To perform a real-time capture, a client may instantiate AVCaptureSession and add appropriate AVCaptureInputs, such as AVCaptureDeviceInput, and outputs, such as AVCaptureMovieFileOutput. [AVCaptureSession startRunning] starts the flow of data from the inputs to the outputs, and [AVCaptureSession stopRunning] stops the flow. A client may set the sessionPreset property to customize the quality level or bitrate of the output

AVCaptureDevice* inputDevice =
      [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
  AVCaptureDeviceInput *captureInput =
      [AVCaptureDeviceInput deviceInputWithDevice:inputDevice error:nil];
  AVCaptureVideoDataOutput *captureOutput = [[AVCaptureVideoDataOutput alloc] init]; 
  captureOutput.alwaysDiscardsLateVideoFrames = YES; 
  [captureOutput setSampleBufferDelegate:self queue:dispatch_get_main_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] autorelease];