WEBVTT 00:00.520 --> 00:07.540 In this laboratory lesson, we will implement our first rose to node in Python that will use the publisher 00:07.540 --> 00:12.820 subscriber communication protocol to publish a message within a ros2 topic. 00:13.210 --> 00:18.790 If you are only interested in the cplusplus software development, then you can skip directly to the 00:18.790 --> 00:24.430 next lesson where we will implement the same node with the same functionalities in C plus plus. 00:25.150 --> 00:28.300 Let's start by opening Visual Studio code. 00:28.300 --> 00:31.710 And here let's open the robots workspace. 00:31.720 --> 00:36.490 So let's open the folder that is called Arduino Bot Workspace. 00:39.170 --> 00:42.980 Inside the workspace within the source folder. 00:44.250 --> 00:51.900 So here we created already the two packages, Arduino bot, CP examples and the Arduino bot Pi examples 00:51.900 --> 00:59.100 with the Ros two package create command in the previous lesson and this already created also several 00:59.100 --> 01:02.550 folders and several files within the two packages. 01:03.250 --> 01:11.920 The files and the folder created in the Arduino Pi example is the typical structure of any Python package 01:11.920 --> 01:13.390 used in Ros2. 01:13.480 --> 01:16.960 So there is a sub folder which has the same name of the package. 01:16.960 --> 01:22.240 So in this case Arduino Pi examples and this will contain the source code. 01:22.240 --> 01:25.840 So all the Python scripts that we are going to develop. 01:25.840 --> 01:36.640 So here let's create also a new file and let's call it simple publisher dot pi to indicate that it is 01:36.640 --> 01:38.890 a python file here. 01:38.890 --> 01:43.390 Let's start by importing the Pi library. 01:46.020 --> 01:51.730 Which will allow us to use all the ros2 functionalities within our Python script. 01:51.750 --> 01:56.670 Then still from the rql py library. 01:56.700 --> 01:59.610 This time from the node module. 01:59.610 --> 02:07.560 Let's import the node class and we can use this class to create a new ros2 node. 02:07.740 --> 02:18.540 So let's create a new class called for example Simple publisher and let's make this class inherit from 02:18.540 --> 02:21.690 the node class of the Pi library. 02:22.170 --> 02:26.100 So in this class, let's first define the constructor of the class. 02:26.100 --> 02:32.250 So the function that is automatically executed when a new object of the class is created. 02:33.120 --> 02:36.150 So this is called the init function. 02:39.320 --> 02:40.370 And here. 02:40.370 --> 02:47.330 So when we initialize a new object of type simple publisher, the first thing we do is to initialize 02:47.330 --> 02:49.520 the components of the base class. 02:49.520 --> 02:55.670 So namely the components of the node class from which the simple publisher is inheriting. 02:55.850 --> 03:05.900 So let's call the super init that will call the constructor of the node class and let's assign a name 03:05.900 --> 03:11.570 to our node that we call Simple Publisher. 03:12.440 --> 03:19.130 Now that we have initialized the Ros two node class, let's create a new publisher object. 03:19.160 --> 03:21.020 So let's create a new variable. 03:24.120 --> 03:26.940 And to initialize a publisher object. 03:26.970 --> 03:33.930 Let's use the create publisher function that is defined in the node class from which we are inheriting. 03:34.410 --> 03:36.960 So let's use the create. 03:39.030 --> 03:40.440 Publisher function. 03:40.560 --> 03:44.220 And in this function we need to indicate the message type. 03:44.220 --> 03:49.650 So the type of the interface that we need to use for the message exchange with the topic. 03:49.770 --> 03:56.880 In practice, we need to specify which type of messages the publisher object will send through the topic. 03:57.150 --> 04:02.280 For example, let's say that we want to send text messages, so we want to send strings. 04:02.640 --> 04:08.640 The string message is contained, is defined in another Ros two library. 04:08.850 --> 04:17.080 So let's import it from the standard messages library from the messages that are defined in this library. 04:17.100 --> 04:20.820 Let's import the string message. 04:22.770 --> 04:30.990 And let's use it to indicate that within the create publisher function we are going to use string messages 04:30.990 --> 04:33.000 to interact with the topic. 