iOS扫码App触摸屏幕触发自动对焦时偶现崩溃求助

我的App实现了触摸屏幕对应位置自动对焦的二维码扫描功能，但偶发崩溃，崩溃点在device.focusPointOfInterest = focusPoint这一行。

相关代码 #

/** Touch the screen for autofocus */
public override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    guard touches.first?.view == view,
          let touchPoint = touches.first,
          let device = videoCaptureDevice
    else { return }
    
    let videoView = view
    let screenSize = videoView!.bounds.size
    let xPoint = touchPoint.location(in: videoView).y / screenSize.height
    let yPoint = 1.0 - touchPoint.location(in: videoView).x / screenSize.width
    let focusPoint = CGPoint(x: xPoint, y: yPoint)
    
    do {
        try device.lockForConfiguration()
    } catch {
        return
    }
    
    // Focus to the correct point, make continiuous focus and exposure so the point stays sharp when moving the device closer
    device.focusPointOfInterest = focusPoint
    device.focusMode = .continuousAutoFocus
    device.exposurePointOfInterest = focusPoint
    device.exposureMode = AVCaptureDevice.ExposureMode.continuousAutoExposure
    device.unlockForConfiguration()
}

崩溃日志（相关部分） #

Last Exception Backtrace:
0   CoreFoundation                  0x180f37c60 __exceptionPreprocess + 216 
(NSException.m:200)
1   libobjc.A.dylib                 0x198767ee4 objc_exception_throw + 56 (objc- 
exception.mm:565)
2   AVFCapture                      0x1a45297f4 -[AVCaptureFigVideoDevice          
setFocusPointOfInterest:] + 276 (AVCaptureFigVideoDevice.m:0)
3   QR Code Creator Scanner         0x1021f13bc specialized 
CodeScannerView.ScannerViewController.touchesBegan(_:with:) + 508 
(CodeScannerView.swift:307)
4   QR Code Creator Scanner         0x1021f010c 
CodeScannerView.ScannerViewController.touchesBegan(_:with:) + 12 (<compiler- 
generated>:0)
5   QR Code Creator Scanner         0x1021f010c @objc 
CodeScannerView.ScannerViewController.touchesBegan(_:with:) + 120
6   UIKitCore                       0x1833c099c forwardTouchMethod + 316 
(UIResponder.m:2039)
7   UIKitCore                       0x183341ed0 -[UIWindow _sendTouchesForEvent:] + 
624 (UIWindow.m:2960)

崩溃原因分析 #

从崩溃栈可以看出，异常来自AVFoundation框架的setFocusPointOfInterest:方法，可能的触发场景包括：

• 设备不支持手动指定对焦兴趣点（比如部分老旧设备或前置摄像头）
• 计算出的对焦坐标超出了[0,1]的有效范围
• 相机设备状态异常（如被其他会话占用、设备断开），导致配置锁失效后仍执行参数设置

修复方案 #

针对上述可能的原因，对代码进行以下优化：

1. 校验设备能力：设置参数前先确认设备是否支持对应功能
2. 限制坐标范围：确保对焦坐标始终在0到1的有效区间内
3. 确保配置锁正确释放：使用defer语句保证锁一定会被释放
4. 捕获参数设置异常：对设备参数设置过程添加异常捕获，避免崩溃

修改后的代码 #

/** Touch the screen for autofocus */
public override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    guard touches.first?.view == view,
          let touchPoint = touches.first,
          let device = videoCaptureDevice
    else { return }
    
    let videoView = view
    let screenSize = videoView!.bounds.size
    let rawX = touchPoint.location(in: videoView).y / screenSize.height
    let rawY = 1.0 - touchPoint.location(in: videoView).x / screenSize.width
    // 限制坐标在[0,1]有效范围内
    let xPoint = max(0.0, min(1.0, rawX))
    let yPoint = max(0.0, min(1.0, rawY))
    let focusPoint = CGPoint(x: xPoint, y: yPoint)
    
    do {
        try device.lockForConfiguration()
    } catch {
        return
    }
    
    // 确保无论是否异常，都会释放配置锁
    defer {
        device.unlockForConfiguration()
    }
    
    do {
        // 检查对焦功能支持性
        if device.isFocusPointOfInterestSupported,
           device.isFocusModeSupported(.continuousAutoFocus) {
            device.focusPointOfInterest = focusPoint
            device.focusMode = .continuousAutoFocus
        }
        
        // 检查曝光功能支持性
        if device.isExposurePointOfInterestSupported,
           device.isExposureModeSupported(.continuousAutoExposure) {
            device.exposurePointOfInterest = focusPoint
            device.exposureMode = .continuousAutoExposure
        }
    } catch {
        print("相机配置设置失败: \(error)")
    }
}

补充说明 #

• 部分设备的前置摄像头或老旧机型不支持手动对焦，必须通过能力校验避免无效操作
• 坐标范围限制可以解决因视图边界计算误差、触摸位置超出视图等情况导致的无效坐标输入
• defer语句能保证设备配置锁被正确释放，避免因异常导致锁未释放，影响后续相机操作

内容的提问来源于stack exchange，提问作者Deepak Gautam