AVCaptureOutput takes dark picture even with flash on

I have come up with an implementation of AVFoundation and ImageIO to take care of the photo taking in my application. I have an issue with it, however. The images I take are always dark, even if the flash goes off. Here's the code I use:

        [[self currentCaptureOutput] captureStillImageAsynchronouslyFromConnection:[[self currentCaptureOutput].connections lastObject]
                                        completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {

                                            [[[blockSelf currentPreviewLayer] session] stopRunning];
                                            if (!error) {
                                                NSData *data            = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
                                                CGImageSourceRef source = CGImageSourceCreateWithData开发者_高级运维((CFDataRef) data, NULL);

                                                if (source) {

                                                    UIImage *image = [blockSelf imageWithSource:source];
                                                    [blockSelf updateWithCapturedImage:image];
                                                    CFRelease(source);

                                                }

                                            }

                                        }];

Is there anything there that could cause the image taken to not include the flash?

I found I sometimes got dark images if the AVCaptureSession was set up immediately before this call. Perhaps it takes a while for the auto-exposure & white balance settings to adjust themselves.

The solution was to set up the AVCaptureSession, then wait until the AVCaptureDevice's adjustingExposure and adjustingWhiteBalance properties are both NO (observe these with KVO) before calling -[AVCaptureStillImageOutput captureStillImageAsynchronouslyFromConnection: completionHandler:].