特定iPhone XR设备上captureOutput(_:didOutput:from:)代理未触发问题排查

问题背景 #

我正在使用captureOutput(_:didOutput:from:)代理方法捕获相机帧（CVPixelBuffers），用于将帧传入CoreML进行图像分类。该功能在目前测试过的所有iOS设备上均正常运行，但某一台特定iPhone XR设备出现异常，无法捕获缓冲帧，代理方法完全不触发。

设备规格 #

• iPhone XR
• 问题出现在iOS 16.3和iOS 16.7系统版本（未测试其他版本）

已尝试操作与限制 #

• 在亚马逊购买的翻新XR设备及同事的XR设备上，该功能均可正常运行。
• 无法持续访问出现问题的客户个人XR设备。
• Xcode未抛出任何错误，苹果文档仅定义了两个相关代理方法（captureOutput和丢帧相关方法）。

相关代码 #

视图控制器定义 #

import Foundation
import UIKit
import AVFoundation
import Vision


class TestCameraViewController: UIViewController {
    
    var bufferSize: CGSize = .zero
    private var previewView: UIView = UIView()
    private var previewLayer: AVCaptureVideoPreviewLayer! = nil

    private let session = AVCaptureSession()
    private let videoDataOutput = AVCaptureVideoDataOutput()
    private var currentVideoDevice: AVCaptureDevice?
                
    override func viewDidLoad() {
        super.viewDidLoad()
        setupView()
        setupAVCapture()
        startCaptureSession()
    }
        
    func setupView() {
        previewView.translatesAutoresizingMaskIntoConstraints = false
        
        view.addSubview(previewView)
        previewView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        previewView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        previewView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        previewView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
    }
    
    func setupAVCapture() {

        var deviceInput: AVCaptureDeviceInput!
        
        // Select a video device, make an input
        let videoDevice = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera], mediaType: .video, position: .back).devices.first
        
        if videoDevice == nil {
            print("Device Does Not Exist, properties: nil, level: .error, skipAnalytics: true")
            return
        }

        do {
            deviceInput = try AVCaptureDeviceInput(device: videoDevice!)

        } catch {
            print("Could not create video device input")
            return
        }
        
        session.beginConfiguration()
        session.sessionPreset = .vga640x480 // Model image size is smaller.
        
        // Add a video input
        guard session.canAddInput(deviceInput) else {
            session.commitConfiguration()
            print("Could not add video device input")
            return
        }
        
        session.addInput(deviceInput)
        
        if session.canAddOutput(videoDataOutput) {
            session.addOutput(videoDataOutput)
            videoDataOutput.alwaysDiscardsLateVideoFrames = true
            videoDataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)]
            videoDataOutput.setSampleBufferDelegate(self, queue: .global())
            
        } else {
            session.commitConfiguration()
            print("Could not add video data output.")
            return
        }
                
        let captureConnection = videoDataOutput.connection(with: .video)
        captureConnection?.isEnabled = true
        
        do {
            try  videoDevice!.lockForConfiguration()
            let dimensions = CMVideoFormatDescriptionGetDimensions((videoDevice?.activeFormat.formatDescription)!)
            bufferSize.width = CGFloat(dimensions.width)
            bufferSize.height = CGFloat(dimensions.height)
            videoDevice!.unlockForConfiguration()
        } catch {
            print("Could not lock video device configuration.")
            return
        }
        
        session.commitConfiguration()
        previewLayer = AVCaptureVideoPreviewLayer(session: session)
        previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
        self.currentVideoDevice = videoDevice
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        if previewLayer != nil {
            previewLayer.frame = previewView.bounds
            previewView.layer.addSublayer(previewLayer)
        }
    }
    
    func startCaptureSession() {
        DispatchQueue.global(qos: .background).async {
            self.session.startRunning()
        }
    }
        
    // Clean up capture setup
    func teardownAVCapture() {
        session.stopRunning()
        previewLayer.removeFromSuperlayer()
        previewLayer = nil
    }
}

代理扩展（问题设备上未触发） #

extension TestCameraViewController: AVCaptureVideoDataOutputSampleBufferDelegate {
    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        
        //NOT CALLED on one particular iPhone XR physical device
        
        guard let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else {
            return
        }
    }
}

排查建议与后续步骤 #

1. 检查相机权限状态：
   用户可能在系统设置中关闭了相机权限，或权限请求流程存在遗漏。建议启动相机前主动检查权限状态，添加权限请求回调日志，确认设备是否授予相机访问权限。
2. 验证视频连接有效性：
   不要直接强制使用captureConnection，添加空值判断和日志输出，确认连接是否成功启用：

   if let captureConnection = videoDataOutput.connection(with: .video) {
       captureConnection.isEnabled = true
       print("Video connection enabled successfully")
   } else {
       print("Failed to get valid video connection")
   }
   

3. 调整会话预设与像素格式：
   部分设备对特定像素格式或会话预设的支持存在差异。尝试将会话预设改为.high，或调整像素格式为kCVPixelFormatType_32BGRA，测试是否能触发代理方法。
4. 使用专用代理队列：
   当前使用.global()队列作为代理队列，建议创建专用串行队列，避免队列优先级冲突或被意外释放：

   private let videoProcessingQueue = DispatchQueue(label: "com.yourapp.video-processing", qos: .userInitiated)
   // 替换setSampleBufferDelegate的队列参数
   videoDataOutput.setSampleBufferDelegate(self, queue: videoProcessingQueue)
   

5. 添加丢帧代理日志：
   实现丢帧代理方法，确认是否因丢帧导致代理不触发：

   func captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
       print("Frame dropped at \(Date())")
   }
   

6. 排查硬件与系统异常：
   引导用户在系统相机APP中测试相机是否正常工作，排除硬件故障；同时建议用户重启设备或更新系统到最新可用版本，排查系统层面的异常。
7. 监听会话运行时错误：
   添加会话错误监听，捕获可能的运行时异常：

   override func viewDidLoad() {
       super.viewDidLoad()
       // 其他初始化代码
       NotificationCenter.default.addObserver(self, selector: #selector(handleSessionError), name: .AVCaptureSessionRuntimeError, object: session)
   }
   
   @objc private func handleSessionError(notification: NSNotification) {
       if let error = notification.userInfo?[AVCaptureSessionErrorKey] as? NSError {
           print("Capture session error: \(error.localizedDescription)")
       }
   }
   
   deinit {
       NotificationCenter.default.removeObserver(self)
   }
   

内容的提问来源于stack exchange，提问作者Archid