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Let's get to work inside of our ticket service.

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We're going to create a listener that is going to watch for that order created events and then any time

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we see that event as I just mentioned we're going to lock that ticket down the back inside my editor

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I'll find my tickets directory inside there I'll find the events folder and inside of events I'll make

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a new directory of listeners and then inside that I'm going to make a listener file called order created

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listener thought to yes.

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Now I don't know about you but it's kind of hard to remember how to create a listener.

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You might recall that we've gone through the process of creating a listener twice now and you might

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recall that it's something around extending that base listener class that we had defined inside of our

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common module.

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But the actual details of it kind of easy to forget.

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So here's how we would approach this using typescript to kind of help us through development at the

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very minimum.

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We probably want to remember that because we are trying to create a listener.

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We're going to want to extend or subclass the listener base class we had put together that a very minimum.

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Hopefully we can remember that we need to import the listener base class from our common module don't

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import that and then I will export a new class called order read listener and that's going to extend

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our listener base class.

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Now if we can just remember this part right here typescript is going to help us the rest of the way.

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So already we can see that typescript is saying hey this thing right here is a generic type and you

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have to provide a type argument.

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So we need to stick in as the generic type the event that we want to be listening for.

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That's kind of our hint.

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That's how we're gonna get hints along the way on how to actually implement this thing so from our common

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module let's import the event that we want to listen for.

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In this case it is as you guests by the name of our class right here the order created that

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don't stick that in as the generic type and then after that typescript still has our back it's going

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to tell us K Well if you want to do this here's what you have to do.

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You have to implement that on message function a cougar name and a subject as well.

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Well let's start off with the subject.

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If you want to add a subject we probably need that subjects enum that we had previously declared inside

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of our common module as well so from the com module I also get these subjects in the and then I'll define

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the subject we want to listen for.

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In this case I'm going to annotate the type of subject as subject not order created and then set its

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value as subject start order created.

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Next up we have to set the user group name.

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Remember what we did back inside of our order service.

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We created that separate file and we defined the Q Group name that was supposed to be used among all

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of our different listeners.

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Now it's just going to help us from or prevent us from making any silly typos.

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So we'll take that same pattern once again inside of our listeners directory.

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We'll make a new file of Q Group Name that T.S. and inside of here I will export a concept of Q Group

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Name and because we are inside the ticket service organizing a Q Group Name of tickets service.

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Simple enough going to save that then back inside of our listener at the very top.

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I will import Q Group Name from same directory Q group name.

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Now we can set that property on our class though.

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Q Group Name is you group name and then finally our last step we have to implement the on message function

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so we will define on message and remember this is where a touch script is not going to quite help us

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out until we start to list out some arguments right here.

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This thing is going to have to argue this provided the first is gonna be the data property of the event

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though we're listening for and second it will be the message type from the no Nat streaming library

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no put in here data and that's gonna be of type order created event specifically the data property of

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that interface.

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So order created event at data and then the second argument will be our message which is subtype message

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again from the no Nat streaming library we definitely need to import that at the top as well.

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No not streaming chances are on message function is going to have some async code inside of it.

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So let's mark this boat this function as a sink as well.

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So there we go again easy to forget what's going on inside of here.

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But as long as we can just remember that we have to extend that base class that script is going to pretty

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much help us along the way.

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All right.

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So we've got the listener put together.

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Let's take a pause right here in the next video.

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We're going to figure out some strategy some way of figuring out how to mark a ticket as being locked

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down and prevent any changes from being made to it.