import AVFoundation
import SwiftUI

/// Full-screen QR scanner; calls `onCode` once with the first detected payload.
struct QRScannerView: View {
    var onCode: (String) -> Void
    @Environment(\.dismiss) private var dismiss
    @State private var errorText: String?

    var body: some View {
        NavigationStack {
            ZStack {
                QRScannerRepresentable(
                    onCode: { code in
                        onCode(code)
                        dismiss()
                    },
                    onError: { errorText = $0 }
                )
                .ignoresSafeArea()

                VStack {
                    Spacer()
                    Text("将相机对准电脑上的分享二维码")
                        .font(.subheadline)
                        .foregroundStyle(.white)
                        .padding(.horizontal, 16)
                        .padding(.vertical, 10)
                        .background(.black.opacity(0.55), in: Capsule())
                        .padding(.bottom, 36)
                }
            }
            .background(Color.black)
            .navigationTitle("扫描二维码")
            .navigationBarTitleDisplayMode(.inline)
            .toolbar {
                ToolbarItem(placement: .cancellationAction) {
                    Button("取消") { dismiss() }
                }
            }
            .alert("无法扫码", isPresented: Binding(
                get: { errorText != nil },
                set: { if !$0 { errorText = nil } }
            )) {
                Button("好", role: .cancel) { dismiss() }
            } message: {
                Text(errorText ?? "")
            }
        }
    }
}

private struct QRScannerRepresentable: UIViewControllerRepresentable {
    var onCode: (String) -> Void
    var onError: (String) -> Void

    func makeUIViewController(context: Context) -> QRScannerViewController {
        let vc = QRScannerViewController()
        vc.onCode = onCode
        vc.onError = onError
        return vc
    }

    func updateUIViewController(_ uiViewController: QRScannerViewController, context: Context) {}
}

final class QRScannerViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
    var onCode: ((String) -> Void)?
    var onError: ((String) -> Void)?

    private let session = AVCaptureSession()
    private var previewLayer: AVCaptureVideoPreviewLayer?
    private var didEmit = false

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .black
        checkPermissionAndStart()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        previewLayer?.frame = view.bounds
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        if session.isRunning {
            DispatchQueue.global(qos: .userInitiated).async { [session] in
                session.stopRunning()
            }
        }
    }

    private func checkPermissionAndStart() {
        switch AVCaptureDevice.authorizationStatus(for: .video) {
        case .authorized:
            configureSession()
        case .notDetermined:
            AVCaptureDevice.requestAccess(for: .video) { [weak self] granted in
                DispatchQueue.main.async {
                    if granted {
                        self?.configureSession()
                    } else {
                        self?.onError?("需要相机权限才能扫描二维码，请在设置中开启。")
                    }
                }
            }
        default:
            onError?("需要相机权限才能扫描二维码，请在设置中开启。")
        }
    }

    private func configureSession() {
        session.beginConfiguration()
        session.sessionPreset = .high

        guard
            let device = AVCaptureDevice.default(for: .video),
            let input = try? AVCaptureDeviceInput(device: device),
            session.canAddInput(input)
        else {
            session.commitConfiguration()
            onError?("当前设备无法使用相机扫码。")
            return
        }
        session.addInput(input)

        let output = AVCaptureMetadataOutput()
        guard session.canAddOutput(output) else {
            session.commitConfiguration()
            onError?("当前设备不支持二维码扫描。")
            return
        }
        session.addOutput(output)
        output.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
        if output.availableMetadataObjectTypes.contains(.qr) {
            output.metadataObjectTypes = [.qr]
        } else {
            session.commitConfiguration()
            onError?("当前设备不支持二维码扫描。")
            return
        }
        session.commitConfiguration()

        let preview = AVCaptureVideoPreviewLayer(session: session)
        preview.videoGravity = .resizeAspectFill
        preview.frame = view.bounds
        view.layer.insertSublayer(preview, at: 0)
        previewLayer = preview

        DispatchQueue.global(qos: .userInitiated).async { [session] in
            session.startRunning()
        }
    }

    func metadataOutput(
        _ output: AVCaptureMetadataOutput,
        didOutput metadataObjects: [AVMetadataObject],
        from connection: AVCaptureConnection
    ) {
        guard !didEmit else { return }
        guard
            let object = metadataObjects.first as? AVMetadataMachineReadableCodeObject,
            object.type == .qr,
            let value = object.stringValue?.trimmingCharacters(in: .whitespacesAndNewlines),
            !value.isEmpty
        else { return }

        didEmit = true
        if session.isRunning {
            DispatchQueue.global(qos: .userInitiated).async { [session] in
                session.stopRunning()
            }
        }
        onCode?(value)
    }
}