Video recording fails on iPhone, but works on iPod Touch and iPad

Question

Here's the scenario: If I perform both video recording and video editing operations simultaneously (asynchronously), one of the two operations fail. The same piece of code works flawlessly on an iPod Touch, but fails on iPhone.

// Method that starts video recording
- (IBAction)recordButtonTapped:(id)sender {
    CamViewController *vc = [[CamViewController alloc] initWithNibName:@"CamViewController" bundle:nil];
    [self presentViewController:vc animated:YES completion:nil];
}

// Method that starts video editing -- more specifically it tries to fix the video orientation
- (IBAction)fixButtonTapped:(id)sender {
    [self fixVideoOrientation];
}

Here's the code that's editing the video:

- (void)fixVideoOrientation {
    NSURL *url = [NSURL fileURLWithPath:[[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"Video.MOV"]];
    AVAsset *firstAsset = [AVAsset assetWithURL:url];

    if(firstAsset !=nil && [[firstAsset tracksWithMediaType:AVMediaTypeVideo] count]>0){
        //Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
        AVMutableComposition* mixComposition = [[AVMutableComposition alloc] init];

        //VIDEO TRACK
        AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
        [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
        AVMutableVideoCompositionInstruction * MainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        MainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, firstAsset.duration);

        if ([[firstAsset tracksWithMediaType:AVMediaTypeAudio] count]>0) {
            //AUDIO TRACK
            AVMutableCompositionTrack *firstAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
            [firstAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];
        }else{
            NSLog(@"warning: video has no audio");
        }

        //FIXING ORIENTATION//
        AVMutableVideoCompositionLayerInstruction *firstLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];

        AVAssetTrack *FirstAssetTrack = [[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

        UIImageOrientation FirstAssetOrientation_  = UIImageOrientationUp;

        BOOL  isFirstAssetPortrait_  = NO;

        CGAffineTransform firstTransform = FirstAssetTrack.preferredTransform;

        if(firstTransform.a == 0 && firstTransform.b == 1.0 && firstTransform.c == -1.0 && firstTransform.d == 0) {
            FirstAssetOrientation_= UIImageOrientationRight; isFirstAssetPortrait_ = YES;
        }

        if(firstTransform.a == 0 && firstTransform.b == -1.0 && firstTransform.c == 1.0 && firstTransform.d == 0) {
            FirstAssetOrientation_ =  UIImageOrientationLeft; isFirstAssetPortrait_ = YES;
        }

        if(firstTransform.a == 1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == 1.0) {
            FirstAssetOrientation_ =  UIImageOrientationUp;
        }

        if(firstTransform.a == -1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == -1.0) {
            FirstAssetOrientation_ = UIImageOrientationDown;
        }

        //        CGFloat FirstAssetScaleToFitRatio = 320.0/FirstAssetTrack.naturalSize.width;
        CGFloat FirstAssetScaleToFitRatio = 1.0f;

        if(isFirstAssetPortrait_) {
            //            FirstAssetScaleToFitRatio = 320.0/FirstAssetTrack.naturalSize.height;
            CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);
            [firstLayerInstruction setTransform:CGAffineTransformConcat(FirstAssetTrack.preferredTransform, FirstAssetScaleFactor) atTime:kCMTimeZero];

        } else {
            CGAffineTransform FirstAssetScaleFactor = CGAffineTransformMakeScale(FirstAssetScaleToFitRatio,FirstAssetScaleToFitRatio);
            //            [FirstlayerInstruction setTransform:CGAffineTransformConcat(CGAffineTransformConcat(FirstAssetTrack.preferredTransform, FirstAssetScaleFactor),CGAffineTransformMakeTranslation(0, 160)) atTime:kCMTimeZero];
            [firstLayerInstruction setTransform:CGAffineTransformConcat(FirstAssetTrack.preferredTransform, FirstAssetScaleFactor) atTime:kCMTimeZero];
        }

        [firstLayerInstruction setOpacity:0.0 atTime:firstAsset.duration];

