WEBVTT 00:00.050 --> 00:05.810 Now that you understand what a ros2 topic is, let's create a publisher with Python. 00:05.810 --> 00:09.680 So what we are going to do here is we are first going to create a node. 00:09.680 --> 00:15.320 And in this node we're going to add a publisher that's going to publish some text twice per second on 00:15.320 --> 00:15.980 a topic. 00:15.980 --> 00:17.060 And how to get started. 00:17.060 --> 00:21.680 Well as I told you, we need to create a node because the publisher is going to be inside the node. 00:21.680 --> 00:23.630 And where do we create nodes? 00:23.630 --> 00:29.270 Well, we are going to go back to our roster workspace in the source folder. 00:29.270 --> 00:31.670 And then we already have a Python package. 00:31.670 --> 00:33.290 So I'm just going to use that one okay. 00:33.320 --> 00:38.150 For the examples we do in this course I'm going to go to my Python package. 00:38.150 --> 00:43.910 And inside this package I also have another folder with the same name okay. 00:43.940 --> 00:49.310 And you can simply see that we already have my first node.py. 00:49.340 --> 00:53.810 I've also added the template okay, that you could download from the section three. 00:53.810 --> 00:56.420 And we are going to create a new file. 00:56.420 --> 01:03.460 So I'm going to name it Robot News station dot p y. 01:03.490 --> 01:09.760 So the ID here is just to imagine we are creating a news station that's going to publish some text on 01:09.760 --> 01:10.420 a topic. 01:10.450 --> 01:10.630 Okay. 01:10.630 --> 01:11.680 Very simple example. 01:11.710 --> 01:17.770 So I create the file and I'm going to make it executable chmod plus x. 01:17.860 --> 01:24.700 That's because for the symlink install to work if you're going to use symlink install for installing 01:24.700 --> 01:28.990 your Python package you have to have the file executable. 01:29.020 --> 01:29.470 Great. 01:29.470 --> 01:35.500 So now we can go back here I'm going to open VSCode once again. 01:35.530 --> 01:38.260 I open it from the source folder of the workspace. 01:38.440 --> 01:42.010 And from there we can edit that file. 01:43.210 --> 01:49.840 So let's open the new file here the robot news station. 01:49.840 --> 01:52.000 And we have a blank page. 01:52.000 --> 01:55.030 But from now on we're going to use this template. 01:55.030 --> 01:59.900 So this is a template we have written in the section on nodes. 01:59.900 --> 02:04.580 So we can just take that template here and put it there. 02:04.610 --> 02:05.240 Okay. 02:05.240 --> 02:08.930 And the only thing we need to do is to modify the names. 02:08.930 --> 02:13.850 So the name of the class here and also the name of the node. 02:13.850 --> 02:23.330 So here my custom node is going to be let's name it Robot News Station node okay. 02:23.360 --> 02:25.550 And that's going to be the same here. 02:26.300 --> 02:29.600 And then I need to give a name to the node. 02:29.600 --> 02:35.210 So let's simply name it Robot News Station. 02:35.210 --> 02:40.070 And as you can see here already I'm using the same name for the file name here. 02:40.070 --> 02:44.960 And for the node name that's something that's quite common okay. 02:44.990 --> 02:46.400 Those are two different things. 02:46.400 --> 02:49.100 Once again this is the file name and this is the node name. 02:49.100 --> 02:51.590 But you can use the same if you want. 02:51.590 --> 02:55.490 Let's just remove those comments that are not needed. 02:56.240 --> 02:56.630 All right. 02:56.660 --> 02:57.850 And let's save the file. 02:57.880 --> 02:58.450 Great. 02:58.450 --> 02:59.800 And now we can start from this. 02:59.800 --> 03:05.470 So we have the imports here we have our class where we can write everything that's related to the node. 03:05.470 --> 03:09.730 And then we have the main function where we initialize ros2 communications. 