WEBVTT 00:00.050 --> 00:05.780 This is an extra lesson I've added here, because I think the parameter callback functionality is quite 00:05.780 --> 00:06.440 interesting. 00:06.440 --> 00:08.630 You will not use that for all your nodes. 00:08.630 --> 00:14.180 This is something that's often optional, and I'm only going to talk about parameter callbacks in this 00:14.180 --> 00:17.300 bonus video, so at least you know how they work. 00:17.330 --> 00:22.370 And let's start with the problem so you can understand why we need them in the first place. 00:22.370 --> 00:31.310 So I'm going to start my number publisher again Ross to run my example the Python one number publisher. 00:31.310 --> 00:35.720 And well I can set some parameters directly or use the default ones. 00:35.750 --> 00:37.490 I'm just going to use the default ones. 00:37.640 --> 00:49.160 So now if I do Ross two param list and then I do Ross two param get number publisher. 00:49.190 --> 00:51.830 Let's just verify what we have. 00:53.360 --> 01:01.200 So it's two and the other one is should be 1.0 as we set in the code. 01:01.200 --> 01:10.290 And then if I do Ros two topic equal number, we should then publish two every one second, which is 01:10.290 --> 01:10.770 the case. 01:10.770 --> 01:12.540 So no surprise until there. 01:12.540 --> 01:20.760 But then what if I do Ros two param I also have a set functionality, which means that I can change 01:20.790 --> 01:24.750 a parameter value after I have started the node. 01:24.780 --> 01:26.280 Okay, so let's try that. 01:26.850 --> 01:33.120 So we will need to provide first the name of the node then the name of the parameter. 01:33.120 --> 01:36.420 So just like for the get functionality. 01:36.420 --> 01:39.810 But then we need to provide another argument which is the new value. 01:39.810 --> 01:41.070 So let's say four. 01:41.070 --> 01:46.890 So with this I want to change the number parameter here inside the number publisher. 01:46.890 --> 01:49.050 And the value is going to be four. 01:49.680 --> 01:51.510 And that's working. 01:51.510 --> 01:56.520 Now if I get the parameter again you see its value is four. 01:56.550 --> 01:57.060 Great. 01:57.090 --> 01:58.260 Now what's the problem. 01:58.260 --> 02:01.190 Well let's equal call the number topic again. 02:01.190 --> 02:03.590 And you see it's still two. 02:03.620 --> 02:04.490 It's not four. 02:04.520 --> 02:05.180 Why is that? 02:05.180 --> 02:08.390 We have changed the parameter but then it doesn't change in the code. 02:08.420 --> 02:10.010 Let's go back to the code. 02:10.880 --> 02:19.040 Let's open the number publisher and let's understand why this new value is not actually used. 02:19.070 --> 02:25.460 So what we do in the constructor when we start the node we declare the parameter and we get the value 02:25.490 --> 02:27.500 when we start the node. 02:27.530 --> 02:27.920 Okay. 02:27.950 --> 02:34.850 So this value is the one that you either get from the default value or when you do rose to run with 02:34.850 --> 02:35.870 the values here. 02:35.870 --> 02:43.790 But then you see that if the parameter value changes, well, there is no way in the code where we reflect 02:43.790 --> 02:48.230 that there is no way in the code where we say, okay, we receive the new value, so we're going to 02:48.260 --> 02:49.190 use that new value. 02:49.190 --> 02:52.190 So you can change the parameter value. 02:52.190 --> 02:54.980 And it's not going to be used in the code. 02:54.980 --> 03:00.720 If you want to be able to do that then you need to add what's called a parameter callback. 03:00.720 --> 03:02.850 So let's add a parameter callback here. 03:02.850 --> 03:08.280 So after we have declared and the parameters and after we get them we can for example do. 03:08.310 --> 03:12.840 Here I'm going to do this here self dot. 03:12.840 --> 03:19.440 And there is a function called add post set parameters callback. 03:19.440 --> 03:23.340 So the name is quite long but well you can use the auto completion. 03:23.340 --> 03:26.790 And then you need to provide a callback function. 03:27.090 --> 03:33.720 So I'm going to create another callback here def um callback. 03:33.750 --> 03:38.640 Actually let's name it parameters callback. 03:39.030 --> 03:40.980 And this is going to receive. 03:40.980 --> 03:44.670 So we have self and is going to receive a list of parameters. 03:44.670 --> 03:45.810 So we have params. 03:45.810 --> 03:51.450 And I'm going to put an explicit type is going to be a list of parameter objects. 