        MainInstruction.layerInstructions = [NSArray arrayWithObjects:firstLayerInstruction,nil];;

        AVMutableVideoComposition *MainCompositionInst = [AVMutableVideoComposition videoComposition];
        MainCompositionInst.instructions = [NSArray arrayWithObject:MainInstruction];
        MainCompositionInst.frameDuration = CMTimeMake(1, 30);

        CGSize naturalSizeFirst;

        if(isFirstAssetPortrait_){
            naturalSizeFirst = CGSizeMake(FirstAssetTrack.naturalSize.height, FirstAssetTrack.naturalSize.width);

        } else {
            naturalSizeFirst = FirstAssetTrack.naturalSize;
        }

        //Adjust Composition render size (width, height)
        float renderWidth, renderHeight;

        renderWidth = naturalSizeFirst.width;
        renderHeight = naturalSizeFirst.height;

        MainCompositionInst.renderSize = CGSizeMake(renderWidth, renderHeight);

        // Start recording to a temporary file.
        //        NSString *outputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"Fixed.mov"];
        NSString *outputFilePath = [self pathOfVideoFixOrientationFile];

        if ([[NSFileManager defaultManager] fileExistsAtPath:outputFilePath]) {
            [[NSFileManager defaultManager] removeItemAtPath:outputFilePath error:nil];
        }

        NSURL *url = [NSURL fileURLWithPath:outputFilePath];

        self.exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];

        self.exporter.outputURL=url;
        self.exporter.outputFileType = AVFileTypeQuickTimeMovie;
        self.exporter.videoComposition = MainCompositionInst;
        self.exporter.shouldOptimizeForNetworkUse = YES;

        self.exportTimer = [NSTimer scheduledTimerWithTimeInterval:0.1f target:self selector:@selector(fixingProgress) userInfo:nil repeats:YES];

        [self.exporter exportAsynchronouslyWithCompletionHandler:^ {
            dispatch_async(dispatch_get_main_queue(), ^{
                [self exportDidFinish:self.exporter];
            });
        }];

    } else {
        NSLog(@"Error, video track not found");
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error" message:@"An error occured while preparing to upload. Please try again later." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
        [alertView show];
    }
}

- (void)exportDidFinish:(AVAssetExportSession*)session {
    [self.exportTimer invalidate];

    if (session.status == AVAssetExportSessionStatusCompleted){
        NSLog(@"Completed!");

    } else {
        NSLog(@"ERROR FIXING ORIENTATION : %@", session.error);
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error" message:@"An error occured while preparing to upload. Please try again later." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
        [alertView show];
    }
}

- (NSString *)pathOfVideoFixOrientationFile {
    NSLog(@"%s", __FUNCTION__);
    NSString *directoryName = @"Fix";
    NSString *outputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:directoryName];

    BOOL isDir;
    BOOL exists = [[NSFileManager defaultManager] fileExistsAtPath:outputFilePath isDirectory:&isDir];
    if (exists) {
        /* file exists */
        if (isDir) {
            /* file is a directory */
            outputFilePath = [outputFilePath stringByAppendingPathComponent:@"Fixed.mov"];
        }
    } else {
        if ([[NSFileManager defaultManager] createDirectoryAtPath:outputFilePath withIntermediateDirectories:YES attributes:nil error:nil]) {
            outputFilePath = [outputFilePath stringByAppendingPathComponent:@"Fixed.mov"];
        }
    }
    NSLog(@"%@", outputFilePath);
    return outputFilePath;
}

- (void)fixingProgress {
    CGFloat prog = self.exporter.progress;
    self.progressView.progress = prog;
    prog = prog * 100;
    NSInteger perc = prog;
    self.progressLabel.text = [NSString stringWithFormat:@"%d%%", perc];
}

Here's the code for controller class that's used for video recording (CamViewController.h/.m):

#import <AVFoundation/AVFoundation.h>
#import <AssetsLibrary/AssetsLibrary.h>

#import "CamViewController.h"
#import "AVCamPreviewView.h"

static void * CapturingStillImageContext = &CapturingStillImageContext;
static void * RecordingContext = &RecordingContext;
static void * SessionRunningAndDeviceAuthorizedContext = &SessionRunningAndDeviceAuthorizedContext;

