Interpreting keypresses sent to raspberry-pi through uv4l-webrtc datachannel

Question

I apologize if this doesn't make sense since I'm still a newbie with using a raspberry pi and this is my first time posting on StackOverflow.

I am making a web app that lets me stream video to and from a raspberry pi while also letting me send keycodes. The sent keycodes would ultimately let me control servos on a drone. After scouring the internet, I figured that the simplest way to stream 2-way video is by using uv4l so I have it installed along with uv4l-webrtc on my raspberry pi. I hooked up some GPIO pins to a flight controller and I am using pigpio to send PWM signals to it, which I then monitor using CleanFlight.

Right now, I can manipulate with keypresses the roll, pitch, etc. of the flight controller using a python script if I access the pi remotely using VNC, but I would like to ultimately be able to do this through my custom web page that is being served by the uv4l-server. I am trying to use the WebRTC Data Channels, but I'm having some trouble understanding what I would need to do to recognize the messages sent through the data channels. I know that the data channels are opened when a video call is initiated and I've tried the test in this link to see if I can indeed send keycodes to the pi (and I can).

My problem right now is that I have no idea where those sent messages go or how I can get them so I can incorporate them into my python script. Would I need to make a server that would listen for the keycodes being sent to the pi?

tl;dr I have a python script on a raspberry pi to control servos on a flight controller using keypresses and a separate webpage that streams video using WebRTC, but I have no idea how to combine them together using WebRTC data channels.

Thanks to @adminkiam for the solution. Here's a version of the python script that now listens to the socket. It's essentially a variation of this code by the person who made pigpio:

import socket
import time
import pigpio

socket_path = '/tmp/uv4l.socket'

try:
    os.unlink(socket_path)
except OSError:
    if os.path.exists(socket_path):
        raise

s = socket.socket(socket.AF_UNIX, socket.SOCK_SEQPACKET)

ROLL_PIN     = 13
PITCH_PIN    = 14
YAW_PIN      = 15

MIN_PW = 1000
MID_PW = 1500
MAX_PW = 2000

NONE        = 0
LEFT_ARROW  = 1
RIGHT_ARROW = 2
UP_ARROW    = 3
DOWN_ARROW  = 4
LESS_BTN    = 5
GREATER_BTN = 6

print 'socket_path: %s' % socket_path
s.bind(socket_path)
s.listen(1)

def getch(keyCode):
    key = NONE
    if keyCode == 188:
        key = LESS_BTN
    elif keyCode == 190:
        key = GREATER_BTN
    elif keyCode == 37:
        key = LEFT_ARROW
    elif keyCode == 39:
        key = RIGHT_ARROW
    elif keyCode == 38:
        key = UP_ARROW
    elif keyCode == 40:
        key = DOWN_ARROW
    return key

def cleanup():
    pi.set_servo_pulsewidth(ROLL_PIN, 0)
    pi.set_servo_pulsewidth(PITCH_PIN, 0)
    pi.set_servo_pulsewidth(YAW_PIN, 0)
    pi.stop()

while True:
    print 'awaiting connection...'
    connection, client_address = s.accept()
    print 'client_address %s' % client_address
    try:
        print 'established connection with', client_address

        pi = pigpio.pi()

        rollPulsewidth     = MID_PW
        pitchPulsewidth    = MID_PW
        yawPulsewidth      = MID_PW

        pi.set_servo_pulsewidth(ROLL_PIN, rollPulsewidth)
        pi.set_servo_pulsewidth(PITCH_PIN, pitchPulsewidth)
        pi.set_servo_pulsewidth(YAW_PIN, yawPulsewidth)

        while True:
            data = connection.recv(16)
            print 'received message"%s"' % data

            time.sleep(0.01)
            key = getch(int(data))

            rollPW     = rollPulsewidth
            pitchPW    = pitchPulsewidth
            yawPW      = yawPulsewidth

            if key == UP_ARROW:
                pitchPW = pitchPW + 10
                if pitchPW > MAX_PW:
                    pitchPW = MAX_PW
            elif key == DOWN_ARROW:
                pitchPW = pitchPW - 10
                if pitchPW < MIN_PW:
                    pitchPW = MIN_PW
            elif key == LEFT_ARROW:
                rollPW = rollPW - 10
                if rollPW < MIN_PW:
                    rollPW = MIN_PW
            elif key == RIGHT_ARROW:
                rollPW = rollPW + 10
                if rollPW > MAX_PW:
                    rollPW = MAX_PW
            elif key == GREATER_BTN:
                yawPW = yawPW + 10
                if yawPW > MAX_PW:
                    yawPW = MAX_PW
            elif key == LESS_BTN:
                yawPW = yawPW - 10
                if yawPW < MIN_PW:
                    yawPW = MIN_PW

            if rollPW != rollPulsewidth:
                rollPulsewidth = rollPW
                pi.set_servo_pulsewidth(ROLL_PIN, rollPulsewidth)
            if pitchPW != pitchPulsewidth:
                pitchPulsewidth = pitchPW
                pi.set_servo_pulsewidth(PITCH_PIN, pitchPulsewidth)
            if yawPW != yawPulsewidth:
                yawPulsewidth = yawPW
                pi.set_servo_pulsewidth(YAW_PIN, yawPulsewidth)

            if data:
                print 'echo data to client'
                connection.sendall(data)
            else:
                print 'no more data from', client_address
                break

    finally:
        # Clean up the connection
        cleanup()
        connection.close()

