there is the "aelaflmode" config variable, ... However it seems as if the D3300 does not expose it via its API . :/

Thank you for your reply and i am so sorry for that i didn't notice your anwser. It has been confirmed that the D3300 does not support AFL via its API. So i tried to capture the image in LiveView mode and it did work for me. But now i meet some problem when i using gp_camera_capture_preview to get preview image data in a while loop and i get the error message below:
// error log start
errordumper error:Writing 16 = 0x10 bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x1015 sending req failed: Error writing to the port (-35)
errordumper error:Writing 16 = 0x10 bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x1015 sending req failed: Error writing to the port (-35)
errordumper error:Writing 16 = 0x10 bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x1016 sending req failed: Error writing to the port (-35)
errordumper error:Writing 16 = 0x10 bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x1016 sending req failed: Error writing to the port (-35)
errordumper error:'ptp_setdevicepropvalue (params, PTP_DPC_NIKON_RecordingMedia, &value, PTP_DTC_UINT8)' failed: PTP I/O Error (0x02ff)
errordumper error:Writing 12 = 0xc bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x9201 sending req failed: Error writing to the port (-35)
errordumper error:Writing 12 = 0xc bytes to port failed: Error writing to the port (-35)
errordumper error:PTP_OC 0x9201 sending req failed: Error writing to the port (-35)
errordumper error:'ret' failed: Nikon enable liveview failed (0x02ff: PTP I/O Error)
errordumper error:Nikon enable liveview failed (0x02ff: PTP I/O Error)

*** Contexterror ***
Nikon enable liveview failed (0x02ff: PTP I/O Error)
errordumper error:'camera->functions->capture_preview ( camera, file, context)' failed: -7
// error log end
The code is sometimes normal but sometimes i just meet the error presented above.
That is my code snippet:
while(this->mCaptureFlag)
{
CameraFile file;
char * imageData;
retval = gp_file_new(&file);
retval = gp_camera_capture_preview(this->mCamera, file, this->mContext);
if (retval != GP_OK)
{
Log::getInstance()->error(TAG, "gp_camera_capture_preview failed...");
break;
}
else
{
retval = gp_file_get_data_and_size(file, (const char*)&imageData, &imageSize);
if(retval != GP_OK)
{
Log::getInstance()->error(TAG, "gp_file_get_data_and_size failed...");
}
char *data = (char *) malloc(imageSize);
memcpy(data, imageData, imageSize);
gp_file_unref(file);

    if(this->isFinishedFrame)
    {
        this->isFinishedFrame = false;
        emit signalSendToVideoThread(data, imageSize);
    }
    else
    {
        delete data;
    }
}
QThread::msleep(20);

}

I wonder if i didn't use the api well, thus the application went wrong ? Thank you for your reply.