WEBVTT 00:00.050 --> 00:01.280 What are Ros2? 00:01.310 --> 00:02.510 Interfaces. 00:02.540 --> 00:08.900 In the previous sections on topics and services, you have used some common interfaces to communicate 00:08.900 --> 00:11.240 between nodes with topics. 00:11.240 --> 00:17.540 You have seen that all nodes publishing are subscribing to a topic must use the same data type with 00:17.570 --> 00:18.140 services. 00:18.140 --> 00:24.200 You have seen that the client must send a message respecting a certain data type, and the server must 00:24.200 --> 00:29.120 respond with another message, also respecting another data type. 00:29.120 --> 00:37.220 So a topic is defined by two things a name so you can recognize it, and an interface based on a message 00:37.220 --> 00:41.930 definition which is the data structure of the information that you send. 00:42.020 --> 00:49.190 A service is also defined by two things a name so you can recognize it, and an interface or service 00:49.190 --> 00:50.000 definition. 00:50.030 --> 00:56.240 The service definition contains one message definition for the request and one message definition for 00:56.240 --> 00:56.900 the response. 00:56.900 --> 00:58.700 So it's a pair of messages. 00:58.700 --> 01:05.950 And well you can see topics and services as the communication layer tool, and the interfaces or the 01:05.950 --> 01:09.550 messages are the actual content that you send. 01:09.550 --> 01:13.000 Let's just make another very simple real life analogy. 01:13.030 --> 01:17.380 When you send a letter, the transport company will carry your letter. 01:17.410 --> 01:24.640 The content of this letter is the analogy of a raster interface here using a message definition. 01:24.820 --> 01:30.580 When you send a letter and you wait for a response, then the first letter contains a request message, 01:30.580 --> 01:34.990 and the letter that you receive back contains a response message. 01:34.990 --> 01:40.210 And the combination of the two message definitions is the service definition. 01:40.210 --> 01:40.660 All right? 01:40.660 --> 01:45.070 As you can see, message and service definitions are quite useful. 01:45.070 --> 01:50.140 Without them, you would have nothing to send over a topic or a service. 01:50.140 --> 01:56.980 So now, technically speaking, how is it possible that you can use those interfaces directly in your 01:56.980 --> 01:57.520 code? 01:57.520 --> 02:01.300 Because for now it just looks like a bunch of simple text. 02:01.300 --> 02:04.870 Well, let's say that you create a message definition. 02:04.870 --> 02:12.370 When you use the build command line, the message will be passed by the build system, and then a source 02:12.370 --> 02:18.640 code will be generated for this message in any Ros2 supported language C plus plus Python. 02:18.640 --> 02:20.980 But it could also be another language. 02:21.010 --> 02:25.960 Your interface is now ready and that's why you can, for example, include the message header in your 02:25.990 --> 02:33.910 C plus plus code or import the library in Python because the build system generated that file. 02:33.940 --> 02:36.190 Now how to write an interface. 02:36.190 --> 02:42.550 Well, you will be able to use primitive types that we will see in a minute and also include other message 02:42.550 --> 02:43.420 definitions. 02:43.420 --> 02:49.090 And they are already a lot of existing interfaces in for example, those package that we are going to 02:49.120 --> 02:50.470 browse right now. 02:50.770 --> 02:56.620 So what kind of data types can you use to create a message or a service definition? 02:56.620 --> 03:00.280 If you go on the Ros2 interfaces page on the Ros documentation. 03:00.280 --> 03:06.480 So you can just basically type Ros to interfaces, you will get a page like that, and you can see all 03:06.480 --> 03:08.910 the primitive data types that you can use. 03:08.940 --> 03:10.350 It's not really complicated. 03:10.380 --> 03:12.630 Among them you can see for example we have boolean. 03:12.630 --> 03:14.460 So you will need to write bool. 03:14.460 --> 03:16.050 You have integer numbers. 03:16.080 --> 03:22.110 For example with int 64 float numbers it's the same float 64 for example. 03:22.140 --> 03:28.650 Then you can use string etc. and you can also create arrays of those types. 03:28.680 --> 03:30.630 So can you have an example here. 03:31.230 --> 03:34.410 So that's what you're going to write in the message definition. 03:34.410 --> 03:36.450 We're going to create one in the next lesson. 