03:09.730 --> 03:10.900 We start the node. 03:10.900 --> 03:13.930 We make the node spin and then shut down. 03:13.930 --> 03:17.770 And how to create a publisher and also where to create it. 03:17.800 --> 03:21.850 Well we're going to do that in the constructor of the class. 03:21.880 --> 03:22.060 Okay. 03:22.090 --> 03:26.590 So we have the class robot new station node here after the super. 03:26.590 --> 03:28.960 So the super is going to be the first line okay. 03:28.990 --> 03:32.020 After this I can create a publisher. 03:32.020 --> 03:37.990 So let's also do self dot publisher okay. 03:38.020 --> 03:43.090 We're going to save it as an attribute of the class and how to create a publisher. 03:43.090 --> 03:47.920 I'm going to do self dot create publisher. 03:47.920 --> 03:51.760 And as you can see I have the auto completion here okay. 03:51.760 --> 03:56.300 So I use the create publisher method from the node class. 03:57.050 --> 04:01.460 And if you remember from the previous video for a topic, we need two things. 04:01.460 --> 04:05.270 We need a name and we also need a data type. 04:05.270 --> 04:07.760 So what type are we going to use. 04:07.760 --> 04:11.000 Because you see we have to provide the type here as the first argument. 04:11.000 --> 04:15.200 And well here to get started I'm going to give you directly the type we're going to use. 04:15.200 --> 04:22.160 So there are already some data types for messages for topics that are existing and already installed 04:22.160 --> 04:23.360 for you to use. 04:23.390 --> 04:26.660 And then we will be able to also create our own types. 04:26.660 --> 04:32.390 But this is going to be the focus on one full section just a bit later in this course. 04:32.480 --> 04:36.350 So I'm just going to show you I'm going to go back to the terminal and I'm just going to show you, 04:36.380 --> 04:39.830 let's just clear that we have a comment. 04:39.860 --> 04:43.910 It's called Ross to Interface show. 04:43.970 --> 04:44.240 Okay. 04:44.270 --> 04:45.740 So still a rose to command line. 04:45.740 --> 04:52.010 And then interface show to display an interface that you can use and that's already installed. 04:52.010 --> 04:56.620 And you can type Example interfaces. 04:56.620 --> 05:04.840 You can press tab, you see, and you should have the auto completion and then MSG slash and then string 05:04.870 --> 05:07.600 with an S uppercase okay. 05:07.630 --> 05:11.470 If you type this and you press enter you see we have here. 05:11.470 --> 05:13.840 So it's found we have an interface. 05:13.840 --> 05:15.550 So we have an interface. 05:15.550 --> 05:19.750 We have a message called string which contains that. 05:19.750 --> 05:21.970 So we have a few commented lines you see. 05:21.970 --> 05:24.280 So you don't need to worry about those lines. 05:24.280 --> 05:26.800 And then we have string data. 05:26.830 --> 05:34.210 This means that we have a field named data inside this message and of type string. 05:34.210 --> 05:37.480 And this is exactly what we want for this topic. 05:37.480 --> 05:40.450 We want to publish a string on a topic. 05:40.450 --> 05:42.760 So we're going to use that message. 05:42.760 --> 05:45.640 So you see the example interface package. 05:45.640 --> 05:46.750 So that's a package. 05:46.750 --> 05:49.720 And that's an interface inside the package okay. 05:49.750 --> 05:52.060 So this package is quite useful for you to get started. 05:52.060 --> 05:58.520 So we can use all kinds of basic data types without having to create messages by ourselves, which is 05:58.520 --> 06:00.710 going to be the focus later in this course. 06:00.740 --> 06:06.680 So let's import that one inside our code and how to import that. 06:06.710 --> 06:12.890 Well we will need to do here from example interfaces. 06:12.890 --> 06:16.310 And as you can see it's also found because of the Ross extension. 06:16.310 --> 06:21.260 So from example interfaces dot msg. 06:21.710 --> 06:30.680 Okay we have an MSG folder that's for topics import and then string you see as uppercase. 