WEBVTT 00:00.050 --> 00:06.320 Let's now create a subscriber to listen to the publisher we have just created in the previous lesson. 00:06.320 --> 00:12.230 So we will create a node for a smartphone here and inside it create a topic subscriber. 00:12.260 --> 00:17.720 As you can guess, a node which is handling a smartphone can have many features such as reading and 00:17.720 --> 00:23.300 publishing the battery state, sending a text message to a contact, increase or decrease the speaker 00:23.300 --> 00:30.380 volume, etc., etc. the subscriber that we create here is one part is just one part of this node. 00:30.380 --> 00:34.400 So let's go back to our workspace source. 00:34.400 --> 00:37.670 And then in the Python package again. 00:38.150 --> 00:43.790 And there we have the robot new station I will create. 00:44.270 --> 00:55.580 So I will create a smartphone dot Pi and make it executable okay for the Simulink install to work. 00:56.720 --> 01:02.570 Also if you have a you see if you have a Pi cache folder like that, you might want to remove it. 01:03.260 --> 01:03.440 Okay. 01:03.460 --> 01:05.470 We don't really need that. 01:06.190 --> 01:06.610 All right. 01:06.610 --> 01:11.770 So now we have our file here and we can open VSCode again. 01:11.770 --> 01:13.420 So it was already open here. 01:14.140 --> 01:16.300 Let's go to smartphone Dot Pi. 01:16.300 --> 01:23.230 And let's use the template again in smartphone dot Pi. 01:24.040 --> 01:24.430 Okay. 01:24.460 --> 01:34.330 So we have my custom node that's going to become smartphone node which inherits from node and then smartphone 01:34.330 --> 01:35.830 node here again. 01:35.830 --> 01:37.990 And let's keep things simple. 01:37.990 --> 01:40.870 Let's name the node smartphone. 01:40.870 --> 01:42.760 And maybe I've said that already. 01:42.760 --> 01:43.270 Maybe not. 01:43.270 --> 01:48.640 But don't name your nodes for example smartphone node okay. 01:48.670 --> 01:51.040 Because it's already a node name okay. 01:51.070 --> 01:54.250 So you don't say it's the smartphone node node. 01:54.280 --> 01:57.340 You say it's the smartphone node okay. 01:57.370 --> 02:00.370 So don't add underscore node or something like that. 02:00.370 --> 02:03.190 You might see this in existing packages okay. 02:03.220 --> 02:05.990 But that's basically not a good practice. 02:06.020 --> 02:07.580 All right, let's modify. 02:08.210 --> 02:10.640 Let's remove the modify commands. 02:10.640 --> 02:13.880 And we are done with the node structure. 02:13.880 --> 02:16.670 So you can see once we use the template it's very easy. 02:16.670 --> 02:18.800 Just make sure you correctly understand this template. 02:18.830 --> 02:22.670 Of course now we are going to create a Python subscriber. 02:22.670 --> 02:25.550 Where do we create the subscriber. 02:25.580 --> 02:29.780 Well we are also going to create it here in the constructor. 02:29.780 --> 02:32.540 And of course after the super. 02:33.170 --> 02:39.200 So let's do self dot subscriber I'm going to save it as an attribute of the class. 02:39.260 --> 02:41.390 And we do self dot. 02:41.570 --> 02:47.180 And once again we're going to use the functionalities of the node class create sub. 02:47.180 --> 02:50.270 And you see we have create subscription. 02:50.270 --> 02:54.260 So it's not create subscriber it's create subscription. 02:54.260 --> 03:00.350 Then I open the parentheses and you see the first thing again that we need to provide is the message 03:00.350 --> 03:01.100 type. 03:01.100 --> 03:07.010 So message type that's going to be the same as the one we have on the publisher okay. 03:07.040 --> 03:12.580 If you want a publisher and a subscriber to be able to communicate, then you need to use the exact 03:12.580 --> 03:14.320 same message type. 03:14.320 --> 03:19.780 And in the previous lesson I've already explained to you what is this example interfaces message with 03:19.780 --> 03:20.350 the string. 03:20.350 --> 03:22.630 So we're going to use the exact same here. 03:22.630 --> 03:32.860 So from example interfaces dot msg import string okay. 03:32.860 --> 03:38.950 Just make sure that it's the exact same here between the publisher and the subscriber. 03:38.950 --> 03:45.280 And then as we use the example interfaces we make sure that in package dot XML we have the dependency 03:45.280 --> 03:50.020 which I already set that because I wrote the publisher before. 03:50.950 --> 03:51.400 Great. 03:51.400 --> 03:54.850 So now I can provide string. 