WEBVTT 00:00.050 --> 00:04.880 Let's now improve this node with object oriented programming or OOP. 00:05.180 --> 00:11.240 This will make your node more scalable, and it's the recommended way to write code in Ros2 as per the 00:11.240 --> 00:12.620 official guidelines. 00:12.620 --> 00:17.870 And any way you will see that all the codes well everywhere is using OOP. 00:17.900 --> 00:20.990 So let's create a class here for our node. 00:20.990 --> 00:26.270 And after we have this class, we will have the template that we can use for any nodes that we create 00:26.270 --> 00:27.050 in the future. 00:27.050 --> 00:28.880 So how do we create a class here. 00:28.880 --> 00:35.690 Well I'm going to use the class keyword and let's name it my node. 00:35.930 --> 00:37.820 So here I don't have any context. 00:37.820 --> 00:40.100 So I use my node. 00:40.220 --> 00:45.830 But for example if you would create a temperature sensor node well you would name it temperature sensor 00:45.860 --> 00:46.370 node. 00:46.370 --> 00:51.050 So that's the class that's going to inherit from the node class. 00:51.080 --> 00:52.310 So that's very important. 00:52.310 --> 00:59.900 So you inherit from the node class which means that you will be able to use all the Ros2 node functionality 00:59.930 --> 01:01.100 from our CLP. 01:01.100 --> 01:02.210 So we'll do this. 01:02.210 --> 01:04.480 And then inside we have a def underscore. 01:04.510 --> 01:05.320 Underscore init. 01:05.320 --> 01:09.370 So that's the constructor for Python self. 01:09.400 --> 01:09.640 Okay. 01:09.670 --> 01:12.550 So this is the basic Python OOP stuff. 01:12.550 --> 01:15.610 We create a constructor in this constructor. 01:15.640 --> 01:19.420 The first thing we need to do is super we call the super. 01:20.080 --> 01:23.170 So we call the parent constructor of the node class. 01:23.170 --> 01:27.820 And we do init as well here like that. 01:27.820 --> 01:31.960 And we pass here the node name. 01:31.960 --> 01:35.380 So that's going to be pi test. 01:35.710 --> 01:36.160 All right. 01:36.160 --> 01:41.170 So we call the parent constructor here to initialize the node from the node class. 01:41.170 --> 01:43.600 And that's it for the minimum code for a node. 01:43.630 --> 01:44.050 Okay. 01:44.080 --> 01:48.490 Now I'm also going to add this line here to use the logging functionality. 01:48.490 --> 01:53.740 So how to use the logging functionality I can directly use self okay. 01:53.740 --> 02:01.570 Because we are in the class we use self as this is a node, so we are inheriting from the node. 02:01.570 --> 02:05.380 I can use self dot get logger. 02:05.410 --> 02:13.820 You see it's also working and then.info and I can just use the same log here. 02:14.240 --> 02:14.630 All right. 02:14.630 --> 02:16.040 And that's going to do the same thing. 02:16.040 --> 02:19.340 So this here is doing the same thing as that. 02:19.340 --> 02:22.070 So now I'm going to remove that. 02:22.070 --> 02:31.310 And instead instead of creating a node directly here I'm going to create a my node instance okay. 02:31.340 --> 02:35.360 So from the class my node that I have created with the parentheses. 02:35.360 --> 02:35.960 And that's it. 02:35.990 --> 02:38.180 We don't have any argument here to pass. 02:38.210 --> 02:38.630 All right. 02:38.660 --> 02:40.250 So now I have created the node. 02:40.250 --> 02:42.080 So you see the main is the same. 02:42.200 --> 02:44.900 We still initialize rows to communication. 02:44.900 --> 02:46.040 We create the node. 02:46.040 --> 02:47.270 We make the node spin. 02:47.270 --> 02:48.560 And we have the shutdown. 02:48.590 --> 02:51.650 Now the node is not just using the node class. 02:51.650 --> 02:56.270 It's using our own class which inherits from the node class. 02:56.600 --> 03:03.530 And then we get all of the Ros2 functionalities for the node directly from the class with self dot and 03:03.530 --> 03:04.670 then the functionality. 03:04.670 --> 03:07.070 So let's save this. 03:07.070 --> 03:13.440 And now I can go back here and well let's go back to our Ros2 workspace. 03:13.710 --> 03:14.220 Clear. 03:14.250 --> 03:16.680 Let's do the call code build again. 03:16.680 --> 03:24.000 So let's come back to this command call build with the Python package in this folder. 03:27.660 --> 03:29.850 And then I need to source the workspace. 03:29.850 --> 03:32.250 So once again I can source the bashrc. 03:32.490 --> 03:35.640 I could also close and open a new terminal. 03:35.670 --> 03:39.180 I can just do because here I'm already in this place. 03:39.180 --> 03:42.870 So I can do source install setup bash. 03:43.050 --> 03:48.150 Okay I go inside the install folder and I have the setup bash script. 03:48.780 --> 03:49.020 Great. 