03:36.450 --> 03:39.030 And then how to interact with this in the code. 03:39.030 --> 03:42.540 Well you see you have the simple plus and the Python equivalent. 03:42.540 --> 03:47.760 So if you're going to use for example a string then in Python it's just going to be a normal string. 03:47.760 --> 03:51.120 And in Cplusplus it's going to be an STD string. 03:51.150 --> 03:51.690 Okay. 03:51.690 --> 03:54.120 For most of it is really, really simple. 03:54.120 --> 03:58.050 So those are all the primitive types that you can use. 03:58.050 --> 04:03.120 Now where to find the existing packages, for example the ones that we have used already. 04:03.150 --> 04:05.690 Well you can find most of them on GitHub. 04:05.690 --> 04:09.770 For example, here I am on GitHub with the example interfaces. 04:09.770 --> 04:11.300 So you have the URL here. 04:11.300 --> 04:15.710 But you could also just type ros2 and then example interfaces GitHub. 04:15.710 --> 04:17.510 You will easily find this page. 04:17.840 --> 04:21.560 Same thing I don't necessarily give you URLs because they might change stuff. 04:21.560 --> 04:22.910 They might move stuff around. 04:22.910 --> 04:24.950 So a very simple Google search will do. 04:24.980 --> 04:27.800 Now I am in this example interfaces package. 04:27.800 --> 04:29.750 So that's the code inside the package. 04:29.750 --> 04:31.460 You see we have a cmakelists.txt. 04:31.490 --> 04:33.140 We have a package dot XML. 04:33.140 --> 04:35.510 And then we have for example in MSG. 04:35.510 --> 04:43.160 So in message we have here if I go down we have int 64 dot msg. 04:43.190 --> 04:44.600 That's the one we have used already. 04:44.600 --> 04:48.590 You see that the definition we have seen which contains an int 64. 04:48.590 --> 04:52.100 So that's an integer number with the field data okay. 04:52.130 --> 04:53.990 So that's something we've already seen. 04:54.740 --> 04:56.060 So you can browse around. 04:56.060 --> 04:58.910 You can find different messages and services here. 04:58.910 --> 05:04.370 And let's go to the Common Interfaces repository on GitHub okay. 05:04.400 --> 05:09.620 This one contains actually it's not just one package, it's a collection of packages. 05:09.650 --> 05:11.690 Okay, so each folder is a different package. 05:11.690 --> 05:15.050 Some of them are already installed when you install Ros2. 05:15.080 --> 05:19.220 For example, the geometry messages were already installed for us. 05:19.250 --> 05:24.440 And if you want to install one that's not installed yet, it's very easy. 05:24.470 --> 05:27.740 For example, let's say you want these shape messages. 05:27.740 --> 05:32.720 I don't know if it's installed, but if it's not you're going to do a sudo apt install and then ros 05:32.720 --> 05:38.150 dash the distribution dash and then shape dash msgs okay. 05:38.180 --> 05:42.290 If you have an underscore for the package name, you're going to replace it with a dash. 05:42.290 --> 05:43.910 So it's very easy to install. 05:43.910 --> 05:45.500 Now let's check some of them. 05:45.500 --> 05:47.810 So let's go to sensor messages. 05:47.810 --> 05:50.900 This one is quite useful okay. 05:50.930 --> 05:53.300 And this one for example we go to message. 05:54.110 --> 05:57.350 And there are a lot of stuff okay. 05:57.380 --> 06:00.500 And let's go for example to joint state. 06:02.390 --> 06:04.640 So you see lots of comments. 06:04.640 --> 06:08.230 And you see also that it's a bit bigger. 06:08.230 --> 06:10.240 So what do we have in this message? 06:10.240 --> 06:12.910 You see we have an STD message header. 06:12.910 --> 06:16.030 So that's a message from another package okay. 06:16.060 --> 06:20.350 You can include a message inside a message that's completely possible. 06:20.500 --> 06:26.050 So this one if you go to check that definition of that message inside this package is going to contain 06:26.050 --> 06:27.970 a timestamp and some other stuff. 06:27.970 --> 06:32.740 And then in this one you can see for example we have an array of strings for name. 06:32.740 --> 06:40.210 So basically for each joint of a robot and then three arrays with float numbers for the position, velocity 06:40.210 --> 06:40.870 and effort. 06:40.870 --> 06:45.790 So as an example, let's say you have a robotic arm with 5 or 6 joints. 06:45.790 --> 06:48.520 And you want to publish the state for the joints. 06:48.520 --> 06:54.250 So you read data from the encoders, and then you want to create a message to publish the state with 06:54.250 --> 06:56.770 the position, maybe velocity, maybe effort. 