ContentView.swift

//
//  THIS DOESN'T WORK, see the final comment for fix
//

import SwiftUI
import AVFoundation

struct ContentView: View {
    @StateObject private var cameraManager = CameraManager()
    
    var body: some View {
        ZStack {
            if cameraManager.isSessionInitialized {
                VideoPreviewView().edgesIgnoringSafeArea(.all)
            } else {
                Text("loading cam")
            }
            RecordButton()
        }
        .environmentObject(cameraManager)
    }
}

class RecordingHandler: NSObject, AVCaptureFileOutputRecordingDelegate {
    func fileOutput(_ output: AVCaptureFileOutput, didStartRecordingTo fileURL: URL, from connections: [AVCaptureConnection]) {}
    func fileOutput(_ output: AVCaptureFileOutput, didFinishRecordingTo outputFileURL: URL, from connections: [AVCaptureConnection], error: Error?) {}
}

class CameraManager: ObservableObject {
    var captureSession = AVCaptureSession()
    @Published var isSessionInitialized = false
    private var movieOutput: AVCaptureMovieFileOutput?
    private var recordingHandler: RecordingHandler

    init() {
        self.recordingHandler = RecordingHandler()
        DispatchQueue.global(qos: .background).async { [weak self] in
            guard let self = self else { return }
            
            guard let camera = AVCaptureDevice.default(for: .video) else { return }

            do {
                let input = try AVCaptureDeviceInput(device: camera)
                self.captureSession.addInput(input)

                self.movieOutput = AVCaptureMovieFileOutput()
                if self.captureSession.canAddOutput(self.movieOutput!) {
                    self.captureSession.addOutput(self.movieOutput!)
                }

                // Start running the capture session on the background queue
                self.captureSession.startRunning()
                DispatchQueue.main.async {
                    self.isSessionInitialized = true
                }
            } catch {
                print(error.localizedDescription)
            }
        }
    }
    
    func startRecording() {
        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let outputURL = documentsDirectory.appendingPathComponent(UUID().uuidString).appendingPathExtension("mov")
        movieOutput?.startRecording(to: outputURL, recordingDelegate: recordingHandler)
    }

    func stopRecording() {
        movieOutput?.stopRecording()
    }
}

struct RecordButton: View {
    @EnvironmentObject private var cameraManager: CameraManager
    @State private var isRecording = false
    
    var body: some View {
        Button(action: {}) {
            Image(systemName: "circle.fill")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .frame(width: 75, height: 75)
                .foregroundColor(isRecording ? .white : .red)
                .gesture(DragGesture(minimumDistance: 0, coordinateSpace: .local)
                    .onChanged({ _ in
                        if !isRecording {
                            cameraManager.startRecording()
                            isRecording = true
                        }
                    })
                    .onEnded({ _ in
                        isRecording = false
                        cameraManager.stopRecording()
                    })
                )
        }
    }
}

struct VideoPreviewView: UIViewRepresentable {
    @EnvironmentObject var cameraManager: CameraManager

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        let previewLayer = AVCaptureVideoPreviewLayer(session: cameraManager.captureSession)
        previewLayer.frame = view.layer.bounds
        previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
        view.layer.addSublayer(previewLayer)
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {}
}

To test this code, use XCode 14.3.1 (14E300c) or compatible to create a new iOS app. Then replace the ContentView that is generated for you with this, and it should allow you to experiment. For code that works (but runs the preview layer attach code multiple times), see my stack overflow question.

loremipsum suggested: Pass it as a “let” from the parent from the View that has the conditional. You are just removing the wrapper that is what triggers update.

Here is the new code:

// ContentView
var body: some View {
    ZStack {
        VideoPreviewView(cameraManager: cameraManager).edgesIgnoringSafeArea(.all)
        RecordButton()
    }
    .environmentObject(cameraManager)
}

and

struct VideoPreviewView: UIViewRepresentable {
    let cameraManager: CameraManager
    
    init(cameraManager: CameraManager) {
        self.cameraManager = cameraManager
    }

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {
        if cameraManager.isSessionInitialized {
            print("now")
            let previewLayer = AVCaptureVideoPreviewLayer(session: cameraManager.captureSession)
            previewLayer.frame = uiView.layer.bounds
            previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
            uiView.layer.addSublayer(previewLayer)
        }
    }
}

This doesn't seem to work either (the view is still blank). Please let me know what I've done wrong.
EDIT: the "now" statement never fires.

Another interpretation of that comment is this, which also still has a blank view (but does make the "now" print statement fire).

// ContentView
var body: some View {
    ZStack {
        if cameraManager.isSessionInitialized {
            VideoPreviewView(cameraManager: cameraManager).edgesIgnoringSafeArea(.all)
        } else {
            Text("nope not in this house")
        }
        RecordButton()
    }
    .environmentObject(cameraManager)
}

and

struct VideoPreviewView: UIViewRepresentable {
    let cameraManager: CameraManager
    
    init(cameraManager: CameraManager) {
        self.cameraManager = cameraManager
    }

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        if cameraManager.isSessionInitialized {
            print("now")
            let previewLayer = AVCaptureVideoPreviewLayer(session: cameraManager.captureSession)
            previewLayer.frame = view.layer.bounds
            previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
            view.layer.addSublayer(previewLayer)
        }
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {
    }
}

The issue as @vadian pointed out is the bounds were zero since I the view I was taking the bounds from was just initialized. The correct code is a tiny change from my desired modifications. Use UIScreen.main.bounds rather than view.layer.bounds. In the initial code.