WEBVTT 00:00.920 --> 00:08.060 In this laboratory lesson, we will implement our first Ros two node in C plus plus that will use the 00:08.060 --> 00:13.670 publisher subscriber communication protocol to publish a message within our Ros two topic. 00:14.800 --> 00:17.860 Let's start by opening Visual Studio code. 00:17.860 --> 00:21.090 And here let's open the robots workspace. 00:21.160 --> 00:24.160 So let's open a folder. 00:25.860 --> 00:29.490 And let's open the folder that we called Arduino Bot Workspace. 00:30.560 --> 00:36.890 Inside the workspace in the source folder, we already find the two packages we created in the previous 00:36.890 --> 00:42.680 lesson and so the Arduino Pi examples and the Arduino, but CP examples. 00:42.680 --> 00:50.270 So let's open the CP examples package where the Ros two package create command is already automatically 00:50.270 --> 00:53.210 created various folder and files. 00:53.210 --> 00:55.520 And this is the typical structure. 00:55.520 --> 01:01.040 So this is the typical convention that is used for each C plus plus package in Ros two. 01:01.800 --> 01:03.420 The include sub folder. 01:03.420 --> 01:07.740 So this one contains another sub folder that has the same name of the package. 01:07.740 --> 01:11.100 And so Arduino CP examples. 01:11.100 --> 01:18.750 And this will contain the definitions of function and classes in C plus plus instead, the source folder 01:18.750 --> 01:25.140 contains the C plus plus files that define the behavior of function and classes, which is where we 01:25.140 --> 01:28.230 will insert the logic of each Ros two node. 01:28.320 --> 01:38.640 So within this source folder, let's create a new file and let's call this one simple publisher dot 01:38.640 --> 01:40.530 CP here. 01:40.530 --> 01:44.880 Let's start by including the CP library. 01:44.880 --> 01:56.280 So let's include the CP, and from this one, let's take the CP CP. 01:57.160 --> 02:04.120 And this library will allow us to use all the functionalities of Ros two within our script so we can 02:04.120 --> 02:13.540 use it to create a new Ros two node by creating a new class called, for example, Simple Publisher 02:13.540 --> 02:22.000 that inherits from the node class that is defined in the CPP package. 02:22.000 --> 02:25.030 And so let's inherit from the node class. 02:27.600 --> 02:32.310 First among the public methods of the simple publisher class. 02:33.480 --> 02:34.590 Among the public one. 02:34.620 --> 02:37.710 Let's start by defining the constructor of the class. 02:37.710 --> 02:40.590 That is the function that has the same name of the class. 02:40.680 --> 02:42.450 So simple publisher. 02:42.450 --> 02:46.800 And that is automatically executed when we create a new object of this class. 02:46.950 --> 02:52.980 And so when initializing a new object of the simple publisher class, the first step that we want to 02:52.980 --> 02:56.100 do is to initialize the component of the base class. 02:56.100 --> 02:58.110 So of the node class. 02:58.350 --> 03:03.480 By executing the constructor of the node class, that is the base class. 03:03.600 --> 03:10.980 And let's assign it a name to our node that we call simple publisher. 03:11.340 --> 03:14.150 Then among the private attributes. 03:14.160 --> 03:21.450 So here, let's add some support variables here. 03:22.490 --> 03:24.880 Let's open and close the parentheses. 03:25.090 --> 03:28.870 And here among the private variables, let's add some support variables. 03:28.870 --> 03:36.310 So let's start by adding an unsigned int called counter. 03:36.310 --> 03:41.310 And this will count the number of messages that are published within the Ros2 topic. 03:41.320 --> 03:45.760 So let's initialize this 1 to 0 when we execute the constructor. 03:45.760 --> 03:50.710 So let's set the counter variable to zero. 03:51.420 --> 03:58.290 Now that we have initialized the Ros two Node class, let's create a new publisher object using the 03:58.290 --> 04:00.590 Create Publisher function. 04:00.600 --> 04:10.800 So here in the constructor let's use the Create publisher function that is defined in the node class 04:10.800 --> 04:12.390 from which we are inheriting. 04:12.750 --> 04:18.060 And let's save the content so the output of this function in a new private variable. 04:18.060 --> 04:27.240 So here let's declare a new private variable as an object of the CPP publisher class. 