swift 。如何获取函数外变量的值

Question

我正在研究机器人项目，并尝试将跟踪对象的位置作为 yTrack 变量的值(请参阅代码)。我可以从 func handleVisionRequestUpdate 打印 yTrack，但我需要访问此函数之外的 yTrack 以便将其与其他函数一起使用。以 getCoord() 为例。请帮忙!

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

问题出在您的 handleVisionRequestUpdate 函数上。

首先，@Azat 在评论中是正确的:当您声明 let yTrack = Double(self.transformedRect?.origin.y ?? 0.5) 时，您在此函数的范围内创建了新变量，其中与 ViewController 范围内的 var yTrack 没有任何联系。

因此，当您在此函数中打印 yTrack 时，您会打印函数的内部变量，该变量将在函数返回后被销毁。为了能够在函数外部使用 yTrack，您需要为 ViewController 的 youTrack 分配新值，然后您就可以在您想要的任何功能中使用它

yTrack = Double(self.transformedRect?.origin.y ?? 0.5)

第二个问题与DispatchQueue.main.async有关。大多数情况下，该 block 内的代码将在该 block 之后执行

let yTrack = Double(self.transformedRect?.origin.y ?? 0.5)
print(yTrack as Any)

那是因为使用这个 block 你告诉编译器“为这个代码块创建单独的“队列”，并在你能够做到的时候异步执行它”所以这一行self.transformedRect = newObservation.boundingBox 在大多数情况下都会在这行之后完成 let yTrack = Double(self.transformedRect?.origin.y ?? 0.5) 并且您将拥有之前的 transformedRect如果我理解正确的话，这一行里面的内容是nil。因此，请从此函数中删除 DispatchQueue.main.async 或将 self.yTrack = Double(self.transformedRect?.origin.y ?? 0.5) 移入其中。