Swift. How to get value of variable outside the function - swift

Question

I am working on robotics project and trying to get position of tracking object as a value of the yTrack variable (see the code). I can print yTrack from the func handleVisionRequestUpdate, but I need access to yTrack outside this function in order to use it with other function. getCoord () as an example. Please help!
import AVFoundation
import Vision
import UIKit
import Foundation
class ViewController: UIViewController, AVCaptureVideoDataOutputSampleBufferDelegate {
var protocolString: String?
var inputStream: InputStream?
var outputStream: OutputStream?
var dataAsString: String?
var yTrack: Double?
#IBOutlet private weak var cameraView: UIView?
#IBOutlet private weak var highlightView: UIView? {
didSet {
self.highlightView?.layer.borderColor = UIColor.red.cgColor
self.highlightView?.layer.borderWidth = 4
self.highlightView?.backgroundColor = .clear
}
}
private let visionSequenceHandler = VNSequenceRequestHandler()
private lazy var cameraLayer: AVCaptureVideoPreviewLayer = AVCaptureVideoPreviewLayer(session: self.captureSession)
private lazy var captureSession: AVCaptureSession = {
let session = AVCaptureSession()
session.sessionPreset = AVCaptureSession.Preset.photo
guard
let backCamera = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back),
let input = try? AVCaptureDeviceInput(device: backCamera)
else { return session }
session.addInput(input)
return session
}()
override func viewDidLoad() {
super.viewDidLoad()
self.highlightView?.frame = .zero
self.cameraView?.layer.addSublayer(self.cameraLayer)
let videoOutput = AVCaptureVideoDataOutput()
videoOutput.setSampleBufferDelegate(self, queue: DispatchQueue(label: "MyQueue"))
self.captureSession.addOutput(videoOutput)
self.captureSession.startRunning()
}
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
self.cameraLayer.frame = self.cameraView?.bounds ?? .zero
}
public var lastObservation: VNDetectedObjectObservation?
func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
guard
let pixelBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer),
let lastObservation = self.lastObservation
else { return }
let request = VNTrackObjectRequest(detectedObjectObservation: lastObservation, completionHandler: self.handleVisionRequestUpdate)
request.trackingLevel = .fast
do {
try self.visionSequenceHandler.perform([request], on: pixelBuffer)
} catch {
print("Throws: \(error)")
}
}
public func handleVisionRequestUpdate(_ request: VNRequest, error: Error?) {
test {(yTrack) in
DispatchQueue.main.async {
guard let newObservation = request.results?.first as? VNDetectedObjectObservation else { return }
self.lastObservation = newObservation
guard newObservation.confidence >= 0.3 else {
self.highlightView?.frame = .zero
return
}
self.transformedRect = newObservation.boundingBox
self.transformedRect!.origin.y = 1 - self.transformedRect!.origin.y
let convertedRect = self.cameraLayer.layerRectConverted(fromMetadataOutputRect: self.transformedRect!)
self.highlightView?.frame = convertedRect
}
}
let yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
// HERE IT WORKS, yTrack IS PRINTING, BUT I NEED IT OUTSIDE THIS FUNCTION
print(yTrack as Any)
}
public func test (returnCompletion: #escaping (AnyObject) -> () ){
DispatchQueue.global(qos: .background).async {
self.yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
returnCompletion(self.yTrack as AnyObject)
}
}
public func getCoord () {
//HERE IT DOESN'T WORK. NOTHING IS PRINTING FROM HERE.
print(yTrack)
}

Answer 1

The problem is with your handleVisionRequestUpdate function.
First of all #Azat was right in comments: when you declare let yTrack = Double(self.transformedRect?.origin.y ?? 0.5) you create new variable in scope of this function which has no connection with var yTrack from ViewController's scope.
So when you print yTrack in this function you print function's internal variable which will be destroyed after function returns. To be able to use yTrack outside of a function you need to assign new value to ViewController's youTrack and then you'll be able to use it in any function you want
yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
The second problem is with DispatchQueue.main.async. The code inside this block will be performed in most cases after this block
let yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
print(yTrack as Any)
that's because using this block you tell compiler "create separate «queue» for this block of code and asynchronously perform it when you'll be able to do it" so this line self.transformedRect = newObservation.boundingBox will be done in most cases after this line let yTrack = Double(self.transformedRect?.origin.y ?? 0.5) and you'll have previous transformedRect inside this line which is nil if I understand correctly. So remove DispatchQueue.main.async from this function or move self.yTrack = Double(self.transformedRect?.origin.y ?? 0.5) into it.

Answer 2

Check out the code below. I added a lot of comments but I didn't test it out. But it should fix things. There were a couple confusing things going on. Specifically with the "test" function. Also, you may want to practice using self.myVar or self.myFunc() so that you start to get a better understanding of which variables are local and which are properties on the view controller:
public func handleVisionRequestUpdate(_ request: VNRequest, error: Error?) {
// test {(yTrack) in /// remove this since we removed it below
DispatchQueue.main.async {
guard let newObservation = request.results?.first as? VNDetectedObjectObservation else { return }
self.lastObservation = newObservation
guard newObservation.confidence >= 0.3 else {
self.highlightView?.frame = .zero
return
}
self.transformedRect = newObservation.boundingBox
self.transformedRect!.origin.y = 1 - self.transformedRect!.origin.y
let convertedRect = self.cameraLayer.layerRectConverted(fromMetadataOutputRect: self.transformedRect!)
self.highlightView?.frame = convertedRect
//
// Move these into the Dispatch closure
// let yTrack = Double(self.transformedRect?.origin.y ?? 0.5) // delete this one
yTrack = Double(self.transformedRect?.origin.y ?? 0.5) // replace it with this one
print(yTrack as Any)
}
}
//
// Remove this. I'm not sure what it does, but its making things more complex
//
// public func test (returnCompletion: #escaping (AnyObject) -> () ){
// DispatchQueue.global(qos: .background).async {
// self.yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
// returnCompletion(self.yTrack as AnyObject)
// }
// }