04:33.750 --> 04:39.390 We also need to specify the name of the topic in which we want the messages to be published. 04:39.420 --> 04:43.290 So, for example, let's call this one chatter. 04:43.710 --> 04:49.320 Furthermore, we also need to indicate the size of the queue, namely the size of the message queue 04:49.350 --> 04:51.000 that works as a buffer. 04:51.000 --> 04:55.890 If the subscriber to this topic is not receiving and processing messages fast enough. 04:56.530 --> 04:58.900 Let's set this 1 to 10. 04:59.800 --> 05:06.460 Let's add some more supporting variables to the simple publisher class, and let's call the first one, 05:06.490 --> 05:14.140 the variable counter that we initialize to zero, and it will count the number of messages that we are 05:14.140 --> 05:16.060 going to publish within the topic. 05:16.330 --> 05:19.540 Then let's also define a frequency variable 05:22.840 --> 05:29.590 that we initialize to 1.0, so to one hertz, and this one will define the frequency at which we want 05:29.590 --> 05:32.080 to publish messages within the topic. 05:32.080 --> 05:35.650 So we are going to publish them at a frequency of one hertz. 05:35.650 --> 05:40.270 So we are going to publish one message per second within the chapter topic. 05:41.150 --> 05:44.090 At this point during the initialization of the node. 05:44.120 --> 05:50.210 Let's also print an informative message in the terminal using the get logger function, which we are 05:50.210 --> 05:52.940 still inheriting from the node class. 05:53.690 --> 05:58.220 So let's use the get logger function. 05:58.220 --> 06:04.440 And then from this one let's use the info function to print an informative message in the terminal. 06:04.460 --> 06:09.770 And the message says publishing at. 06:11.360 --> 06:14.630 And then the number of earths that we are using. 06:14.630 --> 06:20.210 So followed by the variable frequency. 06:21.520 --> 06:28.300 Lastly, inside the constructor of the Simple Publisher class, let's also create a new timer object 06:28.330 --> 06:30.790 using the create timer function. 06:30.790 --> 06:35.260 So let's call this one timer. 06:35.650 --> 06:42.490 And still from the node class, let's use the create timer function. 06:43.550 --> 06:50.750 So they create timer function takes as input the frequency at which we want to execute a certain function 06:50.750 --> 07:00.260 and we store this one in the frequency variable and it also takes the name of the function to be executed 07:00.260 --> 07:01.880 at regular time intervals. 07:01.880 --> 07:10.430 So at each expiration of the timer, so at a frequency of one hertz and let's call this function timer 07:11.390 --> 07:12.350 callback. 07:12.800 --> 07:18.620 With this, the constructor of the simple publisher class is completed and we can proceed to define 07:18.620 --> 07:24.410 the timer callback function, which will be the one that is repeatedly executed with a frequency of 07:24.410 --> 07:26.600 one hertz by the timer object. 07:26.750 --> 07:32.630 So let's define this function, the timer callback function. 07:32.870 --> 07:39.800 And this is a member of the simple publisher class and the goal is to publish a new message within the 07:39.800 --> 07:42.650 chapter topic every time this function is called. 07:42.650 --> 07:46.110 So every time this function is executed by the timer. 07:47.190 --> 07:52.590 So here, let's start by creating a new message to be published within the topic. 07:52.590 --> 07:57.150 So let's call this variable message and this is of type string. 07:57.150 --> 08:02.100 So from the string interface that is defined in the standard messages. 08:02.250 --> 08:15.360 And now let's publish, for example, the message that says Hello Ross to and then let's also print 08:15.780 --> 08:17.130 the counter. 08:19.500 --> 08:25.200 And so let's print the variable that we called counter. 08:27.030 --> 08:32.970 Then with this done, we can use the publish function of the publisher object. 08:32.970 --> 08:35.280 So of this object here. 08:37.850 --> 08:43.820 Let's call the publish function to publish the message we just created. 08:43.820 --> 08:46.220 So the variable MSG. 08:46.850 --> 08:53.330 And then once we have published the message within a topic, let's also increment the counter variable 08:53.360 --> 08:56.960 by one since we have just published a new message. 