利用AVKit检测光度

Question

我正在开发一个使用SwiftUi的应用程序，它利用设备摄像头来检测光度，如这个职位的顶部答案所述。用顶部答案中的captureOutput(_:didOutput:from:)函数计算光度。根据苹果文档，这个函数的目的是通知委托编写了一个新的视频帧，所以我把这个函数放在一个VideoDelegate类中。然后在VideoStream类中设置此委托，该类处理请求权限和设置AVCaptureSession的逻辑。我的问题是如何访问在我的luminosity视图中的委托中计算出来的SwiftUI值？

struct ContentView: View {
    @StateObject var videoStream = VideoStream()
    
    var body: some View {
        Text("\(videoStream.luminosityReading) ?? Detecting...")
            .padding()
    }
}

class VideoStream: ObservableObject {
    
    @Published var luminosityReading : Double = 0.0 // TODO get luminosity from VideoDelegate
    var session : AVCaptureSession!
    
    init() {
        authorizeCapture()
    }
    
    func authorizeCapture() {
        // permission logic and call to beginCapture()
    }
    
    func beginCapture() {        
        session = AVCaptureSession()
        session.beginConfiguration()
        let videoDevice = bestDevice() // func definition omitted for readability 
        guard
            let videoDeviceInput = try? AVCaptureDeviceInput(device: videoDevice),
            session.canAddInput(videoDeviceInput)
        else {
            print("Camera selection failed")
            return
        }
        
        let videoOutput = AVCaptureVideoDataOutput()
        guard
            session.canAddOutput(videoOutput)
        else {
            print("Error creating video output")
            return
        }
        
        session.sessionPreset = .high
        session.addOutput(videoOutput)
        
        let queue = DispatchQueue(label: "VideoFrameQueue")
        let delegate = VideoDelegate()
        videoOutput.setSampleBufferDelegate(delegate, queue: queue)
        
        session.commitConfiguration()
        session.startRunning()
    }
}

class VideoDelegate: NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {
    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        //Retrieving EXIF data of camara frame buffer
        let rawMetadata = CMCopyDictionaryOfAttachments(allocator: nil, target: sampleBuffer, attachmentMode: CMAttachmentMode(kCMAttachmentMode_ShouldPropagate))
        let metadata = CFDictionaryCreateMutableCopy(nil, 0, rawMetadata) as NSMutableDictionary
        let exifData = metadata.value(forKey: "{Exif}") as? NSMutableDictionary
        
        let FNumber : Double = exifData?["FNumber"] as! Double
        let ExposureTime : Double = exifData?["ExposureTime"] as! Double
        let ISOSpeedRatingsArray = exifData!["ISOSpeedRatings"] as? NSArray
        let ISOSpeedRatings : Double = ISOSpeedRatingsArray![0] as! Double
        let CalibrationConstant : Double = 50
        
        //Calculating the luminosity
        let luminosity : Double = (CalibrationConstant * FNumber * FNumber ) / ( ExposureTime * ISOSpeedRatings )
         
        // how to pass value of luminosity to `VideoStream`? 
    }
}

Answer 1

正如注释中所讨论的，最小的摩擦选项是让VideoStream符合AVCaptureVideoDataOutputSampleBufferDelegate并在那里实现委托方法。