@interface CamViewController () <AVCaptureFileOutputRecordingDelegate>

@property (weak, nonatomic) IBOutlet UILabel *recordingTimeLabel;
@property (nonatomic, weak) IBOutlet AVCamPreviewView *previewView;
@property (nonatomic, weak) IBOutlet UIButton *recordButton;
@property (nonatomic, weak) IBOutlet UIButton *cameraButton;
@property (nonatomic, weak) IBOutlet UIButton *stillButton;

// Session management.
@property (nonatomic) dispatch_queue_t sessionQueue; // Communicate with the session and other session objects on this queue.
@property (nonatomic) AVCaptureSession *session;
@property (nonatomic) AVCaptureDeviceInput *videoDeviceInput;
@property (nonatomic) AVCaptureMovieFileOutput *movieFileOutput;
@property (nonatomic) AVCaptureStillImageOutput *stillImageOutput;

// Utilities.
@property (nonatomic) UIBackgroundTaskIdentifier backgroundRecordingID;
@property (nonatomic, getter = isDeviceAuthorized) BOOL deviceAuthorized;
@property (nonatomic, readonly, getter = isSessionRunningAndDeviceAuthorized) BOOL sessionRunningAndDeviceAuthorized;
@property (nonatomic) BOOL lockInterfaceRotation;
@property (nonatomic) id runtimeErrorHandlingObserver;

@end

@implementation CamViewController

- (BOOL)isSessionRunningAndDeviceAuthorized {
    return [[self session] isRunning] && [self isDeviceAuthorized];
}

+ (NSSet *)keyPathsForValuesAffectingSessionRunningAndDeviceAuthorized {
    return [NSSet setWithObjects:@"session.running", @"deviceAuthorized", nil];
}

- (void)viewDidLoad {
    [super viewDidLoad];

    // Create the AVCaptureSession
    AVCaptureSession *session = [[AVCaptureSession alloc] init];
    [self setSession:session];

    // Setup the preview view
    [[self previewView] setSession:session];

    // Check for device authorization
    [self checkDeviceAuthorizationStatus];

    // In general it is not safe to mutate an AVCaptureSession or any of its inputs, outputs, or connections from multiple threads at the same time.
    // Why not do all of this on the main queue?
    // -[AVCaptureSession startRunning] is a blocking call which can take a long time. We dispatch session setup to the sessionQueue so that the main queue isn't blocked (which keeps the UI responsive).

    dispatch_queue_t sessionQueue = dispatch_queue_create("session queue", DISPATCH_QUEUE_SERIAL);
    [self setSessionQueue:sessionQueue];

    dispatch_async(sessionQueue, ^{
        [self setBackgroundRecordingID:UIBackgroundTaskInvalid];

        NSError *error = nil;

        AVCaptureDevice *videoDevice = [CamViewController deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionBack];
        AVCaptureDeviceInput *videoDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];

        if (error) {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:videoDeviceInput]) {
            [session addInput:videoDeviceInput];
            [self setVideoDeviceInput:videoDeviceInput];

            dispatch_async(dispatch_get_main_queue(), ^{
                // Why are we dispatching this to the main queue?
                // Because AVCaptureVideoPreviewLayer is the backing layer for AVCamPreviewView and UIView can only be manipulated on main thread.
                // Note: As an exception to the above rule, it is not necessary to serialize video orientation changes on the AVCaptureVideoPreviewLayer’s connection with other session manipulation.