03:54.940 --> 03:57.010 Then I need to give the topic name. 03:57.010 --> 04:03.940 And here again it is very important that you use the same topic name for the publisher and the subscriber, 04:03.940 --> 04:09.460 because if you use the wrong name, then this is going to publish to one topic. 04:09.490 --> 04:15.600 This is going to to try to receive data from another topic and then the communication will not work. 04:15.600 --> 04:22.440 So make sure here I'm just going to copy robot News and paste it here to make sure it's the same name. 04:22.470 --> 04:23.100 All right. 04:23.100 --> 04:30.120 And then after that you see we need to provide a callback because, well, a publisher, when you have 04:30.120 --> 04:32.820 a publisher you decide when to publish. 04:32.820 --> 04:34.860 So in this case you create a publisher. 04:34.860 --> 04:41.760 And when you want to publish, you do publisher dot publish, but a subscriber doesn't know when it's 04:41.760 --> 04:44.040 going to receive messages, okay. 04:44.070 --> 04:47.250 It's going to receive messages when the publisher is publishing it. 04:47.250 --> 04:50.580 But on the subscriber side, we have no control over that. 04:50.580 --> 04:56.490 So that's why we're going to provide a callback function that's going to be called whenever we receive 04:56.520 --> 04:57.630 a message. 04:57.660 --> 05:01.770 And so let's create a function here a method inside the class. 05:01.800 --> 05:07.200 And let's name it Callback robot news with self. 05:07.200 --> 05:09.810 And we are going to receive a message. 05:09.810 --> 05:14.750 I can also use a colon here And type string. 05:14.780 --> 05:16.880 Okay, that's not mandatory. 05:16.880 --> 05:17.990 That's optional with Python. 05:17.990 --> 05:23.690 But this way you make the type a bit more explicit, which is, I would say, never a bad thing. 05:23.720 --> 05:26.930 And so you see I named this function. 05:26.930 --> 05:33.950 You can name it as you want, but usually for subscriber callbacks I will start with callback and then 05:33.950 --> 05:35.750 the name of the topic. 05:35.870 --> 05:36.410 Okay. 05:36.440 --> 05:37.460 First callback. 05:37.460 --> 05:42.140 Because then it's a good way not to make mistakes in the future because you don't want to call this 05:42.140 --> 05:44.030 function by yourself from the class. 05:44.060 --> 05:44.210 Okay. 05:44.240 --> 05:46.880 This is something that's going to be called automatically. 05:46.880 --> 05:49.130 And then this is to recognize the topic. 05:49.130 --> 05:53.180 So when we receive a message what do we do with that message. 05:53.210 --> 05:54.560 Well you can do whatever you want here. 05:54.560 --> 05:55.910 We are just going to print it. 05:55.910 --> 06:02.870 So self dot get logger and then info message. 06:02.870 --> 06:04.760 And actually we don't just print the message. 06:04.760 --> 06:06.170 The message is an object. 06:06.200 --> 06:10.670 We're going to print the message dot data. 06:10.670 --> 06:16.280 And you see I have the auto completion here because I have provided the type string and message dot 06:16.310 --> 06:23.030 data once again, because in you see in the string object we have a data field. 06:23.300 --> 06:24.680 So that's why it's data. 06:24.680 --> 06:26.060 So we just print it. 06:26.060 --> 06:31.280 But for example I don't know let's say you have let's say this is a temperature sensor that's going 06:31.310 --> 06:34.820 to publish the temperature of whatever in your robot. 06:34.850 --> 06:40.250 Then this could be a node that monitors the temperature of several sensors. 06:40.250 --> 06:44.330 And then if the temperature is too high, maybe you need to take some actions. 06:44.330 --> 06:48.950 So in this case in this callback you could monitor the temperature. 06:48.950 --> 06:53.990 And if it's too high then you could well you could do whatever you want and take some actions here from 06:53.990 --> 06:54.650 that node. 06:54.650 --> 06:55.100 Great. 06:55.100 --> 06:56.870 So we have the callback. 06:56.900 --> 06:59.270 You can see just one line here. 06:59.270 --> 07:02.690 We need to provide the callback here in the create subscription. 07:02.690 --> 07:07.850 So I'm going to do self dot callback robot news. 07:07.850 --> 07:13.250 And finally we have one last parameter here which is the queue size. 07:13.250 --> 07:16.520 So exact same thing as for the publisher. 07:16.670 --> 07:19.180 We have a queue size just in case. 07:19.210 --> 07:20.740 Well, there are too many messages. 07:20.740 --> 07:22.000 Messages are too big. 07:22.000 --> 07:27.610 The network is lossy, so you have kind of a buffer and make sure that the messages can arrive. 07:27.640 --> 07:27.880 All right. 