04:28.060 --> 04:31.420 And we need to indicate here the message type. 04:31.420 --> 04:35.450 So the type of interface to be used for the message exchange. 04:35.470 --> 04:42.610 In practice, we need to specify the type of message the publisher object will send through the topic. 04:42.760 --> 04:49.180 For example, let's assume that we want to send text messages, so we want to send strings and the string 04:49.180 --> 04:52.730 message type is defined in another rescue library. 04:52.750 --> 05:01.790 So let's include from the standard messages library from the messages that are defined in this library. 05:01.810 --> 05:06.310 Let's include the string dot Http. 05:08.190 --> 05:09.390 And let's use it. 05:09.390 --> 05:16.290 So here when we are declaring the publisher object among the angular parentheses of this template class, 05:16.320 --> 05:22.020 let's use the message type standard messages. 05:22.680 --> 05:28.680 And from the messages of this library, let's take the string message. 05:29.650 --> 05:39.250 And let's take a shared pointer to this class and let's call this shared pointer Pope. 05:39.460 --> 05:46.630 Now we can finally store the output of the create publisher function within the Pope. 05:46.660 --> 05:47.380 Shared pointer. 05:47.380 --> 05:54.430 So within the Pope variable and also inside the angle brackets of the Create publisher class, we still 05:54.430 --> 05:58.540 need to indicate the type of message that we want to publish in the topic. 05:58.540 --> 06:05.740 So still from the standard messages library, from the messages of this library, let's use the string. 06:05.740 --> 06:11.590 And then instead in the parentheses we need to set the name of the topic in which we want the messages 06:11.590 --> 06:12.750 to be published. 06:12.760 --> 06:15.520 So let's call this one for example chapter. 06:15.520 --> 06:21.370 And furthermore, we also need to indicate the size of the queue, namely the size of the message queue 06:21.370 --> 06:22.930 that works as a buffer. 06:22.930 --> 06:29.570 If the subscriber to this topic is not receiving and processing messages fast enough, let's set this 06:29.570 --> 06:31.010 one, for example, to ten. 06:32.380 --> 06:34.660 At this point of the node initialization. 06:34.660 --> 06:40.210 Let's create a new timer object using the create wall timer function. 06:40.210 --> 06:44.590 So create wall timer. 06:44.590 --> 06:50.260 And this is still a function that is available from the node class from which we are inheriting. 06:50.410 --> 06:58.210 And let's call this function and again, among the private variables of the simple publisher class. 06:58.210 --> 07:09.820 So here let's define a new shared pointer to a class of type RCL, CP timer base, and let's take a 07:09.820 --> 07:16.360 shared pointer to this class and let's call this one timer. 07:16.360 --> 07:18.520 And now we can use this object. 07:18.520 --> 07:24.910 So this variable timer we just created to store the output of the create world timer function. 07:25.480 --> 07:28.450 So timer equals to create world timer. 07:29.320 --> 07:36.400 This function allows us to define a Ros two timer whose task is to repeatedly execute a specific function 07:36.400 --> 07:37.960 at a certain frequency. 07:37.990 --> 07:42.010 So the create timer function takes as input. 07:42.010 --> 07:47.420 So here between the parentheses, the frequency at which we want to execute the function. 07:47.440 --> 07:51.880 For example, let's assume that we want to execute it with a frequency of one second. 07:52.000 --> 07:59.440 So first, let's start by including here, the Chrono Library. 08:00.130 --> 08:07.300 And this is a plain cplusplus library and let's use so using namespace. 08:09.550 --> 08:10.090 Chrono 08:12.370 --> 08:18.550 literals in order to use the definition of the one second. 08:18.550 --> 08:22.660 So in order to create a world timer that will expire each second. 08:23.350 --> 08:30.490 Then in order to create this timer function, we also need to pass as input the name of the function 08:30.490 --> 08:36.460 that we want to execute at regular time intervals so that we want to execute every time the timer expires. 08:36.730 --> 08:44.890 To do so, let's use the bind function from the standard Cplusplus library and we want to execute the 08:44.890 --> 08:46.810 timer callback function. 08:46.810 --> 08:53.650 So timer callback, which we are going to define inside the same class. 