06:30.680 --> 06:31.880 And it should be found. 06:32.060 --> 06:39.170 So now what I can do in my create publisher I can provide the message type which is string. 06:39.200 --> 06:39.500 Okay. 06:39.530 --> 06:41.270 That's the class here that I'm using. 06:41.270 --> 06:46.040 Now before we go further here, as you can see we have a new import. 06:46.070 --> 06:48.410 And the first import was RCL pipe. 06:48.440 --> 06:48.800 Okay. 06:48.830 --> 06:52.750 With rcl p we already had if I go to package XML. 06:52.750 --> 06:59.050 We already had a depend tag for this package for this dependency, but now we are using a new one. 06:59.050 --> 07:03.070 We are using the package named example interfaces. 07:03.070 --> 07:10.090 So because we are depending on a new package, I'm going to add the dependency to the package dot XML. 07:10.090 --> 07:18.790 So I'm going to add depend example interfaces just like that. 07:18.820 --> 07:19.300 Okay. 07:19.300 --> 07:26.440 So that will specify that this package Mypy pkg depends on the example interfaces. 07:26.860 --> 07:29.890 So I'm going to save and go back here. 07:29.890 --> 07:33.040 And that's basically it for the package dot XML. 07:33.070 --> 07:33.250 Okay. 07:33.280 --> 07:39.910 So whenever you import a new package here make sure you also add the depend tag on the package dot XML. 07:39.940 --> 07:40.330 Great. 07:40.330 --> 07:41.950 And now let's finish this publisher. 07:41.950 --> 07:44.410 So to create a publisher we have a data type. 07:44.410 --> 07:46.540 And we have a name okay. 07:46.570 --> 07:48.700 So what name do we use. 07:48.700 --> 07:54.680 Well here we can create a name Because we create a publisher, we can publish to whatever name we want. 07:54.680 --> 07:58.940 And as a reminder, you need to start the name with a letter. 07:58.970 --> 08:01.910 Then you can use some numbers, some underscores, etc.. 08:01.910 --> 08:06.290 Here I'm going to name it Robot News just like that. 08:06.290 --> 08:10.550 And there is one more thing actually you need to add for the publisher. 08:10.550 --> 08:12.140 Otherwise you might get an error. 08:12.140 --> 08:14.930 You need to add a queue size. 08:14.930 --> 08:16.340 So I'm just going to put ten. 08:16.340 --> 08:24.290 So very basically that can be useful if you are going to publish let's say big messages like images 08:24.320 --> 08:30.410 that are very big or if you're going to publish very fast in a network, in a Wi-Fi network that's not 08:30.410 --> 08:35.510 very stable, maybe publishers and subscribers can't keep up with the messages. 08:35.510 --> 08:43.070 So this is going to create kind of a buffer so that the publisher can have a buffer of up to ten messages 08:43.070 --> 08:43.880 to send. 08:43.910 --> 08:44.390 Okay. 08:44.390 --> 08:47.660 So you can choose for example, one you could put ten 1000. 08:47.690 --> 08:49.810 Well it doesn't really matter at this point. 08:49.840 --> 08:50.110 Okay. 08:50.140 --> 08:56.650 As I told you, maybe if you have a lousy network or big messages, you're going to have to tweak this. 08:56.650 --> 09:01.690 But for everything we do in this course and most things you will do anyway with roster, you don't need 09:01.690 --> 09:02.680 to worry about that. 09:02.680 --> 09:04.810 So I'm just going to keep ten. 09:04.840 --> 09:05.290 Okay. 09:05.320 --> 09:07.540 For everything we do in this course. 09:07.570 --> 09:08.050 All right. 09:08.050 --> 09:09.760 And now we have our publisher. 09:09.760 --> 09:14.380 So create publisher with a data type and a name. 09:14.500 --> 09:18.070 The thing is this publisher is not going to do anything because we create the node. 09:18.070 --> 09:18.820 We have a publisher. 09:18.850 --> 09:19.120 Great. 09:19.120 --> 09:20.770 But it's not doing anything. 09:20.800 --> 09:25.150 I'm going to add another method in the class here. 09:25.180 --> 09:30.370 I'm going to call it for example publish news. 09:31.240 --> 09:39.250 And I need to put self because we are in a Python class and how to publish a message on that topic. 