AVCaptureDeviceInput返回nil致部分用户相机功能崩溃求助

Hey there, let's break down the two main issues you're facing—crash in 10% of devices and deviceInputWithDevice returning nil for some users. Here's what I've debugged in similar cases and how to fix them:

This is almost always tied to permissions, device availability, or unhandled errors. Let's cover each scenario:

• Missing Camera Permissions
  If the user hasn't granted camera access, or has denied/restricted it, creating a capture input will fail silently (or return nil). Always check permission status before initializing your capture session:

  AVAuthorizationStatus status = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
  if (status == AVAuthorizationStatusDenied || status == AVAuthorizationStatusRestricted) {
      // Show an alert asking the user to enable camera access in Settings
      UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Camera Access Required" message:@"Please enable camera access in your device settings to use this feature." preferredStyle:UIAlertControllerStyleAlert];
      [alert addAction:[UIAlertAction actionWithTitle:@"Go to Settings" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
          [[UIApplication sharedApplication] openURL:[NSURL URLWithString:UIApplicationOpenSettingsURLString]];
      }]];
      [self presentViewController:alert animated:YES completion:nil];
      return;
  } else if (status == AVAuthorizationStatusNotDetermined) {
      // Request permission first
      [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL granted) {
          if (granted) {
              // Initialize camera on the main thread
              dispatch_async(dispatch_get_main_queue(), ^{
                  [self setupCameraSession];
              });
          } else {
              // Handle denied permission
          }
      }];
      return;
  }
  

• Device Already in Use
  If another app (or even another part of your own app) is using the camera, deviceInputWithDevice will return nil. Always capture the error to diagnose this:

  NSError *inputError = nil;
  AVCaptureDeviceInput *deviceInput = [AVCaptureDeviceInput deviceInputWithDevice:device error:&inputError];
  if (!deviceInput) {
      NSLog(@"Failed to create capture input: %@", inputError.localizedDescription);
      // Show an alert telling the user to close other apps using the camera
      return;
  }
  

• Invalid Device or Preset
  Rare, but some older devices might not support AVCaptureSessionPresetPhoto, or defaultDeviceWithMediaType:AVMediaTypeVideo could return nil (e.g., on devices without a camera, though these are rare now). Add checks:

  AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
  if (!device) {
      // Alert user their device doesn't support camera functionality
      return;
  }
  
  // Verify the session preset is supported
  if (![_session canSetSessionPreset:AVCaptureSessionPresetPhoto]) {
      _session.sessionPreset = AVCaptureSessionPresetHigh; // Fallback to a supported preset
  }
  

Looking at your code snippet, the most likely culprit is unlocked device configuration and potential thread safety issues:

• Forgot to Unlock Device Configuration
  You're calling lockForConfiguration:nil, but if the lock succeeds, you never call unlockForConfiguration! Leaving the device locked can cause race conditions, crashes, or failed operations later. Fix this immediately:

  if ([device lockForConfiguration:nil]) {
      if (device.autoFocusRangeRestrictionSupported) {
          device.autoFocusRangeRestriction = AVCaptureAutoFocusRangeRestrictionNear; // Or your desired value
      }
      // Critical: Unlock the device after making changes
      [device unlockForConfiguration];
  } else {
      NSLog(@"Failed to lock device configuration");
  }
  

• AVCaptureSession Operations on Wrong Thread
  All configuration and state changes for AVCaptureSession (like adding inputs/outputs, starting/stopping the session) must be done on a serial queue (or the main thread—serial queue is better for performance). If you're modifying the session from a background thread, this will cause crashes. Use a dedicated queue:

  // Declare a serial queue as an instance variable
  @property (nonatomic, strong) dispatch_queue_t cameraQueue;
  
  // Initialize it in your setup
  self.cameraQueue = dispatch_queue_create("com.yourapp.cameraQueue", DISPATCH_QUEUE_SERIAL);
  
  // Use it for all session operations
  dispatch_async(self.cameraQueue, ^{
      [_session beginConfiguration];
      // Add inputs/outputs, adjust presets, etc.
      [_session commitConfiguration];
      [_session startRunning];
  });
  

• Session Not Properly Retained
  If _session is a weak reference, it might get deallocated unexpectedly, leading to crashes. Ensure it's a strong instance variable:

  @property (nonatomic, strong) AVCaptureSession *session;
  

• Always validate all objects (device, input, output) before using them—never assume they'll be non-nil.
• Test on older iOS versions (e.g., iOS 13/14) if you're supporting them, as some APIs behave differently.
• Check crash logs for specific stack traces—they'll point you to the exact line causing the crash (e.g., if it's a null pointer dereference or a thread violation).

内容的提问来源于stack exchange，提问作者Dipen Sekhsaria