08:53.650 --> 08:56.620 So inside the simple publisher class. 08:56.620 --> 08:57.460 So. 08:58.350 --> 09:06.450 Let's use the simple publisher in order to declare that the timer callback will be defined in the simple 09:06.450 --> 09:07.370 publisher class. 09:07.380 --> 09:13.260 And then let's also use the this pointer to indicate that we want to use the version of this class that 09:13.260 --> 09:15.450 is defined in this current object. 09:16.120 --> 09:19.360 As last step for the constructor. 09:19.360 --> 09:28.750 So here let's also print an informative message in the terminal using the CP info function which is 09:28.750 --> 09:32.940 defined still in the CP library that we have already imported. 09:32.950 --> 09:39.610 So let's use the RCL CP info. 09:40.740 --> 09:50.310 And this function takes as input the logger scope which we can obtain with the get logger function which 09:50.310 --> 09:53.310 we have already inherits from the node class. 09:53.310 --> 09:55.560 And it also takes the message. 09:55.560 --> 09:58.770 So the text that we want to print in the terminal. 09:58.770 --> 10:09.120 And so for example, let's print the message publishing at one hertz and this will be the one that we 10:09.120 --> 10:11.760 are going to view to visualize in the terminal. 10:12.360 --> 10:18.780 With this, the constructor of the Simple Publisher class is complete and now we have to define the 10:18.780 --> 10:20.250 timer callback function. 10:20.250 --> 10:21.210 In fact, here. 10:21.240 --> 10:26.160 Visual Studio code is telling us that the timer callback function is not yet defined. 10:26.250 --> 10:30.030 So let's define this function here. 10:30.330 --> 10:36.300 Still, within the simple publisher class, and the goal of this function is to publish a new message 10:36.300 --> 10:40.200 within the chapter topic every time this function is executed. 10:40.230 --> 10:46.350 So first we create the message to be published in the topic, which is a string message. 10:47.400 --> 10:57.540 Let's call this variable message and let's create an instance of the string message that is defined 10:57.540 --> 10:59.730 in the Standard Messages library. 11:01.450 --> 11:14.410 And let's change the content of this message to Hello Ross to followed by the counter and then followed 11:14.410 --> 11:16.300 by the variable counter. 11:16.330 --> 11:18.550 So let's convert first. 11:21.220 --> 11:30.460 To string the counter and then also let's increase its value for the next execution of this function. 11:31.150 --> 11:36.730 Then we can use the publish function that is available in the publisher object. 11:36.730 --> 11:40.060 So let's take the put object. 11:40.060 --> 11:48.910 And on this let's execute the publish function in order to publish the message that we have just created. 11:49.660 --> 11:54.550 With this, the simple publisher class is complete and now we can move on. 11:54.550 --> 11:58.750 So at the end of the class definition to define the main function. 11:58.750 --> 12:02.680 So the one that is automatically executed when the script starts. 12:02.890 --> 12:06.130 So let's define the main function. 12:16.310 --> 12:20.390 And let's return zero if everything goes well. 12:20.390 --> 12:30.830 And here as a first step, we need to initialize the Ros two with the function init from the CPP library 12:30.830 --> 12:34.190 and also to this one we pass the argument of the main. 12:36.490 --> 12:41.470 Then we can also create a pointer to an object of the simple publisher class. 12:41.800 --> 12:53.800 So let's call this one node and let's use the function make shared to create a shared pointer of the 12:54.820 --> 12:57.220 simple publisher class. 12:58.330 --> 13:04.900 And then let's keep this node up and running, thus keeping the timer and the publisher active so that 13:04.900 --> 13:14.260 they can continuously send messages within the chapter topic using the Rql CPP spin function. 13:14.470 --> 13:18.310 And so here, let's pass the node. 13:18.910 --> 13:25.030 Finally, if we terminate the execution of the node with control C, we need to ensure that the simple 13:25.030 --> 13:32.680 publisher node is correctly destroyed by using the shut down function. 13:34.110 --> 13:38.140 Now this is all and we can proceed to execute this node. 13:38.160 --> 13:44.070 But first we need to instruct the compiler on how it should build our script and make it an executable 13:44.070 --> 13:44.970 in Ros two. 13:45.270 --> 13:52.800 To do this, we need to modify the file cmakelists.txt, which was automatically created within the 13:52.800 --> 13:54.990 Arduino CP examples package. 