WEBVTT 00:00.110 --> 00:05.930 In the second Python laboratory lesson, we will implement a subscriber node that uses the publisher 00:05.930 --> 00:12.290 subscriber communication protocol to receive and subscribe to a topic and read all the messages that 00:12.290 --> 00:13.310 pass through it. 00:14.210 --> 00:19.670 If you are interested in the plus cplusplus software development, then you can skip directly to the 00:19.670 --> 00:25.160 next lesson where we will implement the same node with the same functionalities in C plus plus. 00:25.970 --> 00:32.110 Let's start by adding this new node inside the Arduino board Pi examples package. 00:32.120 --> 00:38.000 And so within the Arduino board, Pi examples subfolder that was automatically created and where we 00:38.000 --> 00:40.880 have already created the simple publisher node. 00:40.970 --> 00:45.560 So let's add a new file and let's call this one simple 00:48.260 --> 00:50.750 subscriber dot pi here. 00:50.750 --> 01:00.170 Let's start by importing the Pi library that will allow us to use the Ros two functionalities within 01:00.170 --> 01:01.100 this script. 01:01.100 --> 01:10.880 And then still from the RCL Pi library from the node modules, let's import the node class and we can 01:10.880 --> 01:14.390 use now this class to create a Ros two node. 01:14.390 --> 01:24.860 So let's create a new class that is called Simple Subscriber that inherits from the node class that 01:24.860 --> 01:28.650 is defined in the RCL Pi library. 01:29.160 --> 01:32.760 So first let's define the constructor of this class. 01:32.760 --> 01:38.130 So the function that is automatically executed when a new object of this class is created. 01:38.160 --> 01:46.920 So let's define the init function and the first thing that we do when we initialize an object of the 01:46.920 --> 01:51.300 simple subscriber class is to call the constructor of the base class. 01:51.300 --> 01:54.720 So of the node class from which we are inheriting. 01:54.720 --> 02:04.050 So let's use, let's call the init function of the base class and let's assign a name that is simple 02:05.490 --> 02:07.230 subscriber to this node. 02:08.030 --> 02:15.560 Now that we have initialized the two node class, let's proceed to create a new subscriber object using 02:15.560 --> 02:17.630 the Create subscription function. 02:17.630 --> 02:26.930 So let's call the create subscription function that we inherited from the node class and let's save 02:27.380 --> 02:31.930 the output of this function into a new variable called soap. 02:32.710 --> 02:38.980 In this function, we need to indicate the message type that is the type of the interface to be used 02:38.980 --> 02:40.420 for the message exchange. 02:40.420 --> 02:46.720 So basically we need to specify which type of messages the subscriber object will receive within the 02:46.720 --> 02:47.390 topic. 02:47.410 --> 02:52.500 For example, let's assume again that we want to transmit string messages. 02:52.510 --> 03:03.220 So let's import from the standard messages library, from the messages, let's import the definition 03:03.220 --> 03:09.340 of the string message and let's use it inside the create subscription function. 03:09.340 --> 03:13.210 To define that, our subscriber is going to read string messages. 03:13.510 --> 03:19.240 Then let's also specify the name of the topic on which we want to listen and receive messages. 03:19.240 --> 03:22.270 And let's call this one for example chapter. 03:22.890 --> 03:30.150 When creating a subscriber to a ros2 topic, it is necessary to define a callback function, namely 03:30.150 --> 03:34.710 a function to be executed upon receiving each new message within the topic. 03:34.710 --> 03:39.960 And we will define this callback function as always within the simple publisher class. 03:39.960 --> 03:47.700 So let's call this one message callback. 03:47.970 --> 03:53.970 And furthermore, we also need to indicate the size of the queue, namely the size of the message queue 03:53.970 --> 03:55.380 that works as a buffer. 03:55.380 --> 04:00.390 If the subscriber to this topic is not receiving and processing messages fast enough. 04:00.540 --> 04:03.570 For example, let's set this 1 to 10. 04:04.470 --> 04:11.160 With this, the constructor of the simple subscriber class is completed and now we can proceed to define 04:11.160 --> 04:12.870 the message callback function. 04:12.870 --> 04:19.440 So this one here, that is the one that is called upon receiving a new message in the Ros2 topic. 