06:56.800 --> 06:59.830 Well, the stuff you compute for all of the joints of your robot. 06:59.830 --> 07:01.570 Then do you need to create a new message? 07:01.570 --> 07:03.430 No, you can just use that one. 07:03.460 --> 07:03.640 Okay. 07:03.670 --> 07:04.750 You just use that one. 07:04.750 --> 07:09.210 And then you fill the different fields here and you just publish with this message. 07:09.240 --> 07:09.540 Okay. 07:09.570 --> 07:13.380 So you can check this sensor message package. 07:13.410 --> 07:15.720 It's quite useful whenever you have a sensor. 07:15.750 --> 07:18.510 You see here we can use Joy for joystick. 07:18.540 --> 07:20.100 You have an IMU. 07:20.130 --> 07:21.510 You have image okay. 07:21.540 --> 07:23.310 Laser scan point cloud. 07:23.310 --> 07:27.150 So it contains a lot of useful things for common sensors. 07:27.150 --> 07:31.890 And of course, well after you've installed this package, the messages are going to be built. 07:31.890 --> 07:34.290 And you can just use them in Python or C plus plus. 07:34.290 --> 07:42.120 Now let's go back to the root of this repository and let's go to geometry Messages. 07:43.260 --> 07:46.770 And in geometry messages we also have an MSG folder. 07:47.220 --> 07:50.010 And you see so we have a lot of stuff. 07:50.010 --> 07:52.830 And one that we've used here is twist. 07:53.610 --> 07:55.110 And you can see the twist. 07:55.110 --> 07:56.760 So we have a comment. 07:56.760 --> 08:00.540 And then we have vector three linear and vector three angular. 08:00.540 --> 08:01.860 And what is vector three. 08:01.860 --> 08:07.920 So here we don't see a package name in front because we are using a message from that same package. 08:08.040 --> 08:12.950 So if we go back to the message here and we find vector three. 08:12.980 --> 08:14.000 It's here. 08:14.780 --> 08:21.440 You see that vector three contains three primitive data types, which are each a float number with x, 08:21.440 --> 08:22.250 y, and z. 08:22.280 --> 08:22.580 Okay. 08:22.610 --> 08:25.970 So if we come back to the twist. 08:27.800 --> 08:33.260 So you see we have linear x, y and z and angular x, y and z. 08:33.260 --> 08:37.130 And usually this one as you can see it's used for velocity. 08:37.130 --> 08:42.050 And so the twist message is used with the turtle sim simulation that we've used. 08:42.050 --> 08:47.750 But also if you're going to use for example, robots like Turtlebot or basically any mobile robot, 08:47.780 --> 08:50.390 you will use this twist message. 08:50.570 --> 08:51.530 It's very useful. 08:51.560 --> 08:51.890 Okay. 08:51.920 --> 08:57.530 And you can see here combination of several messages inside one message. 08:57.530 --> 09:03.080 And then to finish let's go back here and let's go to standard. 09:03.080 --> 09:06.200 So STD solves standard services. 09:06.320 --> 09:13.760 You see we have service folder and we have used the set bull previously. 09:13.790 --> 09:15.740 You can find set bull is here. 09:15.770 --> 09:20.870 Of course, we have the three dashes to separate the request from the response. 09:20.900 --> 09:21.200 Okay. 09:21.230 --> 09:26.870 And as you can see, this is exactly what we saw in the terminal when we did Ros2 interface show. 09:26.870 --> 09:29.330 And so we have also primitive data types. 09:29.330 --> 09:31.250 If you want you could also include messages. 09:31.250 --> 09:34.010 So you cannot include a service inside the service. 09:34.040 --> 09:34.190 Okay. 09:34.220 --> 09:37.010 It's very important you just include messages. 09:37.010 --> 09:41.900 So any message you create for a topic you can use it for topics, but you can also include it inside 09:41.900 --> 09:43.670 a service definition. 09:43.700 --> 09:44.060 Okay. 09:44.090 --> 09:47.750 You could have a vector three here as a request. 09:47.780 --> 09:48.050 Okay. 09:48.080 --> 09:51.320 So you can include messages inside services. 09:51.320 --> 09:53.660 But you cannot include services inside services. 09:53.660 --> 09:54.800 And well that's about it. 09:54.800 --> 10:00.680 Now we're going to practice of course and see how to create all that by ourselves with our own packages 10:00.680 --> 10:01.880 inside the workspace. 10:01.880 --> 10:07.430 So what you've seen for now is a bit more about what Ros2 interfaces are, how to create them with message 10:07.430 --> 10:08.570 and service definitions. 10:08.570 --> 10:13.220 And you've also discovered some of the most common existing interfaces.