                [[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] setVideoOrientation:(AVCaptureVideoOrientation)[self interfaceOrientation]];
            });
        }

        AVCaptureDevice *audioDevice = [[AVCaptureDevice devicesWithMediaType:AVMediaTypeAudio] firstObject];
        AVCaptureDeviceInput *audioDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:&error];

        if (error) {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:audioDeviceInput]) {
            [session addInput:audioDeviceInput];
        }

        AVCaptureMovieFileOutput *movieFileOutput = [[AVCaptureMovieFileOutput alloc] init];

        if ([session canAddOutput:movieFileOutput]) {
            [session addOutput:movieFileOutput];
            AVCaptureConnection *connection = [movieFileOutput connectionWithMediaType:AVMediaTypeVideo];
            if ([connection isVideoStabilizationSupported])
                [connection setEnablesVideoStabilizationWhenAvailable:YES];
            [self setMovieFileOutput:movieFileOutput];
        }

        AVCaptureStillImageOutput *stillImageOutput = [[AVCaptureStillImageOutput alloc] init];

        if ([session canAddOutput:stillImageOutput]) {
            [stillImageOutput setOutputSettings:@{AVVideoCodecKey : AVVideoCodecJPEG}];
            [session addOutput:stillImageOutput];
            [self setStillImageOutput:stillImageOutput];
        }
    });
}

- (void)viewWillAppear:(BOOL)animated {
    dispatch_async([self sessionQueue], ^{
        [self addObserver:self forKeyPath:@"sessionRunningAndDeviceAuthorized" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:SessionRunningAndDeviceAuthorizedContext];
        [self addObserver:self forKeyPath:@"stillImageOutput.capturingStillImage" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:CapturingStillImageContext];
        [self addObserver:self forKeyPath:@"movieFileOutput.recording" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:RecordingContext];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(subjectAreaDidChange:) name:AVCaptureDeviceSubjectAreaDidChangeNotification object:[[self videoDeviceInput] device]];

        __weak CamViewController *weakSelf = self;
        [self setRuntimeErrorHandlingObserver:[[NSNotificationCenter defaultCenter] addObserverForName:AVCaptureSessionRuntimeErrorNotification object:[self session] queue:nil usingBlock:^(NSNotification *note) {
            CamViewController *strongSelf = weakSelf;
            dispatch_async([strongSelf sessionQueue], ^{
                // Manually restarting the session since it must have been stopped due to an error.
                [[strongSelf session] startRunning];
                [[strongSelf recordButton] setTitle:NSLocalizedString(@"Record", @"Recording button record title") forState:UIControlStateNormal];
            });
        }]];
        [[self session] startRunning];
    });
}

- (void)viewDidDisappear:(BOOL)animated {
    dispatch_async([self sessionQueue], ^{
        [[self session] stopRunning];

        [[NSNotificationCenter defaultCenter] removeObserver:self name:AVCaptureDeviceSubjectAreaDidChangeNotification object:[[self videoDeviceInput] device]];
        [[NSNotificationCenter defaultCenter] removeObserver:[self runtimeErrorHandlingObserver]];

        [self removeObserver:self forKeyPath:@"sessionRunningAndDeviceAuthorized" context:SessionRunningAndDeviceAuthorizedContext];
        [self removeObserver:self forKeyPath:@"stillImageOutput.capturingStillImage" context:CapturingStillImageContext];
        [self removeObserver:self forKeyPath:@"movieFileOutput.recording" context:RecordingContext];
    });
}

- (BOOL)prefersStatusBarHidden {
    return YES;
}

- (BOOL)shouldAutorotate {
    // Disable autorotation of the interface when recording is in progress.
    return ![self lockInterfaceRotation];
}

- (NSUInteger)supportedInterfaceOrientations {
    //  return UIInterfaceOrientationMaskAll;
    return UIInterfaceOrientationMaskLandscape;
}

- (void)viewDidLayoutSubviews {
    UIDeviceOrientation orientation = [[UIDevice currentDevice] orientation];

    if (orientation == UIDeviceOrientationLandscapeLeft) {
        [[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] setVideoOrientation:AVCaptureVideoOrientationLandscapeLeft];

    } else if (orientation == UIDeviceOrientationLandscapeLeft) {
        [[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] setVideoOrientation:AVCaptureVideoOrientationLandscapeRight];
    }
}    