04:19.890 --> 04:28.800 So let's define this function as a member of the simple subscriber class and as this is a callback function, 04:28.800 --> 04:33.930 it receives as input the content of the message that was just received on the topic. 04:33.930 --> 04:36.090 So a variable message. 04:36.940 --> 04:37.960 In this function. 04:37.960 --> 04:44.080 Every time a new message is received by the subscriber, we simply decide to print it in the terminal 04:44.080 --> 04:46.540 using the get logger function. 04:46.540 --> 04:53.050 So still, this is a function that we inherited from the base class, the node class. 04:53.320 --> 05:01.450 And from this one, let's print an informative message in the terminal that says I earth, followed 05:01.450 --> 05:04.780 by the content of the message that we want to print. 05:05.170 --> 05:13.300 So for example, let's print the data, so the content of the string that we just received. 05:14.020 --> 05:16.930 With this, The simple subscriber class is complete. 05:16.960 --> 05:21.610 We don't need to do anything else and we just need to define the main function. 05:21.610 --> 05:26.350 So basically the one that is automatically executed when the script starts. 05:32.290 --> 05:35.410 And this one, let's execute another function that is called main. 05:35.860 --> 05:40.360 And first, let's define this main function. 05:40.630 --> 05:42.790 And here let's the first step. 05:42.790 --> 05:52.240 Let's initialize the Ros two with the rcl pi init function, and then let's create a new object of the 05:52.240 --> 05:53.710 simple subscriber class. 05:53.710 --> 06:02.410 So a new instance that we call simple subscriber as an instance of the simple subscriber class. 06:02.410 --> 06:08.200 And then in order to keep this node up and running and thus keep the subscriber active so that it can 06:08.200 --> 06:17.080 continuously receive the messages that pass through the chatter topic, Let's use the RCL pi spin function 06:18.550 --> 06:21.490 to keep the simple subscriber up and running. 06:21.490 --> 06:26.980 And finally, if we terminate the execution of the node with control C, let's ensure that the simple 06:26.980 --> 06:37.180 subscriber node is properly destroyed by calling the function destroy node on the simple subscriber. 06:37.180 --> 06:43.570 And also in this case, let's shut down Ros with the shutdown function. 06:44.620 --> 06:46.660 Let's save this script. 06:46.660 --> 06:52.750 And now to execute this node, first we need to instruct the compiler on how it should build our script 06:52.750 --> 06:55.000 and make it an executable in Ros two. 06:55.510 --> 07:01.420 To do so as we did for the simple publisher, let's modify the file setup.py. 07:01.420 --> 07:08.650 And still in the entry point section where we have installed the simple publisher node, let's add a 07:08.650 --> 07:09.330 new line. 07:09.340 --> 07:14.290 So here let's add a new executable called Simple 07:16.420 --> 07:17.560 subscriber. 07:17.560 --> 07:22.030 And this is still in the Arduino, but by. 07:24.210 --> 07:29.520 Examples in the simple subscriber file. 07:29.520 --> 07:31.770 And from this one we want to start. 07:31.770 --> 07:34.140 So the entry point should be the main function. 07:34.140 --> 07:37.350 We want to start the main function when we start the node. 07:37.950 --> 07:43.410 Let's save also this script and now we can build again our workspace and launch our node. 07:43.950 --> 07:45.810 So open a terminal. 07:46.650 --> 07:53.940 Let's go to the Arduino board workspace and let's use the command called Build to build again our workspace. 07:53.940 --> 07:59.040 And so to compile the new node, once this is done, we can split the terminal. 07:59.040 --> 08:03.300 So let's open a new window and here let's source the workspace. 08:03.450 --> 08:08.880 So source the file setup dot bash that is in the install folder. 08:08.880 --> 08:18.090 And now we can proceed to run the new node so we can still use the command Ros to run to execute a node. 08:18.720 --> 08:21.930 And from the Arduino Pi examples package. 08:21.930 --> 08:27.250 Now if we press tab twice, we can see that we have two nodes, the simple publisher and the simple 08:27.250 --> 08:28.120 subscriber. 08:28.360 --> 08:31.600 So let's start with the simple subscriber node. 08:31.870 --> 08:33.700 Let's press enter. 