iPhone SDK 4 AVFoundation - Как правильно использовать captureStillImageAsynchronouslyFromConnection?

Я пытаюсь использовать новую платформу AVFoundation для фотосъемки на iPhone.

Этот метод вызывается нажатием кнопки. Я слышу звук затвора, но не вижу вывода журнала. Если я вызову этот метод несколько раз, предварительный просмотр камеры остановится.

Есть ли какой-нибудь учебник по использованию captureStillImageAsynchronouslyFromConnection ?

[[self stillImageOutput] captureStillImageAsynchronouslyFromConnection:
                [[self stillImageOutput].connections objectAtIndex:0]
                     completionHandler:^(CMSampleBufferRef imageDataSampleBuffer,
                            NSError *error) {
                                              NSLog(@"inside");
                            }];

- (void)initCapture {
    AVCaptureDeviceInput *captureInput = [AVCaptureDeviceInput 
                                          deviceInputWithDevice:[AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo] 
                                          error:nil];

    AVCaptureVideoDataOutput *captureOutput = [[AVCaptureVideoDataOutput alloc] init];

    captureOutput.alwaysDiscardsLateVideoFrames = YES; 

    dispatch_queue_t queue;
    queue = dispatch_queue_create("cameraQueue", NULL);
    [captureOutput setSampleBufferDelegate:self queue:queue];
    dispatch_release(queue);

    NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey; 
    NSNumber* value = [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32BGRA]; 
    NSDictionary* videoSettings = [NSDictionary dictionaryWithObject:value forKey:key]; 
    [captureOutput setVideoSettings:videoSettings]; 

    self.captureSession = [[AVCaptureSession alloc] init];
    self.captureSession.sessionPreset = AVCaptureSessionPresetLow;

    [self.captureSession addInput:captureInput];
    [self.captureSession addOutput:captureOutput];

    self.prevLayer = [AVCaptureVideoPreviewLayer layerWithSession: self.captureSession];

    [self.prevLayer setOrientation:AVCaptureVideoOrientationLandscapeLeft];

    self.prevLayer.frame = CGRectMake(0.0, 0.0, 480.0, 320.0);
    self.prevLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;

    [self.view.layer addSublayer: self.prevLayer];


    // Setup the default file outputs
    AVCaptureStillImageOutput *_stillImageOutput = [[[AVCaptureStillImageOutput alloc] init] autorelease];
    NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys:
                                    AVVideoCodecJPEG, AVVideoCodecKey,
                                    nil];
    [_stillImageOutput setOutputSettings:outputSettings];
    [outputSettings release];
    [self setStillImageOutput:_stillImageOutput];   

    if ([self.captureSession canAddOutput:stillImageOutput]) {
        [self.captureSession addOutput:stillImageOutput];
    }

    [self.captureSession commitConfiguration];
    [self.captureSession startRunning];

}