03:49.020 --> 04:00.210 So now my workspace is sourced again with all the new changes and I can do Ros2 run my pi pkg with pi 04:00.270 --> 04:03.120 node, which is the executable name. 04:04.020 --> 04:10.800 And no surprise we have the same thing and I press ctrl c and this here what you see here. 04:10.800 --> 04:15.460 That's the template that you can write for any node. 04:15.460 --> 04:17.260 So that's about 20 lines of code. 04:17.260 --> 04:20.650 And in one of the following lessons I will give you the template directly. 04:20.650 --> 04:23.410 You can just copy and paste it if you want okay. 04:23.440 --> 04:28.330 So it's important that you understand how to write this how to build this code. 04:28.330 --> 04:31.000 And then well you don't need to write it every time. 04:31.000 --> 04:37.540 Again you just take this well without the log, but you just take this and you just change the node 04:37.570 --> 04:39.760 name here, maybe the class name. 04:39.760 --> 04:41.890 And that's going to be the same for all your nodes. 04:41.890 --> 04:45.580 And before we continue I want to show you something else here okay. 04:45.610 --> 04:48.430 So we have the node template. 04:48.430 --> 04:54.100 But I want to add one more functionality just to do something a bit more interesting with this node. 04:54.100 --> 04:56.800 Because for now we just print one hello world. 04:56.800 --> 04:58.720 That's not really interesting. 04:58.720 --> 05:03.850 What if we want to, for example, print something every one second? 05:03.880 --> 05:05.680 As long as the node is alive? 05:05.710 --> 05:07.840 That's a very common behavior. 05:07.840 --> 05:13.120 For example, if you create a temperature sensor node and you run to read from the sensor at ten hertz, 05:13.150 --> 05:16.330 so basically ten times per second, how can you do that? 05:16.350 --> 05:19.770 Well, we are going to add what's called a timer. 05:19.770 --> 05:24.600 And the timer functionality is one of the most important ones that you will use in Ros2. 05:24.630 --> 05:30.630 With a timer, you can say, I want to execute this function every x amount of time. 05:30.630 --> 05:37.110 So let's add a timer and to add a timer I'm first going to create a function here inside the class. 05:37.110 --> 05:44.820 So I'm going to add a method named let's say timer callback with self. 05:45.180 --> 05:55.050 And in this timer callback I do self dot get logger.info with hello. 05:55.050 --> 05:56.640 Let's just say hello for now. 05:56.670 --> 06:00.840 I'm going to keep this log here and then I will create. 06:00.840 --> 06:05.580 So in the constructor of the class I will create a timer. 06:05.610 --> 06:07.170 How to create a timer. 06:07.170 --> 06:08.940 Well you can do self. 06:09.570 --> 06:15.330 So you use self always because then you get the functionalities from the node class. 06:15.330 --> 06:18.900 And we can do create timer. 06:18.900 --> 06:21.070 You see we have a create timer function here. 06:21.100 --> 06:23.860 This function will take two parameters. 06:23.860 --> 06:26.770 First is the timer period. 06:26.770 --> 06:29.230 In second that's a float number. 06:29.230 --> 06:32.560 So here let's say 1.0. 06:32.560 --> 06:33.880 So that's one second. 06:33.910 --> 06:39.100 That means we want to call a function every one second. 06:39.130 --> 06:43.510 And the second parameter is the callback. 06:43.540 --> 06:46.210 So we need to provide a callback function. 06:46.210 --> 06:48.700 And the callback function is already implemented here. 06:48.700 --> 06:50.980 That's called timer callback. 06:50.980 --> 06:57.340 So that's going to be self dot timer callback. 06:57.670 --> 07:01.690 And note that we don't provide any parentheses here okay. 07:01.720 --> 07:05.140 If I do this I'm actually calling the function. 07:05.170 --> 07:10.450 If I don't provide the parentheses I'm just giving the reference to the function because we don't want 07:10.450 --> 07:11.320 to call the function. 07:11.320 --> 07:15.670 Now we are just registering the function inside the timer. 07:15.670 --> 07:18.940 And as you can see, so I have named it like that on purpose. 07:18.940 --> 07:24.840 That's actually going to be called a callback function because we call it back later. 07:24.870 --> 07:25.350 All right. 07:25.350 --> 07:26.910 And what's going to happen here? 07:26.970 --> 07:29.730 What's going to happen is that let's look at the main. 07:29.730 --> 07:31.800 So that's what's going to be executed. 07:31.950 --> 07:35.760 We first initialize rows to communication and then we create the node. 07:35.760 --> 07:38.400 So if we create the node we go to the constructor. 07:38.580 --> 07:44.310 We use the parent constructor to initialize the node and give it a name pi test. 07:44.340 --> 07:47.520 Then we print something with the logger functionality. 