08:33.790 --> 08:40.690 And actually we can verify that this node is running by opening another terminal and using the command 08:40.690 --> 08:43.450 Ros2 topic list. 08:43.450 --> 08:47.020 And actually we can see that the chapter topic is there. 08:47.020 --> 08:53.650 And so actually this node is listening to this topic and we can also confirm this by using the command 08:53.650 --> 09:04.450 Ros2 topic info on the chapter topic, let's add the verbose flag and we can see that there is only 09:04.450 --> 09:10.330 one node that is interacting with this topic and actually that is subscribed to this topic. 09:10.330 --> 09:13.120 And this is the simple subscriber node. 09:14.180 --> 09:20.630 Now let's try to publish a new messenger on this topic to observe that actually the node is listening 09:20.630 --> 09:24.980 on this topic and triggers an event upon receiving the message. 09:25.250 --> 09:33.890 To do so, we can use the command Ros2 topic pub followed by the name of the topic in which we want 09:33.890 --> 09:40.010 to publish the message that is the chatter topic, followed by the type of the message that is being 09:40.040 --> 09:43.200 transmitted and accepted in this topic. 09:43.220 --> 09:47.660 So in this case we can use the Ros2 Autocompletion features. 09:47.660 --> 09:54.860 So let's press tab twice and so we can see that it is automatically detected the message that is accepted 09:54.860 --> 09:55.850 in this topic. 09:56.120 --> 10:03.140 So let's use the standard message string and then we need to insert the content of this message. 10:03.140 --> 10:11.480 So let's use the double quotes and then press tab twice so we can see that here it is also present an 10:11.480 --> 10:14.420 empty prototype of the string message. 10:14.420 --> 10:17.840 So let's type data then let's press tab. 10:18.200 --> 10:21.890 And here with this we are going to publish an empty message. 10:22.040 --> 10:26.990 Let's change the content of this string to Hello Ros. 10:27.890 --> 10:34.880 So let's press enter to publish this message and as long as we do so, we can see that this is starting 10:34.880 --> 10:37.760 publishing the same message multiple times. 10:37.760 --> 10:39.950 So let's interrupt this one. 10:39.950 --> 10:46.220 But the interesting thing is that our node, so the simple subscriber node was able to receive to to 10:46.220 --> 10:51.080 listen all the messages that we were publishing inside the chatter topic. 10:51.080 --> 10:53.090 And so it is printed correctly. 10:53.090 --> 10:57.590 The message Hello Ros2 that we were publishing here. 10:58.760 --> 11:04.850 Finally, let me show you another interesting feature of two, which makes this framework a language 11:04.880 --> 11:11.210 agnostic framework, namely that it works independently, so the same way regardless of the programming 11:11.210 --> 11:12.230 language used. 11:12.410 --> 11:18.450 So in this terminal window, let's keep this subscriber node that we just developed in Python Active. 11:18.470 --> 11:26.030 So listening on the chatter topic and in the new terminal, let's still source the workspace. 11:26.270 --> 11:35.330 So the install setup batch file and here let's run the publisher node which we developed in the previous 11:35.330 --> 11:36.860 lesson in C plus plus. 11:36.860 --> 11:43.970 So using a different programming language still, let's run the node with the Ros2 run command. 11:44.390 --> 11:47.450 And from the Arduino bot CP examples. 11:47.720 --> 11:50.240 Let's start the simple publisher node. 11:50.390 --> 11:52.190 So let's press enter. 11:52.340 --> 11:58.530 Now what is happening is that in one terminal window we have a node that is developed in C plus plus. 11:58.530 --> 12:04.290 So this one that is publishing messages on the chatter topic at a frequency of one hertz. 12:04.290 --> 12:08.670 So namely it is publishing one message per second in another window. 12:08.670 --> 12:12.840 So this one above, we have our python node that we just developed. 12:12.840 --> 12:18.840 So the subscriber that is listening on the same chatter topic and that is printing all the messages 12:18.840 --> 12:19.920 that it received. 12:21.370 --> 12:25.030 Among the other things, this is one of the advantages of using Ros two. 12:25.150 --> 12:30.790 That is to enable the communication between different nodes that often perform different operations 12:30.790 --> 12:36.880 and potentially they are developed in different programming languages without any effort from the developer 12:36.880 --> 12:39.160 of these nodes for their integration.