03:51.450 --> 04:04.490 To be able to use parameter I'm going to import here I can do from partial Pi dot parameter import parameter. 04:04.520 --> 04:04.730 Okay. 04:04.760 --> 04:08.810 So that's a different thing than it's not directly from the LCL pi node. 04:08.810 --> 04:11.030 It's from LCL pi parameter. 04:12.530 --> 04:13.160 Okay. 04:13.490 --> 04:16.610 So I provide the type here parameter. 04:17.390 --> 04:19.280 This is just so that it's more explicit. 04:19.280 --> 04:23.420 You don't need to do it, but I like to give explicit types even in Python. 04:24.020 --> 04:25.520 Let's put pass for now. 04:25.520 --> 04:33.830 And we can give the callback function here self dot parameters callback. 04:33.830 --> 04:36.050 So you see in the constructor it's quite easy. 04:36.050 --> 04:42.740 You just add you basically register a new callback that will be triggered whenever something like this 04:42.740 --> 04:43.610 happens. 04:43.640 --> 04:44.180 Okay. 04:44.210 --> 04:47.960 Whenever something like setting a new parameter value happen. 04:47.960 --> 04:53.000 And then you will get to this callback with a list of parameters. 04:53.000 --> 04:57.990 So all the parameters that have been changed because you could also change several parameters at the 04:57.990 --> 04:59.190 same time if you want to. 04:59.220 --> 05:02.610 So what we're going to do here, we are going to use a for loop. 05:02.640 --> 05:06.330 Let's say for param in params. 05:06.960 --> 05:12.390 And then I'm going to do it very basically we can do if param dot. 05:12.390 --> 05:14.670 And you see we have three things. 05:14.670 --> 05:20.010 We have the name, we have the type with an underscore and we have the value. 05:20.010 --> 05:25.830 So if the param name is equal to number okay. 05:25.860 --> 05:28.560 Number is the param name we have used here. 05:29.400 --> 05:37.530 Then we can say self number is equal to the value of the parameter. 05:37.530 --> 05:41.970 So the new value which is param dot value. 05:41.970 --> 05:48.750 And if you want it to do something with the type you also can use param dot and type like this okay. 05:48.780 --> 05:50.370 But we're not going to use it here. 05:51.480 --> 05:54.090 So you can see you do for param in params. 05:54.090 --> 05:58.910 And then if param name is His number and then, well, you can do the same. 05:58.910 --> 06:03.650 You can do if name and you can just do that for every parameter. 06:03.650 --> 06:09.140 So for some parameters like this one, the number that's going to be enough. 06:09.170 --> 06:14.750 You just change the number and then you use the number again in this publisher you might want to validate 06:14.750 --> 06:15.710 the data or not. 06:15.740 --> 06:17.120 It depends here. 06:17.120 --> 06:20.450 We don't here we don't necessarily want to do it. 06:20.480 --> 06:22.670 Well it depends who's going to change the value. 06:22.700 --> 06:24.770 Do you trust the other side for example. 06:24.800 --> 06:27.830 And just to give you an example, I'm not going to add this one here. 06:27.830 --> 06:33.200 But if you were to change the time period here, just changing the time period is not going to be enough. 06:33.200 --> 06:35.570 You would need to stop the timer. 06:35.570 --> 06:44.990 So for example here just to show you if you do self dot and this number timer, you can do cancel for 06:44.990 --> 06:46.070 example to stop it. 06:46.070 --> 06:48.980 Or you can do reset and then you can start it again. 06:48.980 --> 06:51.230 So this you can search on your own. 06:51.230 --> 06:55.980 But just to show you that if you modify a parameter let's say modify the timer period. 06:55.980 --> 06:58.200 I get it inside my parameters callback. 06:58.230 --> 07:01.320 Then it's not just about saving a new variable. 07:01.320 --> 07:07.230 You might need to do more stuff, for example, stopping the timer and creating it again with the new 07:07.260 --> 07:08.430 timer period. 07:08.430 --> 07:14.580 If you were to, let's say you have a camera driver, and then while the camera has disconnected and 07:14.580 --> 07:19.410 you provide the new device name by changing the parameter, maybe you want to restart the node. 07:19.410 --> 07:22.890 Or let's say you want to change the parameter without restarting the node. 07:22.890 --> 07:28.200 Then when you provide the new device name, you will need to do the action of disconnecting the camera 07:28.200 --> 07:29.850 and reconnecting again. 