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {

    if (context == CapturingStillImageContext) {
        BOOL isCapturingStillImage = [change[NSKeyValueChangeNewKey] boolValue];

        if (isCapturingStillImage) {
            [self runStillImageCaptureAnimation];
        }

    } else if (context == RecordingContext) {
        BOOL isRecording = [change[NSKeyValueChangeNewKey] boolValue];

        dispatch_async(dispatch_get_main_queue(), ^{

            if (isRecording) {
                [[self cameraButton] setEnabled:NO];
                [[self recordButton] setTitle:NSLocalizedString(@"Stop", @"Recording button stop title") forState:UIControlStateNormal];
                [[self recordButton] setEnabled:YES];
                self.recordingTimer = [NSTimer scheduledTimerWithTimeInterval:0.33f target:self selector:@selector(updateRecordingTimeLabel) userInfo:nil repeats:YES];
                [self.recordingTimer fire];

            } else {
                [[self cameraButton] setEnabled:YES];
                [[self recordButton] setTitle:NSLocalizedString(@"Record", @"Recording button record title") forState:UIControlStateNormal];
                [[self recordButton] setEnabled:YES];
                [self.recordingTimer invalidate];
                self.recTime = 0;
                self.recordingTimeLabel.text = @"";
            }
        });

    } else if (context == SessionRunningAndDeviceAuthorizedContext) {
        BOOL isRunning = [change[NSKeyValueChangeNewKey] boolValue];

        dispatch_async(dispatch_get_main_queue(), ^{

            if (isRunning) {
                [[self cameraButton] setEnabled:YES];
                [[self recordButton] setEnabled:YES];
                [[self stillButton] setEnabled:YES];

            } else {
                [[self cameraButton] setEnabled:NO];
                [[self recordButton] setEnabled:NO];
                [[self stillButton] setEnabled:NO];
            }
        });

    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

#pragma mark Actions

- (IBAction)toggleMovieRecording:(id)sender {
    [[self recordButton] setEnabled:NO];

    dispatch_async([self sessionQueue], ^{

        if (![[self movieFileOutput] isRecording]) {
            [self setLockInterfaceRotation:YES];

            if ([[UIDevice currentDevice] isMultitaskingSupported]) {
                // Setup background task. This is needed because the captureOutput:didFinishRecordingToOutputFileAtURL: callback is not received until AVCam returns to the foreground unless you request background execution time. This also ensures that there will be time to write the file to the assets library when AVCam is backgrounded. To conclude this background execution, -endBackgroundTask is called in -recorder:recordingDidFinishToOutputFileURL:error: after the recorded file has been saved.
                [self setBackgroundRecordingID:[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil]];
            }

            // Update the orientation on the movie file output video connection before starting recording.
            [[[self movieFileOutput] connectionWithMediaType:AVMediaTypeVideo] setVideoOrientation:[[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] videoOrientation]];

            // Turning OFF flash for video recording
            [CamViewController setFlashMode:AVCaptureFlashModeOff forDevice:[[self videoDeviceInput] device]];

            // Start recording to a temporary file.
            NSString *outputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:[@"movie" stringByAppendingPathExtension:@"mov"]];
            [[self movieFileOutput] startRecordingToOutputFileURL:[NSURL fileURLWithPath:outputFilePath] recordingDelegate:self];

        } else {
            [[self movieFileOutput] stopRecording];
        }
    });
}

- (IBAction)changeCamera:(id)sender {
    [[self cameraButton] setEnabled:NO];
    [[self recordButton] setEnabled:NO];
    [[self stillButton] setEnabled:NO];

    dispatch_async([self sessionQueue], ^{
        AVCaptureDevice *currentVideoDevice = [[self videoDeviceInput] device];
        AVCaptureDevicePosition preferredPosition = AVCaptureDevicePositionUnspecified;
        AVCaptureDevicePosition currentPosition = [currentVideoDevice position];

        switch (currentPosition) {
            case AVCaptureDevicePositionUnspecified:
                preferredPosition = AVCaptureDevicePositionBack;
                break;
            case AVCaptureDevicePositionBack:
                preferredPosition = AVCaptureDevicePositionFront;
                break;
            case AVCaptureDevicePositionFront:
                preferredPosition = AVCaptureDevicePositionBack;
                break;
        }

