How to do auto focus when user moves mobile in iOS?

Question

What I was trying is when user moves I want to do auto focus ,Here I was created camera with ,

AVCaptureSession place because if I open camera through UIImagePickerController then auto focus is not possible so I preferred AVCaptureSession.

 //Capture Session
    AVCaptureSession *session = [[AVCaptureSession alloc]init];
    session.sessionPreset = AVCaptureSessionPresetPhoto;

    //Add device
    AVCaptureDevice *device =
    [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    // SETUP FOCUS MODE
    if ([device lockForConfiguration:nil]) {

        [device setFocusMode:AVCaptureFocusModeAutoFocus];

        [device unlockForConfiguration];
    }
    else{
        NSLog(@"error while configuring focusMode");
    }

    //Input
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:nil];

    if (!input)
    {
        NSLog(@"No Input");
    }

    [session addInput:input];

    //Output
    AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc] init];
    [session addOutput:output];
    output.videoSettings =
    @{ (NSString *)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA) };

    //Preview Layer
    AVCaptureVideoPreviewLayer *previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];
    UIView *myView = viewForCamera;
    previewLayer.frame = myView.bounds;
    previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    [viewForCamera.layer addSublayer:previewLayer];

    //Start capture session
    [session startRunning];

But my code is not working because some fault I did. Please help me to over come this problem.