for try await _ in group { // Wait for the result of the next child task that finished
if nextIndex < pageCount {
group.addTask {
let url = ... // URL to some image
// Perform OCR
let _ = await performOCRRequest(on: url: url)
}
nextIndex += 1
}
}
}
}
// MARK: - ASYNC/AWAIT version with iOS 18
@available(iOS 18, *)
func performOCRRequest(on url: URL) async throws -> [RecognizedText] {
// Create request
var request = RecognizeTextRequest() // Single request: no need for ImageRequestHandler
// Configure request
request.recognitionLevel = .accurate
request.automaticallyDetectsLanguage = true
request.usesLanguageCorrection = true
request.minimumTextHeightFraction = 0.016
// Perform request
let textObservations: [RecognizedTextObservation] = try await request.perform(on: url)
// Convert [RecognizedTextObservation] to [RecognizedText]
return textObservations.compactMap { observation in
observation.topCandidates(1).first
}
}
I also found this Swift forums post mentioning something very similar.
I also opened a feedback: FB17240843
