WEBVTT

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This is the solution for the activity number two.

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In this activity, you have to create some kind of a data pipeline between multiple nodes using topics.

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I will only provide a video solution for Python here, and you can also download the complete solution

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for Cplusplus.

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At the end of this section.

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The Cplusplus solution will be exactly the same just using the Cplusplus syntax and Clcp instead of

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CLP, and I have divided the solution into two steps.

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In this first step in this video, we are going to create the number publisher node, and then in the

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next video, in the second step we will do the number counter node.

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And the reason why I'm doing this is because we are first going to create the publisher, okay.

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We are going to publish a number.

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And then we can validate that this publisher is correctly working using the Ros two command line tool.

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Then when we know that the publisher is correctly working, we can create the subscriber side.

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So let's create a new node which will contain the publisher.

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Let's go to our workspace in.

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So we're going to continue using the My Python package that we've created before we go to the folder

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which has the same name as the package.

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And here we already have our nodes, I'm going to add a new one named number publisher Dot Pi.

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And let's make it executable okay.

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If we want to use the Simulink install.

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So chmod plus x.

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And you see now it's in green right here I will open VSCode.

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So for example here let's go back to the source folder of the workspace.

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And let's do code dot.

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Okay I'm going to clean that a bit.

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We don't need all those files.

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And let's go to our number publisher Dot Pi I will start as we did before.

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I'm just going to take the template and let's put it there.

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Okay.

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So we have the template for the node.

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We want to create a number publisher node okay which is going to be the class name.

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And we need to provide the name here as well.

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And the name of the node is going to be number publisher.

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And let's remove also those comments here.

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Great.

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So now we are going to create a publisher inside the constructor.

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And this publisher is going to publish on a number topic.

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And it's going to publish a number.

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And actually I gave you the type that we're going to use for this topic.

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And this is the.

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So I'm going to type it here.

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So Ross to interface you can do Ross to interface show.

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And this is example interfaces.

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So let's use Autocompletion here and then MSG and then slash int 64.

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Okay so we've already seen a string we have here int 64.

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Which means that 64 bit integer.

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Let's see what we have in this interface because we're going to need to use it.

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And well this is very similar to what we had before.

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So there is a you see we have a few comments here.

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So you can just ignore that.

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And we basically have one field which name is data okay.

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And this field here is an int 64.

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So basically just an integer.

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All right.

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So now that we know this let's include that interface inside our node.

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So right here from example interfaces dot msg import.

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And you can see we find the int.

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So we have int 1632 64 eight okay we have different ones.

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We're just going to use in 64 here.

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And because we use example interfaces we just make sure that in our package dot XML we have the dependency.

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But that's already the case because we've used this package already before for the other nodes in this

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package.

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So let's go back and let's create a publisher.

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I'm going to create the publisher in the constructor.

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So self dot.

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So let's name it number publisher okay.

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Let's try to give explicit names.

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And we're going to use self dot create publisher.

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What's the first thing we need to give.

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You see the message type.

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So that's going to be int 64.

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That's what we've just imported here.

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Then we need to give a name.

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So what name here I gave you.

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The name before is going to be number topic okay.

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The topic doesn't exist yet.

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There is no subscriber or publisher to this topic.

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So here we create the first publisher.

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We decide of the name and then we also have a queue size.

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I'm just going to put ten.

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Then after I create a publisher I'm also going to create a function to publish.

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So that's best practice.

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To do that we create a function that's just going to publish a message on that topic and let's name

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it Publish number with self.

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Because we are in a class and what do we do?

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We create a message.

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So msg is equal to int 64.

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And in this message if you remember we had.

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So in the terminal we saw that we have a data field of type int 64.

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So let's go back and we can do message dot data.

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Usually we find the data here is equal to and well let's give a number.

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Actually I'm going to create a self number here is equal to two for example.

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And let's do message data is equal to self number.

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So we're going to publish the same number every time okay.

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Let's just for this example.

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And then well we have the message.

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What do we do with this message.

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We are going to publish it with the number publisher.

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So self number publisher dot Publish.

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Publish what?

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The message.

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As simple as that.

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So you see, the structure is always the same.

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And then.

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Well, how do we call that?

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So we want to call it, let's say every one second.

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So I don't know if I gave you a frequency to call this function to publish a message, but let's say

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every one second.

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How do we do this.

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We are going to create a timer.

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So self dot let's call it number timer is equal to self create timer.

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We provide a timer period in Saigon.

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So let's say 1.0 and then the reference to the function.

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So that's going to be the callback self dot publish number without any parentheses.

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Once again.

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And that's basically it I'm going to add another log self getlogger just to say so info just to say

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that number publisher has been studied.

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And that's the end of the code.

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So when we create the node here we're going to go to the constructor.

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We're going to create a publisher.

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We're going to create a timer.

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And then when we make the node spin well with the timer we will have one callback every one second.

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So this function is going to be called every one second.

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And we're just going to publish every one second.

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So let's make sure we save the file.

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And then we are going to create an executable.

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So let's go to setup.py.

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Let's add a comma here new line.

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And then well let's name it number.

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Publisher is equal to.

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We have the mypy pkg.

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And then dot what's the file name.

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The file name is number publisher number publisher okay.

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We don't put the extension.

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And then what function we want to call.

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Recall what is always the same main.

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So I put main here.

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I'm going to save this.

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Okay.

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Very important.

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You save all the files.

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Let's go back to the terminal and let's go back here to this folder Ros2 workspace I'm going to do call

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code build packages.

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Select with the Python package and let's use also symlink install.

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Let's press enter.

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Let's build this package.

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So it's going to install the Python executables.

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And actually with symlink installed we can directly run the code that we have written so that if we

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modify it we can just run it again.

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And we don't need to do a build again.

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So let's try that.

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I'm going to try that in this terminal.

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And first because we have actually built something.

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So even if we use all of those options we have built.

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So I need to do a source bash I see, and I'm just going to source all the terminal so you can open

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new ones or just source the existing one.

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Okay.

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And here let's do so.

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Let's clear and let's do Ros to run my Pi PG.

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And that's going to be number publisher.

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You see you should find it with the Autocompletion.

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Let's press enter.

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And we see number publisher has been started here.

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Let's do a Ros to node list.

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We can see the number publisher.

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Let's do a Ros two topic list.

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We can see the number topic okay.

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Plus the two other topics we always have.

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And we can do Ros two topic equal with number.

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And what do we have.

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You see we have data two every one second.

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So it is correctly working.

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And we have finished the first step of this activity with this number publisher.