How do you set a device's flash mode in Swift 4?

I've been facing the same problem. Unfortunately, many useful methods got deprecated in iOS10 and 11. Here is how I managed to resolve it:

AVCapturePhotoSettings object is unique and cannot be reused, so you need to get new settings every time using this method:

/// the current flash mode
private var flashMode: AVCaptureDevice.FlashMode = .auto

/// Get settings
///
/// - Parameters:
///   - camera: the camera
///   - flashMode: the current flash mode
/// - Returns: AVCapturePhotoSettings
private func getSettings(camera: AVCaptureDevice, flashMode: AVCaptureDevice.FlashMode) -> AVCapturePhotoSettings {
    let settings = AVCapturePhotoSettings()

    if camera.hasFlash {
        settings.flashMode = flashMode
    }
    return settings
}

As you can see, lockConfiguration is not needed.

Then simply use it while capturing photo:

 @IBAction func captureButtonPressed(_ sender: UIButton) {
    let settings = getSettings(camera: camera, flashMode: flashMode)
    photoOutput.capturePhoto(with: settings, delegate: self)
}

Hope it will help.

For objective C:

- (IBAction)turnTorchOn: (UIButton *) sender {
    sender.selected = !sender.selected;
    BOOL on;
    if (sender.selected) {
        on = true;
    } else {
        on = false;
    }
    // check if flashlight available
    Class captureDeviceClass = NSClassFromString(@"AVCaptureDevice");
    if (captureDeviceClass != nil) {
        AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
        AVCapturePhotoSettings *photosettings = [AVCapturePhotoSettings photoSettings];
        if ([device hasTorch] && [device hasFlash]) {
            [device lockForConfiguration:nil];
            if (on) {
                [device setTorchMode:AVCaptureTorchModeOn];
                photosettings.flashMode = AVCaptureFlashModeOn;
                //torchIsOn = YES; //define as a variable/property if you need to know status
            } else {
                [device setTorchMode:AVCaptureTorchModeOff];
                 photosettings.flashMode = AVCaptureFlashModeOn;
                //torchIsOn = NO;
            }
            [device unlockForConfiguration];
        }
    }
 }