13:55.170 --> 14:01.770 First, to facilitate reading of this file, let's delete all the commented rows that were automatically 14:01.770 --> 14:02.370 added. 14:02.400 --> 14:04.590 So let's remove all of these. 14:09.960 --> 14:16.200 And you can think of this file as a sort of instruction sheet that tells the compiler how it should 14:16.200 --> 14:24.330 translate our cplusplus scripts into executable files first before building our script and making them 14:24.330 --> 14:25.210 executable. 14:25.230 --> 14:31.770 We need to declare the support packages that we use so the dependencies that we use to develop the functionalities 14:31.770 --> 14:32.760 of our node. 14:32.790 --> 14:42.720 And if you remember correctly, we used the CPP library, so let's use a new instruction, find package. 14:42.840 --> 14:47.610 And so let's declare the usage of the CPP package. 14:48.630 --> 14:50.550 And this is required. 14:50.640 --> 14:54.910 And also we used the definition of the standard messages library. 14:54.930 --> 15:02.970 So let's copy this instruction and let's change the name of the package to standard messages. 15:03.670 --> 15:10.330 And now we can proceed actually to instruct the compiler on how it should build and install our script 15:10.360 --> 15:12.010 with an Add. 15:14.570 --> 15:17.450 Executable instruction. 15:17.450 --> 15:27.500 So let's call our executable simple publisher and its source code is located in the source folder. 15:27.620 --> 15:35.570 And in this simple publisher dot CPP script, Then let's add the dependencies. 15:35.570 --> 15:42.200 Let's declare the dependencies to this executable with the almond target. 15:44.730 --> 15:45.770 Dependencies. 15:45.780 --> 15:50.220 So to the simple publisher executable. 15:50.460 --> 15:54.630 Let's add the dependencies from the CPP library. 15:54.960 --> 15:57.680 The standard messages. 15:57.690 --> 16:03.840 And finally, we can install this script with the instruction install. 16:03.960 --> 16:12.540 And so the target, the executable that we want to install is the one that is called Simple Publisher. 16:13.380 --> 16:16.740 And also we need to specify where we want to install it. 16:16.740 --> 16:23.760 So the destination will be the lib folder and then the subfolder. 16:23.760 --> 16:27.900 We want to install this one in the folder that has the same name of the package. 16:27.900 --> 16:30.840 So the Arduino CPP examples package. 16:30.840 --> 16:39.510 And this is contained in a useful variable of the CMake that is called Project name. 16:39.510 --> 16:45.490 So basically this variable will contain the name of the package that is Arduino, but CPP examples. 16:46.240 --> 16:52.150 Finally before building the workspace so we can save this file, we need also to declare the dependencies 16:52.150 --> 16:58.630 that we use so our xcp standard messages also in the package dot XML file. 16:58.630 --> 17:05.950 So this other file here that was also automatically created when we use the Ros2 package, create command. 17:06.400 --> 17:08.980 So here we need to add two dependencies. 17:09.580 --> 17:21.310 So with the tag depend, let's declare the dependencies from the CPP library and also from the standard 17:21.730 --> 17:22.870 messages library. 17:23.260 --> 17:29.200 Let's save this file and these are the only two libraries that we need in the simple publisher script 17:29.320 --> 17:31.660 so we can finally execute our node. 17:32.330 --> 17:35.300 And let's open a new Linux terminal. 17:35.720 --> 17:42.100 Let's go into the workspace and let's use the command called build. 17:42.590 --> 17:45.320 This will build the entire workspace again. 17:45.500 --> 17:51.890 Here we can see that there is an error since in the CMake list, this should be targets. 17:52.880 --> 17:54.770 Okay, let's save again. 17:56.390 --> 17:58.670 And let's build again our workspace. 18:00.810 --> 18:06.690 And this command will build the entire workspace again, including also the new Cplusplus node. 18:06.870 --> 18:10.950 Now that the build is successfully complete, we can split the terminal. 18:10.950 --> 18:14.820 So let's open a new window and here let's source the workspace. 