Can't start iPhone X flash in touch mode

Question

I have problems with running iPhone X flash in torch mode.

Back AVCaptureDeviceTypeBuiltInTelephotoCamera selected as capture device:

com.apple.avfoundation.avcapturedevice.built-in_video:2' -
AVCaptureDeviceTypeBuiltInTelephotoCamera

After checking touch mode availability with:

[self.captureDevice isTorchModeSupported:AVCaptureTorchModeOn]

I'm trying to switch flash light into torch mode with

[self.captureDevice lockForConfiguration:nil];
BOOL result = [self.captureDevice setTorchModeOnWithLevel:1 error:&error];
[self.captureDevice unlockForConfiguration];

This call is successful. result == YES and error == nil. But flash light blinks once then turns off.

I saw this behaviour on iPhone X myself and there is a reports of the same behaviour from iPhone 8 and iPhone 8 Plus owners. Some users say that this problem appeared after update to iOS 11.1. But I couldn't reproduce it with iPhone 8 myself.

Is there any ideas how to fix or debug this problem?

Full code snippet from my app listed below:

// Retrieve the back camera
    if ([AVCaptureDeviceDiscoverySession class]) {
        DDLogDebug(@"Search camera with AVCaptureDeviceDiscoverySession");
        AVCaptureDevice* camera =
        [AVCaptureDeviceDiscoverySession
         discoverySessionWithDeviceTypes: @[AVCaptureDeviceTypeBuiltInTelephotoCamera]
         mediaType:AVMediaTypeVideo
         position:AVCaptureDevicePositionBack].devices.firstObject;

        if (!camera) {
            camera = [AVCaptureDeviceDiscoverySession
                      discoverySessionWithDeviceTypes: @[AVCaptureDeviceTypeBuiltInTelephotoCamera]
                      mediaType:AVMediaTypeVideo
                      position:AVCaptureDevicePositionBack].devices.firstObject;
        }
        DDLogDebug(@"Did find %@ camera", camera);
        self.captureDevice = camera;
    } else {
        DDLogDebug(@"Haven't found camera device with AVCaptureDeviceDiscoverySession");
    }

    if (!self.captureDevice) {
        DDLogDebug(@"Searching at [AVCaptureDevice devices], where %lu devices available", (unsigned long)AVCaptureDevice.devices.count);
        for (AVCaptureDevice *device in [AVCaptureDevice devices]) {
            if ([device hasMediaType:AVMediaTypeVideo] && [device hasTorch]) {
                self.captureDevice = device;
                break;
            }
        }
    }

    if (!self.captureDevice) {
        NSError* error = [NSError buildError:^(MRErrorBuilder *builder) {
            builder.localizedDescription = NSLocalizedString(@"There is no camera devices able to measure heart rate", nil);
            builder.domain               = kWTCameraHeartRateMonitorError;
            builder.code                 = 27172;
        }];
        DDLogError(@"%@", error);
        self.session = nil;
        self.handler(0, 0, error);
        return NO;
    }

    NSError *error;
    AVCaptureDeviceInput *input = [[AVCaptureDeviceInput alloc] initWithDevice:self.captureDevice
                                                                         error:&error];
    if (error) {
        DDLogError(@"%@", error);
        self.session = nil;
        self.handler(0, 0, error);
        return NO;
    }


    NSString* deviceType = [self.captureDevice respondsToSelector:@selector(deviceType)] ? self.captureDevice.deviceType : @"Unknown";

    DDLogDebug(@"Configurating camera '%@'/'%@' - %@ id %@ at %ld connected: %@", self.captureDevice.localizedName, self.captureDevice.modelID, deviceType, self.captureDevice.uniqueID, (long)self.captureDevice.position, self.captureDevice.connected?@"YES":@"NO");

    self.session               = [[AVCaptureSession alloc] init];
    NSString* preset = [self.session canSetSessionPreset:AVCaptureSessionPresetLow] ? AVCaptureSessionPresetLow : nil;
    if (preset) {
        self.session.sessionPreset = preset;
    }

    [self.session beginConfiguration];
    [self.session addInput:input];

    // Find the max frame rate we can get from the given device
    AVCaptureDeviceFormat *currentFormat;
    for (AVCaptureDeviceFormat *format in self.captureDevice.formats)
    {
        NSArray *ranges = format.videoSupportedFrameRateRanges;
        AVFrameRateRange *frameRates = ranges[0];

        // Find the lowest resolution format at the frame rate we want.
        if (frameRates.maxFrameRate == FRAMES_PER_SECOND && (!currentFormat || (CMVideoFormatDescriptionGetDimensions(format.formatDescription).width < CMVideoFormatDescriptionGetDimensions(currentFormat.formatDescription).width && CMVideoFormatDescriptionGetDimensions(format.formatDescription).height < CMVideoFormatDescriptionGetDimensions(currentFormat.formatDescription).height)))
        {
            currentFormat = format;
        }
    }

    if (![self.captureDevice isTorchModeSupported:AVCaptureTorchModeOn]) {
        NSError* error = [NSError buildError:^(MRErrorBuilder *builder) {
            builder.localizedDescription = NSLocalizedString(@"Torch mode is not supported for your camera", nil);
            builder.domain               = kWTCameraHeartRateMonitorError;
            builder.code                 = 28633;
        }];
        self.session = nil;
        DDLogError(@"%@", error);
        self.session = nil;
        self.handler(0, 0, error);
        return NO;
    }