Answer 1

When a WebRTC data channel is created between UV4L and the other WebRTC peer (i.e. a browser, Janus Gateway, etc...), UV4L creates a full-duplex Unix Domain Socket (/tmp/uv4l.socket by default) from/to which you can receive/send messages on the Raspberry Pi. Your python script should just open, listen and read to the socket for the incoming messages from the e.g. web application and/or write the messages to the same socket for the web app to receive them. An example doing this in C++ is under the link to the tutorial you pointed out in your question:

/*
    Copyright (c) 2016 info@linux-projects.org
    All rights reserved.

    Redistribution and use in source and binary forms are permitted
    provided that the above copyright notice and this paragraph are
    duplicated in all such forms and that any documentation,
    advertising materials, and other materials related to such
    distribution and use acknowledge that the software was developed
    by the linux-projects.org. The name of the
    linux-projects.org may not be used to endorse or promote products derived
    from this software without specific prior written permission.
    THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
    IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
*/

/*
 * This is a simple echo server.
 * It creates to a unix domain socket of type SOCK_SEQPACKET specified by
 * command line, listens to it waiting for incoming messages from clients
 * (e.g. UV4L) and replies the received messages back to the senders.
 *
 * Example:
 *     $ ./datachannel_server /tmp/uv4l.socket
 *
 * To compile this program you need boost v1.60 or greater, for example:
 * g++ -Wall -I/path/to/boost/include/ -std=c++11 datachannel_server.cpp -L/path/to/boost/lib -l:libboost_coroutine.a -l:libboost_context.a -l:libboost_system.a -l:libboost_thread.a -pthread -o datachannel_server
 */

#include <boost/asio/io_service.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/asio/write.hpp>
#include <boost/asio/buffer.hpp>
#include <boost/asio.hpp>
#include <memory>
#include <cstdio>
#include <array>
#include <functional>
#include <iostream>

#if !defined(BOOST_ASIO_HAS_LOCAL_SOCKETS)
#error Local sockets not available on this platform.
#endif

constexpr std::size_t MAX_PACKET_SIZE = 1024 * 16;

namespace seqpacket {

    struct seqpacket_protocol {

        int type() const {
            return SOCK_SEQPACKET;
        }

        int protocol() const {
            return 0;
        }

        int family() const {
            return AF_UNIX;
        }

        using endpoint = boost::asio::local::basic_endpoint<seqpacket_protocol>;
        using socket = boost::asio::generic::seq_packet_protocol::socket;
        using acceptor = boost::asio::basic_socket_acceptor<seqpacket_protocol>;

#if !defined(BOOST_ASIO_NO_IOSTREAM)
        /// The UNIX domain iostream type.
        using iostream = boost::asio::basic_socket_iostream<seqpacket_protocol>;
#endif
    };
}

using seqpacket::seqpacket_protocol;

struct session : public std::enable_shared_from_this<session> {
    explicit session(seqpacket_protocol::socket socket) : socket_(std::move(socket)) {}

    ~session() {
        //std::cerr << "session closed\n";
    }

    void echo(boost::asio::yield_context yield) {
        auto self = shared_from_this();
        try {
            for (;;) {
                seqpacket_protocol::socket::message_flags in_flags = MSG_WAITALL, out_flags = MSG_WAITALL;

                // Wait for the message from the client
                auto bytes_transferred = socket_.async_receive(boost::asio::buffer(data_), in_flags, yield);

                // Write the same message back to the client
                socket_.async_send(boost::asio::buffer(data_, bytes_transferred), out_flags, yield);
            }
        } catch (const std::exception& e) {
            std::cerr << e.what() << '\n';
            socket_.close();
        }
    }

    void go() {
        boost::asio::spawn(socket_.get_io_service(), std::bind(&session::echo, this, std::placeholders::_1));
    }

private:
    seqpacket_protocol::socket socket_;
    std::array<char, MAX_PACKET_SIZE> data_;
};

int main(int argc, char* argv[]) {
    try {
        if (argc != 2) {
            std::cerr << "Usage: datachannel_server <file> (e.g. /tmp/uv4l.socket)\n";
            std::cerr << "*** WARNING: existing file is removed ***\n";
            return EXIT_FAILURE;
        }

        boost::asio::io_service io_service;

        std::remove(argv[1]);

        boost::asio::spawn(io_service, [&](boost::asio::yield_context yield) {
                    seqpacket_protocol::acceptor acceptor_(io_service, seqpacket_protocol::endpoint(argv[1]));
                    for (;;) {
                        boost::system::error_code ec;
                        seqpacket_protocol::socket socket_(io_service);
                        acceptor_.async_accept(socket_, yield[ec]);
                        if (!ec)
                            std::make_shared<session>(std::move(socket_))->go();
                    }
                });

        io_service.run();

    } catch (std::exception& e) {
        std::cerr << "Exception: " << e.what() << "\n";
        return EXIT_FAILURE;
    }
}