07:27.910 --> 07:33.880 So we have here the data type the name the callback and the queue size. 07:33.910 --> 07:37.840 Now this line is quite long I'm just going to do this okay. 07:37.870 --> 07:39.160 To make it a bit more readable. 07:39.160 --> 07:42.310 And that's it for the code for the subscriber. 07:42.310 --> 07:46.870 So what's going to happen is that once again you initialize rows to communications. 07:46.870 --> 07:48.070 Then you create the node. 07:48.070 --> 07:50.770 When you create the node you're going to create a subscriber. 07:50.770 --> 07:55.330 When you create a subscriber, you see we register a callback and then you make the node spin. 07:55.330 --> 07:57.250 So the node is spinning. 07:57.250 --> 07:58.990 We stop the execution here. 07:58.990 --> 08:02.530 The node is still alive and it can process any callbacks. 08:02.530 --> 08:08.770 So this callback is going to be processed when we receive a message from a publisher somewhere else. 08:08.770 --> 08:10.210 So whenever a publisher. 08:10.210 --> 08:15.880 So for example that one is publishing a message on the robot news topic with the string data type, 08:15.880 --> 08:18.040 then this callback is going to be called. 08:18.040 --> 08:23.980 And we receive the message and we can process it inside the node, and this is going to last as long 08:23.980 --> 08:24.910 as the node is alive. 08:24.910 --> 08:28.870 So until we press Ctrl C let's save. 08:28.900 --> 08:32.680 And then let's go back to the terminal and let's go back to. 08:32.710 --> 08:35.890 So let's go back to the roster workspace. 08:35.890 --> 08:42.190 And even though we used Simulink install before, the thing is here this is a new file. 08:42.190 --> 08:44.050 This is going to be a new executable. 08:44.050 --> 08:46.000 So we need to build it anyway. 08:46.180 --> 08:51.280 And actually to build it it's going to be better if we add the instruction to build it. 08:51.280 --> 08:54.280 So here I'm going to add another comma. 08:54.430 --> 08:56.920 And then let's call it smartphone. 08:56.920 --> 09:00.040 That's going to be the name of the executable is equal. 09:00.040 --> 09:04.420 My pi pkg dot. 09:04.900 --> 09:07.030 Uh the name is smartphone dot pi. 09:07.060 --> 09:09.520 So smartphone. 09:09.670 --> 09:14.650 And then the main function as always I'm going to save that. 09:14.680 --> 09:29.280 Go back to the terminal and then run call build with packages Select I select my package so my pi pkg 09:29.280 --> 09:33.270 and then Simulink install. 09:34.770 --> 09:41.910 Okay it was built and now I'm just going to open once again new terminals so that everything is correctly 09:41.910 --> 09:42.360 sourced. 09:42.360 --> 09:45.270 That's probably the easiest way to make sure everything is working fine. 09:45.270 --> 09:48.420 And let's run um let's run it here. 09:48.450 --> 09:55.110 Ros, to run my Pi pkg, if I press tab twice, you can see that. 09:55.110 --> 09:57.660 Now we should have also a smartphone executable. 09:58.020 --> 10:02.280 So let's run smartphone and you can see we have okay. 10:02.310 --> 10:03.810 We don't have any log. 10:03.840 --> 10:09.840 I'm just going to go back and add a log at the end of the constructor. 10:09.840 --> 10:14.400 Self dot get logger info. 10:15.480 --> 10:21.600 Smartphone has been started. 10:21.660 --> 10:28.770 It's not very important but just I like it when you can see that the node has been started and we have 10:28.770 --> 10:30.510 reached the end of the constructor. 10:31.110 --> 10:32.790 So let's run again. 10:32.820 --> 10:38.880 You can see the smartphone has been started and well, nothing happens because well, we have a callback 10:38.910 --> 10:42.960 that's going to print some messages, but we are not publishing any messages. 10:42.960 --> 10:51.750 So now let's start the publisher Ros2 run my pi pkg with Robot News Station. 10:51.750 --> 10:58.680 And as soon as I start you should see we receive messages here on that terminal. 10:59.010 --> 11:04.470 You see, if I stop the publisher, then the subscriber is still running. 11:04.470 --> 11:06.960 But we don't receive any more messages. 11:06.990 --> 11:08.490 I start the publisher again. 11:09.420 --> 11:13.740 You see, we continue to receive messages and congratulations. 11:13.740 --> 11:18.270 You now have two nodes which are communicating with a topic. 11:18.300 --> 11:24.270 As you can see, once you understand how topics work, creating a topic, publisher or subscriber inside 11:24.270 --> 11:28.470 the node is not that hard and it's going to be always the same thing.