07:29.850 --> 07:35.070 So you see, it's going to depend a lot on what you want to do in your application, but you could have 07:35.070 --> 07:39.120 to process some stuff when you get a new value for a parameter. 07:39.120 --> 07:42.960 And then it doesn't mean that because you can do it, that you should do it. 07:42.990 --> 07:43.470 Okay. 07:43.500 --> 07:46.470 This is used in some nodes or with more experience. 07:46.470 --> 07:52.410 You will see where it's useful to use it, but by default I would start basically any node and not use 07:52.410 --> 07:53.610 the parameter callback. 07:53.640 --> 07:57.890 Okay, you just set the parameters when you start the node, and then later on you see that. 07:57.920 --> 08:01.130 Is it useful to actually change the value later on or not? 08:01.160 --> 08:01.520 All right. 08:01.520 --> 08:03.530 So let's save that. 08:03.530 --> 08:06.740 And now we should be able to run it. 08:06.770 --> 08:08.510 We're going to do the test again. 08:08.510 --> 08:11.000 So I start the node again. 08:12.050 --> 08:12.710 Okay. 08:12.770 --> 08:19.010 Let's check that the number is two. 08:19.430 --> 08:20.720 And then let's do Ros two. 08:20.750 --> 08:23.840 Topic eco is going to publish two. 08:23.870 --> 08:27.800 And then let's set the new value with four. 08:28.280 --> 08:32.150 And as you can see as soon as we set the new value now we are publishing four. 08:32.150 --> 08:34.820 So it is correctly working in the code. 08:34.820 --> 08:38.690 And also of course the parameter is going to be four. 08:38.720 --> 08:39.440 Okay. 08:39.650 --> 08:45.860 So what happened is that this is going to register a callback when the node is spinning. 08:46.190 --> 08:48.740 If we change the value of the parameter. 08:48.740 --> 08:52.970 So that's one event that will trigger this callback. 08:52.970 --> 08:55.920 And then we check Do we have the number parameter if. 08:55.950 --> 09:00.750 Yes, we change the value and then the next time we publish is going to use the new value. 09:00.780 --> 09:01.320 All right. 09:01.320 --> 09:02.640 So that's the Python code. 09:02.640 --> 09:06.000 I have also written the cplusplus code that you can check. 09:06.000 --> 09:09.780 So you can download it in number publisher. 09:09.960 --> 09:16.410 So for Cplusplus we save here post set parameters callback handle. 09:16.410 --> 09:19.680 Then we initialize it here. 09:19.830 --> 09:21.450 You see it's basically the same thing. 09:21.450 --> 09:23.130 We register a callback. 09:23.250 --> 09:30.750 And in this callback we receive a list of parameters which is actually a const reference of a vector. 09:30.750 --> 09:34.710 And I just go through the list and you can see we do the same thing. 09:34.710 --> 09:38.220 If param getname is number, then we get the parameter. 09:38.220 --> 09:39.690 And we also need to provide. 09:39.690 --> 09:43.530 Here you see as int okay for specifying the data type. 09:43.530 --> 09:47.340 So this you can download at the end of this section to see the complete code. 09:47.370 --> 09:50.730 But at the same thing as what we did with Python. 09:50.730 --> 09:53.750 And while that's it just to finish Here. 09:54.290 --> 10:03.500 You can see that if I come back to my services Ros2 service list for this node, you see I have. 10:03.500 --> 10:05.930 So we have used previously list parameters. 10:05.930 --> 10:09.680 You can get parameters, but you can also set parameters. 10:09.680 --> 10:14.390 So that action we did here of changing a parameter from the terminal. 10:14.390 --> 10:18.560 If you want to do that from a node using ros2 communications. 10:18.560 --> 10:22.730 So from another node you could call this service. 10:22.760 --> 10:25.310 And for example change that parameter. 10:25.310 --> 10:30.830 And then if it was implemented here in the parameter callback then you can change. 10:30.860 --> 10:36.380 Basically you can change a parameter for a node from another node. 10:36.410 --> 10:36.830 All right. 10:36.830 --> 10:40.250 And that's the end of this parentheses on parameter callbacks. 10:40.250 --> 10:43.520 So this is something that is good to know it exists. 10:43.550 --> 10:45.200 It's good to understand how it works. 10:45.200 --> 10:47.720 And you're not necessarily going to need it quite soon. 10:47.720 --> 10:52.850 But if you need it in case in the future, then you know you can come back to this lesson to get a quick 10:52.850 --> 10:53.690 refresher.