09:39.250 --> 09:40.870 Well, we need to do two things. 09:40.870 --> 09:45.160 First, we need to create the message and then we need to publish it. 09:45.160 --> 09:47.140 So let's create a message. 09:47.140 --> 09:49.140 We are going to use the string here. 09:49.170 --> 09:52.470 The string class and create a string object. 09:52.470 --> 09:56.250 Then inside this string I can do msg dot. 09:56.250 --> 09:59.370 We have you see msg dot data. 09:59.370 --> 10:00.600 Why data? 10:00.630 --> 10:08.730 Because if you remember in the interface we have a field named data which is of type string. 10:08.730 --> 10:13.980 So I will use a string here and let's just say hello. 10:14.010 --> 10:20.640 So I have created a message I have filled all the the fields of the message. 10:20.640 --> 10:22.050 So it's just one here. 10:22.050 --> 10:25.290 And then I can publish it using the publisher. 10:25.290 --> 10:31.560 So I will do self dot publisher dot publish. 10:32.010 --> 10:32.400 Okay. 10:32.430 --> 10:33.780 So it's quite simple. 10:33.780 --> 10:38.490 You publish and you publish what the message that you have just created. 10:38.610 --> 10:41.760 And that's how you publish a message on a topic. 10:41.760 --> 10:47.860 Now once again this code is not going to do anything because well we have a publisher, we have a function, 10:47.860 --> 10:49.510 but we don't call the function. 10:49.540 --> 10:51.340 So how to call the function? 10:51.340 --> 10:54.520 Let's say we want to call the function twice per second. 10:54.550 --> 11:00.760 Well, if you remember what we did previously when we created the first node we added a timer. 11:00.760 --> 11:03.370 So I'm going to add a timer here as well. 11:03.400 --> 11:05.680 Let's do self timer. 11:05.680 --> 11:08.950 So let's save it as also an attribute of the class. 11:08.950 --> 11:13.240 And we can do self dot create timer. 11:13.240 --> 11:16.150 As you can see I also created that in the constructor. 11:16.150 --> 11:19.120 And I will create it also after the publisher. 11:19.120 --> 11:24.100 So the author doesn't necessarily matter in this case okay. 11:24.130 --> 11:27.760 But you see I create a publisher and then I create a timer in the constructor. 11:27.760 --> 11:30.040 So self dot create timer. 11:30.070 --> 11:34.570 The first thing I need to provide is the timer period in second. 11:34.570 --> 11:39.430 So if I want to publish twice per second it means every 0.5 second. 11:39.430 --> 11:42.160 And then what is the function? 11:42.160 --> 11:46.860 What is the method that I need to call back for this timer that one. 11:46.890 --> 11:53.370 So self dot publish news with no parentheses. 11:53.760 --> 11:53.940 Okay. 11:53.940 --> 11:56.640 It's just the reference of the function. 11:56.640 --> 12:02.040 And with this code then that function is going to be called because. 12:02.040 --> 12:03.900 So we initialize Ros to communications. 12:03.900 --> 12:07.770 Then we create the node that's going to create the publisher and the timer. 12:07.770 --> 12:12.060 And then when the node is spinning the node is going to be kept alive. 12:12.090 --> 12:12.270 Okay. 12:12.300 --> 12:13.980 The program is going to be stuck here. 12:13.980 --> 12:15.600 So the node is kept alive. 12:15.600 --> 12:18.930 And we can execute all the callbacks for that node. 12:18.930 --> 12:24.900 And because we have a timer, then we will have a callback that will be triggered every 0.5 second. 12:24.900 --> 12:30.090 So that function will be executed every 0.5 seconds. 12:30.120 --> 12:31.320 That's it for the code. 12:31.320 --> 12:37.770 But there is one thing I like to do is after I finish the constructor here, you see there is no log. 12:37.770 --> 12:43.290 So when we start the node we will not see anything printed on the terminal. 