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Well we've got a 50 percent solution to our problem.

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So in this video I want to explain what the remaining problem is like the remaining 50 percent and then

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describe how to fix it.

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So as usual a quick diagram.

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So here's what's now going inside of use location.

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The very first time we call it are using fact took its ran inside there we call start watching and of

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course inside of start watching at some point time we call watch position async right here.

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So that essentially sets up some listener in our application something and says Hey Expo tell me whenever

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are users location changes we can then imagine that at some point time use location gets called a second

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time maybe this time there's no change in the callback because maybe it was not the recording piece

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of state that changed.

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So maybe use effect won't be called that particular time maybe then it gets re rendered or called a

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third time and maybe during this time the callback function does change.

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So use effectively then run a second time and inside there of course we will call start watching.

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Now here's the issue we have now called start watching twice.

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And between those two calls to it we have invoked watch position async twice.

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So I don't necessarily know what Expo location does behind the scenes but without a doubt we have essentially

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told it that we want to watch for changes in the user's location two separate times.

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So does that mean that we're going to essentially get two separate updates.

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I don't know but it could possibly mean that's the case it might mean that every single time that we

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call watch position async we're always going to say give me another listener.

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Another listener another listener.

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And if we end up Collin's function like ten times we might eventually get like 10 different callbacks

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registered in everyone those 10 might be invoked every single time a user's location gets updated.

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Now again that might not be the case with Expo location but essentially with any other library that

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would definitely be a risk maybe this function doesn't have that kind of vulnerability or I should say

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vulnerability but that issue but it might be that we have some other very similar function that we're

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making use of that has some similar problem.

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So regardless of how watch position async actually works behind the scenes we should definitely make

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sure that when we do a follow up call we somehow stop the previous time we were watching for changes

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to the user's location change.

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So how are we going to do that.

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Well here's the idea the very first time that we call use location we are of course running use effect

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whenever we call a U.S. fact and we pass in a function our inner functionally passing has the option

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to return something called a clean up function.

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This is a function that is meant to solve the exact scenario we are in.

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Essentially if we have a use effect took as we do right here that starts up a listener or starts watching

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something or starts some process then we can return a function from use effect that we can use to stop

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that watching automatically series what goes on behind the scenes.

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The very first time we call a location we'll call these effect.

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We start watching then we're going to return a cleanup function inside that cleanup function we're going

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to put in some amount of code to stop listening to the user's location.

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We can then imagine that use location gets called a second time maybe there's no change to a callback

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so use effect does not get called and then the third time where we do call use effect when we call use

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effect it's going to automatically see that the last time we ran this use affect function like way back

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over here.

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It's going to see that we returned a cleanup function from the first time.

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So before doing anything inside of our use affect function it's going to first call the cleanup function

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that we returned from the very first call back over here so we can use that as our opportunity to do

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a little bit of cleanup and make sure that we stop listening for changes to the user's location before

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we start listening a second time.

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So this essentially gives us the ability to easily clean up after ourselves and it's perfectly suited

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for what we're trying to do Gates.

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Let's make a little change.

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We're going to return a cleanup function from our use effect function.

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We're going to make sure that inside that cleanup function we stop listening to the user's location

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before we start to listen to it.

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Once again all right.

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So inside of use location I'm going to add in a return statement and return a function like so.

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So inside of here we're going to do some amount of cleanup.

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For us our cleanup is going to be to call subscriber dot remove.

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Remember that stops us ladies listening to updates from the user's location.

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So inside of here we're going to first check to make sure that we have a subscriber and then we'll do

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a subscriber dot remove like so.

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Now the reason that we're checking to make sure that we have a subscriber.

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Remember when we first initialize that piece of state we started off with the default value of null.

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So there might be some kind of crazy scenario where we do not actually start listening to users location

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and then actually run that function right there.

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And that's actually very much possible if we ever call use location with a should track default value

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of false.

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That means we're going to fall into this case right here and we're essentially not going to have any

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subscriber around.

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So we might have a subscriber value of null in which case we would not want to call remove okay.

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Let's try saving this I'll go to track rate again start recording.

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Go to my terminal and once again just verify that I'm not like completely spanning this thing with updates.

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And once again I should be able to tap on stop and see a very consistent number of locations have now

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been recorded.

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So this is definitely a good improvement.

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We are now cleaning up after ourselves and making sure that we're not just constantly calling start

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watching start watching start watching again and again and again.

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All right.

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So at this point we've got some code that definitely works but there's one last little thing inside

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of your maybe one or two things.

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I want to make sure it's really clear how we can kind of fix this issue and not run into it in the future.

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And I also want to touch on the topic of the fact that we've got this subscriber piece of state inside

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of here.

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Maybe there would be a better way to handle it subscriber than storing it inside of a piece of state.

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So let's take a quick pause and wrap up on use location in the next video.