18:14.820 --> 18:17.970 So the setup dot bash. 18:18.240 --> 18:26.040 And this way the Arduino CP example package is recognized in the environment and we can proceed to start 18:26.040 --> 18:34.950 the node with the command Ros run followed by the name of the package that is Arduino boot CP examples 18:34.950 --> 18:40.110 so we can press tab to use the Ros2 Autocompletion of the packages. 18:40.110 --> 18:46.980 And also if we press tab twice, we can see that Ros is automatically able to detect that in the Arduino 18:46.980 --> 18:47.130 board. 18:47.130 --> 18:51.300 CP Examples exists the simple publisher node. 18:51.630 --> 18:58.260 So let's run this simple publisher node, let's press enter and we can see that it appears the message 18:58.260 --> 19:01.570 publishing at one hertz in the terminal. 19:01.690 --> 19:09.580 So to verify that indeed the node is running and is publishing messages within the chatter topic, let's 19:09.580 --> 19:16.390 open another window of the terminal and let's take a look to the list of all the topics that are currently 19:16.390 --> 19:22.210 available in Ros two with the Command Ros two topic list. 19:22.210 --> 19:28.240 And as the name of this command suggests, it will show you all the topics that are currently available 19:28.240 --> 19:28.870 in Ros two. 19:29.020 --> 19:34.360 And among these we can see that the chatter topic, which is exactly the name you gave to our topic 19:34.360 --> 19:35.950 in the publisher node appears. 19:36.470 --> 19:42.890 For example, if we stop the execution of the publisher node and we launch again the command Ros2 topic 19:42.890 --> 19:46.460 list, we can see that the chapter topic is no longer available. 19:46.460 --> 19:50.630 Since the node that was publishing it is not longer running. 19:51.320 --> 19:58.220 And by relaunching the node and relaunching again, the command Ros2 topic list, the chapter topic 19:58.220 --> 19:59.480 is back there. 20:00.250 --> 20:09.850 We can also see which messages are now traveling in the topic using the command Ros2 topic Echo, followed 20:09.850 --> 20:12.900 by the name of the topic, which is chatter. 20:12.910 --> 20:19.840 So let's press enter and all the messages that are now published within this channel are printed in 20:19.840 --> 20:20.530 the console. 20:20.530 --> 20:23.590 And so we can see that each message contains the string. 20:23.620 --> 20:26.290 Hello Ros2 followed by a counter. 20:26.290 --> 20:31.360 So a number that increases at each new messages that we are publishing. 20:31.720 --> 20:38.080 So for example, if we stop the execution of the publisher, we can see that in the terminal there are 20:38.080 --> 20:43.540 no longer printed any new messages, since no one is publishing on this topic. 20:44.050 --> 20:48.370 And when we launch it again, the count restarts. 20:50.060 --> 20:57.020 There are two other Ros, two useful commands that provide additional information about a topic, and 20:57.020 --> 21:05.510 these are Ros two topic info followed by the name of the topic from which we want to get more information. 21:05.810 --> 21:12.140 So let's take the chatter topic and here we can see that the message type that is published in this 21:12.140 --> 21:15.050 topic is of type string as we have defined. 21:15.050 --> 21:19.190 And also there is only one node that is publishing in this topic. 21:19.340 --> 21:27.200 Also, if we use the flag variables, we also get more information about this topic. 21:27.200 --> 21:32.180 For example, we can see also the name of the node that is actually interacting with this topic. 21:32.180 --> 21:34.730 That is the simple publisher node. 21:35.520 --> 21:43.380 Finally, one last rose to comment that we are going to use in this lesson is the ros2 topic. 21:44.270 --> 21:48.290 Earth's still followed by the name of the topic that is chatter. 21:48.290 --> 21:50.030 So let's press enter. 21:50.030 --> 21:56.180 And this command analyzes the messages that are published in the topic and calculates the frequency 21:56.180 --> 21:57.410 of the publishing. 21:57.440 --> 22:01.490 In this case, as you can see that it calculates a frequency of one earth. 22:01.490 --> 22:08.090 So the rate, the average rate is one hertz, which is exactly the value that we have set for the timer 22:08.090 --> 22:09.590 in the simple publisher. 22:09.590 --> 22:13.460 So here we set a delay of one second in the timer. 22:13.460 --> 22:16.670 So basically we are publishing one message per second.