        AVCaptureDevice *videoDevice = [CamViewController deviceWithMediaType:AVMediaTypeVideo preferringPosition:preferredPosition];
        AVCaptureDeviceInput *videoDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:nil];

        [[self session] beginConfiguration];

        [[self session] removeInput:[self videoDeviceInput]];

        if ([[self session] canAddInput:videoDeviceInput]) {
            [[NSNotificationCenter defaultCenter] removeObserver:self name:AVCaptureDeviceSubjectAreaDidChangeNotification object:currentVideoDevice];

            [CamViewController setFlashMode:AVCaptureFlashModeAuto forDevice:videoDevice];
            [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(subjectAreaDidChange:) name:AVCaptureDeviceSubjectAreaDidChangeNotification object:videoDevice];

            [[self session] addInput:videoDeviceInput];
            [self setVideoDeviceInput:videoDeviceInput];

        } else {
            [[self session] addInput:[self videoDeviceInput]];
        }

        [[self session] commitConfiguration];

        dispatch_async(dispatch_get_main_queue(), ^{
            [[self cameraButton] setEnabled:YES];
            [[self recordButton] setEnabled:YES];
            [[self stillButton] setEnabled:YES];
        });
    });
}

#pragma mark File Output Delegate

- (void)captureOutput:(AVCaptureFileOutput *)captureOutput didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL
      fromConnections:(NSArray *)connections error:(NSError *)error {

    if (error)
        NSLog(@"%@", error);

    [self setLockInterfaceRotation:NO];

    // Note the backgroundRecordingID for use in the ALAssetsLibrary completion handler to end the background task associated with this recording. This allows a new recording to be started, associated with a new UIBackgroundTaskIdentifier, once the movie file output's -isRecording is back to NO — which happens sometime after this method returns.
    UIBackgroundTaskIdentifier backgroundRecordingID = [self backgroundRecordingID];
    [self setBackgroundRecordingID:UIBackgroundTaskInvalid];

    [[[ALAssetsLibrary alloc] init] writeVideoAtPathToSavedPhotosAlbum:outputFileURL completionBlock:^(NSURL *assetURL, NSError *error) {
        if (error)
            NSLog(@"%@", error);

        [[NSFileManager defaultManager] removeItemAtURL:outputFileURL error:nil];

        if (backgroundRecordingID != UIBackgroundTaskInvalid)
            [[UIApplication sharedApplication] endBackgroundTask:backgroundRecordingID];
    }];
}

#pragma mark Device Configuration

- (void)focusWithMode:(AVCaptureFocusMode)focusMode
       exposeWithMode:(AVCaptureExposureMode)exposureMode
        atDevicePoint:(CGPoint)point
monitorSubjectAreaChange:(BOOL)monitorSubjectAreaChange {

    dispatch_async([self sessionQueue], ^{
        AVCaptureDevice *device = [[self videoDeviceInput] device];
        NSError *error = nil;

        if ([device lockForConfiguration:&error]) {

            if ([device isFocusPointOfInterestSupported] && [device isFocusModeSupported:focusMode]) {
                [device setFocusMode:focusMode];
                [device setFocusPointOfInterest:point];
            }

            if ([device isExposurePointOfInterestSupported] && [device isExposureModeSupported:exposureMode]) {
                [device setExposureMode:exposureMode];
                [device setExposurePointOfInterest:point];
            }

            [device setSubjectAreaChangeMonitoringEnabled:monitorSubjectAreaChange];
            [device unlockForConfiguration];

        } else {
            NSLog(@"%@", error);
        }
    });
}

@end

Note: CamViewController class is inspired by Apple's AVCam sample project.

Answer 1

According to my investigation, this appears to be a limitation while running both video exporting and video recording code simultaneously, on iPhone 4(s) running iOS 7. I ended up adding proper error handling -- to show the user appropriate message and fail the operation gracefully.