09:02.490 --> 09:03.330 With this. 09:03.330 --> 09:09.480 The simple publisher class is complete and now we can proceed to define the main function. 09:09.480 --> 09:14.250 And so the one that is automatically called upon the execution of this script. 09:14.430 --> 09:15.210 So. 09:20.870 --> 09:25.160 And here let's call the main function. 09:25.160 --> 09:27.110 So let's execute the main function. 09:27.110 --> 09:31.730 But first let's define this main function. 09:32.400 --> 09:40.550 And here as our first step, let's initialize rows using the init function from the PY library. 09:40.560 --> 09:43.230 So from rql py. 09:44.040 --> 09:50.970 Let's use the init function to instantiate the communication with rows and then let's create a new object 09:50.970 --> 09:53.010 of the simple publisher class. 09:53.010 --> 10:01.830 So let's call this variable simple publisher and is an instance of the simple publisher class and let's 10:01.830 --> 10:03.330 keep this node up and running. 10:03.330 --> 10:09.600 So keeping the timer and the publisher active so that they can continuously send messages within the 10:09.600 --> 10:10.770 chapter topic. 10:10.770 --> 10:20.220 And to do so we can use still from the PY library the spin function to keep the simple publisher node 10:20.220 --> 10:21.300 up and running. 10:21.660 --> 10:27.720 Finally, if we terminate the execution of the node by pressing Ctrl Z in the terminal, we need to 10:27.720 --> 10:31.110 ensure that also the simple publisher node is properly destroyed. 10:31.470 --> 10:38.800 So let's call the function destroy node on the simple publisher function. 10:38.800 --> 10:43.030 And also in this case, let's shut down Ros two. 10:43.750 --> 10:45.910 Let's save this script. 10:45.910 --> 10:52.270 And now in order to execute this node, we first need to instruct the compiler on how it should build 10:52.270 --> 10:55.180 our script to make it an executable in Ros. 10:55.180 --> 10:55.540 To. 10:56.110 --> 11:03.790 To do so, we need to modify another file that was automatically created within the Arduino bot Pi examples 11:03.790 --> 11:04.450 package. 11:04.450 --> 11:07.180 And this is the Setup.py file. 11:07.700 --> 11:14.390 You can think of this file as a kind of instruction sheet that tells the compiler how to translate our 11:14.390 --> 11:17.090 Python scripts into executable files. 11:18.220 --> 11:24.130 To do this in the entry points section in the console script list. 11:24.160 --> 11:26.440 Let's add a new executable. 11:26.440 --> 11:33.490 So here, let's add a new one and let's call this one simple publisher. 11:33.640 --> 11:44.670 And the script that we want to install is located within the Arduino Boot Pi examples folder. 11:44.680 --> 11:49.900 Then within the simple publisher file. 11:49.900 --> 11:52.360 And from this one we want to start. 11:52.360 --> 11:57.280 So we want the starting point to be the main function. 11:57.730 --> 12:00.130 So let's save also this file. 12:00.130 --> 12:06.100 And finally, before actually building the workspace and starting the node, we also need to declare 12:06.100 --> 12:07.840 the dependencies of the new node. 12:07.840 --> 12:12.400 So namely, we need to declare the libraries that it uses for its execution. 12:12.980 --> 12:18.650 To do this, we need also to modify the file that was automatically created and that is called package 12:18.650 --> 12:23.510 dot XML and that is still in the Arduino dot pi example package. 12:24.250 --> 12:24.820 Here. 12:24.820 --> 12:29.920 We need to add some dependencies, some execution dependencies. 12:30.940 --> 12:38.410 And let's start by adding the one from the Pi library, which we used in our script and which allowed 12:38.410 --> 12:42.160 us to use all the functionalities within the Python script. 12:42.870 --> 12:52.650 Also, let's add another exec dependency from the standard messages library from which we use the declaration 12:52.680 --> 12:54.300 of the string message. 12:54.960 --> 12:57.240 Let's save also this file. 12:57.240 --> 13:03.240 And these are the only two libraries that we need in the simple PowerShell script so we can finally 13:03.240 --> 13:04.500 execute our node. 13:04.620 --> 13:06.660 So let's open a new terminal. 13:06.900 --> 13:14.760 Let's go to the workspace and let's run the command on build. 13:16.350 --> 13:21.030 So this will build the entire workspace again, including also the new node. 13:21.180 --> 13:26.370 And now to execute the node, let's split the terminal. 