12:43.290 --> 12:46.120 So what I like to do usually is to add a log. 12:46.120 --> 12:50.890 So self get logger info. 12:51.130 --> 12:54.430 And just to say that the node has been started. 12:54.430 --> 13:02.110 So let's say robot news station has been started okay. 13:02.140 --> 13:07.390 It could be just uh okay or whatever message, but I like to have a message at the end. 13:07.420 --> 13:09.550 So log at the end of the constructor. 13:09.580 --> 13:10.030 Great. 13:10.030 --> 13:13.300 And now it seems that the code is complete. 13:13.300 --> 13:17.560 What I will do is I will now install this executable. 13:17.560 --> 13:20.080 And to do that I will go to setup.py. 13:20.080 --> 13:26.380 And here you can see at the end we already have in the console script we have our first node. 13:26.380 --> 13:28.600 So I'm going to add a comma. 13:28.630 --> 13:30.130 That's very important here. 13:30.130 --> 13:33.370 Make sure you add a comma and then go back to a new line. 13:33.370 --> 13:35.920 And I will create a new executable. 13:35.920 --> 13:38.620 Now what name to give to the executable. 13:38.620 --> 13:43.140 Well I can name it for example Robot News Station. 13:43.140 --> 13:49.620 So here, as you can see, I'm going to use the same name for the executable, for the file name and 13:49.620 --> 13:51.000 for the node name. 13:51.030 --> 13:51.210 Okay. 13:51.210 --> 13:52.410 That's completely valid. 13:52.410 --> 13:55.350 Once again those are three different things. 13:55.350 --> 13:57.120 But I can use the same name. 13:57.750 --> 14:02.130 So robot new station is equal to what's the folder name. 14:02.130 --> 14:06.120 My pi pkg dot. 14:06.150 --> 14:07.560 What's the file name. 14:07.590 --> 14:08.160 Robot. 14:08.190 --> 14:13.350 New station use station without the extension. 14:13.350 --> 14:17.250 And then what is the function we want to call? 14:17.250 --> 14:23.130 Well it's always going to be main because I'm always going to use the the main with the node template. 14:23.130 --> 14:26.790 So we want to execute the main function. 14:27.360 --> 14:27.840 All right. 14:27.870 --> 14:29.190 As simple as that. 14:29.730 --> 14:34.410 And for every new executable you just add a comma and you create a new one here. 14:34.650 --> 14:38.070 It's not complicated if you ever make a mistake. 14:38.070 --> 14:42.790 For example, if you forgot the comma and then it's going to build something, but it's going to be 14:42.790 --> 14:44.590 weird and you're going to have some errors. 14:44.620 --> 14:44.980 Okay. 14:45.010 --> 14:46.930 In this case I recommend that. 14:46.930 --> 14:53.710 So you go to your workspace and if you have built something that is wrong I recommend that you remove 14:53.740 --> 14:55.960 those build install and log. 14:55.990 --> 14:56.170 Okay. 14:56.200 --> 15:00.160 So our m r build install log. 15:00.160 --> 15:05.590 So in case you mess up with something and you build it, then you can always remove everything that's 15:05.590 --> 15:07.210 going to be a bit cleaner. 15:07.240 --> 15:08.020 Okay. 15:08.500 --> 15:10.450 But now we don't have any mistakes. 15:10.450 --> 15:12.370 So let's save all the files. 15:12.370 --> 15:14.320 Let's make sure we have saved all the files. 15:14.320 --> 15:16.660 You see, we should have crosses here on VSCode. 15:16.660 --> 15:18.160 And let's go back. 15:18.160 --> 15:25.450 And once again, let's go back to our roster workspace and let's do a call on build with I'm going to 15:25.450 --> 15:32.200 do packages select with my pi pkg. 15:32.200 --> 15:42.150 And I'm also going to use the symlink install so that I don't need to compile This anymore. 15:42.450 --> 15:45.930 So I need to use I need to compile it first once. 15:45.960 --> 15:48.120 At least with the Simulink install option. 15:48.120 --> 15:55.260 But then if I modify this file here, I should be able to run it without installing it again. 15:55.260 --> 15:55.620 Great. 15:55.620 --> 15:57.480 So it has been built. 15:57.480 --> 16:02.970 What I'm going to do is I'm going to open a new terminal so that I have everything correctly sourced. 