07:47.520 --> 07:49.500 And then we use a timer. 07:49.500 --> 07:51.330 We create a timer and that's it. 07:51.330 --> 07:52.890 So we don't call this function yet. 07:52.920 --> 07:54.330 We have just created a timer. 07:54.330 --> 07:58.410 But then we are going to do a CLP dot spin with the node. 07:58.410 --> 08:00.480 So spin is going to stop. 08:00.480 --> 08:04.440 The execution here is going to keep this node alive. 08:04.440 --> 08:08.460 And it's going to allow the node to process any callback. 08:08.490 --> 08:12.630 And so here basically with a timer we will have some callbacks. 08:12.660 --> 08:18.390 And because the node is spinning then the timer is saying that every one second we need to call the 08:18.390 --> 08:20.280 timer callback function. 08:20.280 --> 08:25.800 Then because the node is spinning every one second, that function here is going to be called okay. 08:25.830 --> 08:30.640 And that's how this is going to be executed because the node is spinning. 08:30.640 --> 08:34.210 So let's save and then let's go back here. 08:34.990 --> 08:37.720 Let's do build. 08:40.720 --> 08:42.670 And then let's do source. 08:43.270 --> 08:44.890 And then let's do run. 08:44.890 --> 08:48.040 As you can see it's always build source and run. 08:48.040 --> 08:52.450 So remember that you build your source and your run always. 08:52.450 --> 08:55.900 So let's run and you see hello world. 08:55.900 --> 08:57.940 And then you see hello hello hello. 08:57.940 --> 09:00.910 Every one second you can see that with the logs. 09:00.940 --> 09:06.040 Now I press Ctrl C and I stop the execution and I stop the spinning and everything. 09:06.070 --> 09:06.670 All right. 09:06.670 --> 09:12.370 So you see that's quite useful now just for the well for the example, let's just add for example, 09:12.370 --> 09:17.980 we want to say hello zero and then one two, three we could add a counter. 09:17.980 --> 09:20.860 So let's create a counter here. 09:20.890 --> 09:24.070 So self dot counter. 09:24.220 --> 09:25.960 And I'm going to add an underscore here. 09:25.960 --> 09:32.280 So for attributes of the class I'm just going to add a trailing underscore here just as a convention 09:32.430 --> 09:34.050 and let's say zero. 09:34.050 --> 09:37.740 So because the counter is in the class I can use it in that method. 09:37.740 --> 09:39.180 And I can do hello. 09:40.320 --> 09:46.740 Plus as it is an integer I'm going to do str self counter. 09:47.700 --> 09:53.430 And then I can do self dot counter plus one okay. 09:53.430 --> 09:56.820 So I start at zero and then it's going to be 123 etc.. 09:56.850 --> 09:58.380 Just one thing I'm going to show you. 09:58.410 --> 10:00.660 Let's say I have forgotten the plus okay. 10:00.660 --> 10:02.220 So I'm going to have an error. 10:02.250 --> 10:03.840 I'm going to have a Python error here. 10:03.840 --> 10:05.700 But let's say I didn't see that. 10:05.970 --> 10:14.520 So I'm going to come back here and let's do the first step, which is to build the package. 10:15.840 --> 10:19.860 Well, as you can see in this case we have a syntax error. 10:19.860 --> 10:25.380 So if we have a syntax error when you build the package here you're going to get an error. 10:25.380 --> 10:28.140 So it's not going to be correctly built. 10:28.140 --> 10:29.580 You see we have errors. 10:29.610 --> 10:31.800 And in this case it's quite explicit. 10:31.800 --> 10:33.670 So I can fix that. 10:33.700 --> 10:36.280 Add the plus save. 10:36.340 --> 10:40.270 Go back and build again. 10:41.740 --> 10:42.190 Okay. 10:42.220 --> 10:43.180 And now it's working. 10:43.180 --> 10:47.620 So with Python, if you have stuff like syntax errors you will see that when you build. 10:47.620 --> 10:51.310 But then of course you know that with Python you can get extra errors. 10:51.310 --> 10:57.100 For example, if you raise exceptions that are not code in your program, then the build is going to 10:57.100 --> 10:57.730 be successful. 10:57.730 --> 11:01.150 But then when you run the program, maybe you will get an error later on. 11:01.180 --> 11:01.360 Okay. 11:01.390 --> 11:03.100 So something to be aware of. 11:03.370 --> 11:04.360 So I have built. 11:04.390 --> 11:07.810 Now I can source and then I can run. 11:08.740 --> 11:14.410 And now you will see hello 0123 etc.. 11:14.410 --> 11:16.660 So we could add a space or so if we want to. 11:16.690 --> 11:19.000 But that's, that's about it. 11:19.000 --> 11:25.420 And so with this code you have seen the correct way to create a node with a class okay. 11:25.450 --> 11:27.580 So using object oriented programming. 11:27.580 --> 11:34.000 And you have also discovered a quite important functionality which is a timer that we will use again 11:34.000 --> 11:35.920 and again in this course.