13:26.400 --> 13:28.120 So let's take another terminal. 13:28.140 --> 13:30.150 Here, let's source the workspace. 13:30.300 --> 13:36.480 So let's source the file setup dot bash that is in the install folder. 13:37.130 --> 13:41.090 And this way the Arduino board by example is recognized. 13:41.090 --> 13:45.770 And so we can execute now the script, the nodes that it contains. 13:46.560 --> 13:55.230 To start a ros2 node, we can use the command Ros to run followed by the name of the package where the 13:55.230 --> 14:04.290 node is located, which in our case is the Arduino bot pi examples so we can use Tab to exploit the 14:04.290 --> 14:06.000 Ros2 Autocompletion. 14:06.370 --> 14:11.940 Then if we press tab twice we can see that the simple publisher node is recognized. 14:12.030 --> 14:15.780 So let's execute the simple publisher node. 14:15.780 --> 14:18.690 So let's press enter to start the node. 14:18.690 --> 14:26.760 And as you will see, it appears the message publishing at one hertz in the terminal and to verify that 14:26.760 --> 14:34.590 indeed the node is publishing a message within the charter topic, we can split our terminal again and 14:34.590 --> 14:41.310 we can take a look at all the topics that are currently available in Ros2 with the command Ros2. 14:43.330 --> 14:45.150 Topic list. 14:45.160 --> 14:47.110 So let's press enter. 14:47.110 --> 14:53.560 And as the name of the command suggests, it will show all the topics available in Russia at this moment. 14:53.560 --> 15:00.130 And among these topics, we can see that it appears the chapter topic, which is exactly the name we 15:00.130 --> 15:02.620 gave to the topic in the publisher node. 15:03.280 --> 15:09.850 If we stop, for example, the execution of our publisher node and we execute again, the command Ros2 15:09.880 --> 15:10.840 topic list. 15:11.020 --> 15:17.320 Now, as you can see, the chatter topic is no longer available since the node that was publishing messages 15:17.320 --> 15:19.060 on it is no longer running. 15:19.240 --> 15:27.220 So let's relaunch the node again and let's use again the Ros2 topic list and the chatter topic is back 15:27.220 --> 15:27.910 on track. 15:28.180 --> 15:34.570 We can also take a look at which are the messages that are currently traveling within this topic using 15:34.570 --> 15:42.330 the command Ros2 topic Echo followed by the name of the topic, which is chatter. 15:42.340 --> 15:50.020 So let's press enter and all the messages that are now published within this topic are now printed in 15:50.020 --> 15:50.860 the terminal. 15:50.860 --> 15:58.150 And so we can see that each message contains the string l0 ros2 followed by the counter that is increasing 15:58.150 --> 16:00.730 at each new message that is being published. 16:01.590 --> 16:08.010 Now, if we stop again, the execution of the node, we can see that there are no new messages published 16:08.160 --> 16:14.430 within the topic and it will restart as we restart here the node. 16:16.200 --> 16:23.130 There are other two useful ros2 commands that we can use to provide additional information about a topic, 16:23.130 --> 16:31.530 and these are ros2 topic info still followed by the name of the topic from which we want to obtain more 16:31.530 --> 16:32.160 info. 16:32.700 --> 16:34.890 So for example the chatter topic. 16:35.830 --> 16:40.030 And here we can see the type of the message that is being published. 16:40.180 --> 16:47.350 And also, if we use the verbose flag, we get even more information about it. 16:47.350 --> 16:54.520 So for example, we get the name of which Node is interacting with this topic that in this case is the 16:54.520 --> 16:56.200 simple publisher node. 16:57.150 --> 17:04.330 Finally, the last command that we are going to use in this lesson is the Ros2 topic. 17:04.920 --> 17:08.220 Earth's still followed by the name of the topic. 17:08.220 --> 17:15.270 So chatter, let's press enter and this command analyzes the messages that are published in the chatter 17:15.270 --> 17:19.310 topic and calculates the frequency at which they are published. 17:19.320 --> 17:25.920 In this case, as we can see, it is calculating a frequency so an average rate of one hertz, which 17:25.920 --> 17:30.810 is exactly the value that we set for the timer in the Visual Studio code. 17:30.810 --> 17:34.430 So here we set the frequency of one hertz.