    // Tell the device to use the max frame rate.
    [self.captureDevice lockForConfiguration:nil];
    DDLogVerbose(@"Turn on tourch mode with level 0.5");
    self.captureDevice.flashMode = AVCaptureFlashModeOff;
    BOOL result = [self.captureDevice setTorchModeOnWithLevel:0.5 error:&error];
    if (!result) {
        DDLogError(@"%@", error);
        self.session = nil;
        self.handler(0, 0, error);
        return NO;
    }
    [self.captureDevice setFocusMode:AVCaptureFocusModeLocked];
    [self.captureDevice setFocusModeLockedWithLensPosition:1.0
                                         completionHandler:nil];
    self.captureDevice.activeFormat = currentFormat;
    self.captureDevice.activeVideoMinFrameDuration = CMTimeMake(1, FRAMES_PER_SECOND);
    self.captureDevice.activeVideoMaxFrameDuration = CMTimeMake(1, FRAMES_PER_SECOND);
    [self.captureDevice unlockForConfiguration];

    // Set the output
    AVCaptureVideoDataOutput* videoOutput = [AVCaptureVideoDataOutput new];

    // create a queue to run the capture on
    dispatch_queue_t captureQueue=dispatch_queue_create("catpureQueue", DISPATCH_QUEUE_SERIAL);

    // setup our delegate
    [videoOutput setSampleBufferDelegate:self queue:captureQueue];

    // configure the pixel format

    videoOutput.videoSettings = @{(id)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_32BGRA)};
    videoOutput.alwaysDiscardsLateVideoFrames = NO;

    [self.session addOutput:videoOutput];

    if (debugPath) {
        NSError* error;
        [[NSFileManager defaultManager] removeItemAtPath:debugPath
                                                   error:nil];

        BOOL result =
        [[NSFileManager defaultManager] createDirectoryAtPath:debugPath
                                  withIntermediateDirectories:YES
                                                   attributes:nil
                                                        error:&error];
        if (result) {
            [self setupDebugRecordAt:debugPath withFormat:currentFormat];
        } else {
            DDLogError(@"%@", error);
        }

        const char* path = [debugPath cStringUsingEncoding:NSUTF8StringEncoding];
        self.filter->setDebugPath(path);
    }


    // Start the video session
    [self.session commitConfiguration];

    self.frameNumber = 0;
    [self.assetWriter startWriting];
    [self.assetWriter startSessionAtSourceTime:kCMTimeZero];
    [self.session startRunning];

Answer 1

Finally, the problem is fixed. I'm not sure about exact reason. Any information related to this issue are appreciated.

This problem exists on iPhone 8, 8+ and iPhone X running iOS 11.1. I've reproduced this behaviour on iPhone 8 after updating form iOS from 11.0 to 11.1.

What I noticed is that torch turns on after calling

BOOL result = [self.captureDevice setTorchModeOnWithLevel:0.5 error:&error];

and turns off after

[self.captureDevice setFocusMode:AVCaptureFocusModeLocked];

or

[self.session commitConfiguration];

So the solution was to perform torch configuration where ALL other session and device configurations are finished and session is started.

My current implementation is:

// Session configuration ...

[self.session startRunning];

if (![self.captureDevice isTorchModeSupported:AVCaptureTorchModeOn]) {
    NSError* error = [NSError buildError:^(MRErrorBuilder *builder) {
        builder.localizedDescription = NSLocalizedString(@"Torch mode is not supported for your camera", nil);
        builder.domain               = kWTCameraHeartRateMonitorError;
        builder.code                 = 28633;
    }];
    DDLogError(@"%@", error);
    if (self.session) {
       [self.session stopRunning];
    }
    self.session = nil;
    self.handler(0, 0, error);
    return NO;
}

[self.captureDevice lockForConfiguration:nil];
self.captureDevice.flashMode = AVCaptureFlashModeOff;
[self.captureDevice setFocusMode:AVCaptureFocusModeLocked];
[self.captureDevice setFocusModeLockedWithLensPosition:1.0
                                         completionHandler:nil];
self.captureDevice.activeFormat = currentFormat;
self.captureDevice.activeVideoMinFrameDuration = CMTimeMake(1, FRAMES_PER_SECOND);
self.captureDevice.activeVideoMaxFrameDuration = CMTimeMake(1, FRAMES_PER_SECOND);

// This call should be placed AFTER all other configurations

BOOL result = [self.captureDevice setTorchModeOnWithLevel:0.5 error:&error];
if (!result) {
    DDLogError(@"%@", error);
    self.session = nil;
    self.handler(0, 0, error);
    return NO;
}
[self.captureDevice unlockForConfiguration];