16:03.120 --> 16:07.560 Let's just split that for example, and let's run our publisher. 16:07.560 --> 16:14.730 So let's run the node which contains a publisher Ros to run my Pi PG. 16:14.730 --> 16:17.130 And if I press tab twice, you see that. 16:17.130 --> 16:19.530 Now I have two executables. 16:20.070 --> 16:22.590 I have the first one that I've created and Robot New Station. 16:22.590 --> 16:26.430 So I can start robot new station using the auto completion. 16:26.670 --> 16:32.310 And you see so we have this log robot news station has been started. 16:32.490 --> 16:36.870 Now I can use the rose two command line tools that we have seen previously. 16:36.870 --> 16:46.810 For example Ros two Not least I see I have my robot news station Ros2 node info with robot news station. 16:46.840 --> 16:54.940 Don't forget the slash and well, we have a bunch of stuff, but you can see on the publishers we have 16:54.970 --> 16:57.070 robot news. 16:57.100 --> 16:59.350 Okay, you can see there is added slash okay. 16:59.380 --> 17:02.140 There is no slash in the code. 17:02.440 --> 17:02.590 Okay. 17:02.620 --> 17:03.910 You don't need to provide a slash. 17:04.000 --> 17:07.930 If you don't it's going to add automatically a slash for you. 17:07.930 --> 17:16.480 And so you see we have the robot news topic with the string interface from the example interfaces package. 17:16.480 --> 17:21.100 Now to be able to see what we published we need a subscriber. 17:21.100 --> 17:23.020 So we will need to write a subscriber. 17:23.020 --> 17:27.400 But before we do that I'm going to show you there is another command line. 17:27.400 --> 17:31.420 And I'm going to come back to this also in a dedicated lesson in this section. 17:31.480 --> 17:35.710 So we have Ros2 topic list. 17:36.010 --> 17:43.800 If you do Ros2 topic list, you can see we have the robot news topic and if I do rose to topic equal 17:43.830 --> 17:51.960 with the name of the topic here with slash robot News, then I can basically create a subscriber and 17:51.960 --> 17:56.700 I can display what we receive on that topic directly in the terminal. 17:56.700 --> 17:59.100 So make sure that the publisher is running here. 17:59.100 --> 18:01.860 And then you should see that we receive data. 18:01.890 --> 18:02.340 Hello. 18:02.340 --> 18:02.790 Hello. 18:02.790 --> 18:05.280 Every 0.5 seconds okay. 18:05.310 --> 18:07.560 And well it's still receiving. 18:07.560 --> 18:09.900 Now if I kill the publisher. 18:10.170 --> 18:13.440 You see now we don't receive any more data. 18:13.440 --> 18:19.320 So we can see that what we receive here was directly published by that node right here. 18:19.500 --> 18:22.770 And I'm just going to finish with a slight improvement. 18:22.770 --> 18:30.450 So let's say we want to say hi this is and then a name. 18:30.450 --> 18:36.030 So for example C-3po from the Robot News. 18:37.480 --> 18:38.440 station. 18:39.070 --> 18:44.140 And let's also say that instead of hard coding this, I'm going to use a variable. 18:44.140 --> 18:53.380 So I'm going to do this self dot robot name is equal to C-3po. 18:53.980 --> 18:55.630 And then I can use. 18:56.440 --> 19:03.700 So I can use this plus self robot name plus. 19:04.030 --> 19:04.450 All right. 19:04.450 --> 19:08.080 So I construct a string and I put this attribute here. 19:08.080 --> 19:12.010 So that also later on we will see ways to modify that. 19:12.010 --> 19:13.540 Let's save. 19:13.540 --> 19:16.600 And because I have built the workspace with Simulink install. 19:16.600 --> 19:21.220 So this package with Simulink install I should be able to run again. 19:21.220 --> 19:23.830 And we should see you see. 19:23.860 --> 19:27.670 Hi, this is C-3po from the robot news station. 19:27.700 --> 19:30.430 Okay, so now we have the new data. 19:30.460 --> 19:31.240 Great. 19:31.240 --> 19:35.770 And you have created your first Python publisher in Ros two.