在ios中使用AVFoundation捕获图像的问题？

Question

我正在尝试使用AVFoundation捕获图像。我正在使用以下代码。

//in interface
AVCaptureDeviceInput *inputBack;
AVCaptureStillImageOutput *outputBack;
AVCaptureConnection *videoConnection;
AVCaptureVideoPreviewLayer *captureVideoPreviewLayer;

-(void)toStartCameraSession
{
 if (!session)
  {
    NSLog(@"View Will Appear Session Called");

    session = [[AVCaptureSession alloc] init];

    NSArray *devices=[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];

    inputBack = [AVCaptureDeviceInput deviceInputWithDevice:[devices objectAtIndex:0] error:nil];

    [session addInput:inputBack];

    //add the output to session
    outputBack = [[AVCaptureStillImageOutput alloc]init];
    if ([session canAddOutput:outputBack])
    {
        [session addOutput:outputBack];
    }
    else
    {
        NSLog(@"Error on add output, addOutput");
    }

    session.sessionPreset = AVCaptureSessionPresetPhoto;

    // Create the preview layer

    captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];
    CGRect bounds=captureView.layer.bounds;
    captureVideoPreviewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    captureVideoPreviewLayer.bounds=bounds;
    captureVideoPreviewLayer.position=CGPointMake(CGRectGetMidX(bounds), CGRectGetMidY(bounds));

    [captureVideoPreviewLayer setFrame:CGRectMake(0, 0, self.captureView.frame.size.width, self.captureView.frame.size.height)];
    // Set backgrounds colors
    [captureVideoPreviewLayer setBackgroundColor:[UIColor clearColor].CGColor];

    AVCaptureConnection *previewLayerConnection=captureVideoPreviewLayer.connection;

    if ([previewLayerConnection isVideoOrientationSupported])
    {
        [previewLayerConnection setVideoOrientation:[[UIApplication sharedApplication] statusBarOrientation]];
    }
    [self.captureView.layer addSublayer:captureVideoPreviewLayer];
 }

}

-(void)takePhoto
{   
 videoConnection = nil;
 //Find The Connection With Type
 for (AVCaptureConnection *connection in outputBack.connections)
  {
    NSLog(@"**********takephoto*********");
    for (AVCaptureInputPort *port in [connection inputPorts]) {
        if ([[port mediaType] isEqual:AVMediaTypeVideo])
        {
            videoConnection = connection;
            break;
        }
    }
    if (videoConnection)
    {
        break;
    }
}

//Capture the image Asynchronous
[outputBack captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler:
 ^(CMSampleBufferRef imageSampleBuffer, NSError *error)
 {
      NSLog(@"*****fffff*****takephoto*********");
     if (imageSampleBuffer != NULL)
     {
         NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageSampleBuffer];

         UIImage *imageTaked = [[UIImage alloc] initWithData:imageData];

         NSLog(@"captured image in start ---- %@",tempimgview.image);


     }
 }];
[session stopRunning];

}

我在toStartCameraSession中调用ViewWillAppear方法。它有时会工作，有时也不会。

任何帮助都会得到感谢。谢谢。