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Hey everybody what's going on this is Caleb with slopestyle.

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And in this video we're going to continue talking about subjects more specifically the replay subject.

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Now replay subjects are really really cool.

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They work very similarly to a behavior subject but you can give them an explicit buffer size.

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So let's say that I gave it a buffer size of three.

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That means that any future subscribers are going to receive the last three elements that were pushed

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on.

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So it replays a certain amount of elements to all future subscribers.

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It's super duper cool.

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So let's get into it now.

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I want to show you how it works.

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So what we're going to do is we're actually going to create a new playground page by selecting new playground

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page like so go ahead and select all and delete all that stuff and you know what we can rename this

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here replay subject.

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Now of course we're going to need to copy in our imports here as well as our needs indefinite execution

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which is kind of a scary sounding line.

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But anyway that's what it needs.

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So as we've been doing we clean it we build it.

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And most of the time we still have trouble with it importing properly.

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So we may have to quit out unless it's going to let us use it.

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It does not look like it will so I'm going to go ahead and close this stop the task and we're going

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to open it one more time.

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Build it again.

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And we should be able to access all of our arcs swift stuff.

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There it is.

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OK so we are going to create a replay subject that is going to essentially replay the previous three

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bank transactions.

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OK.

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So we're going to call this var let's say a account subject and we're going to make it of type replay

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subject.

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OK.

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Now we're going to make it of type Let's make it of types not string.

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How about double since we're dealing with bank balances and in order to actually create a replaced subject

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you need to call DOT create.

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And you'll notice that there is a buffer size property.

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That's what we want a replay subject has a buffer size and that basically sets the amount of elements

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that will be replayed to future subscribers.

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Or I guess the maximum amount of elements that can be replayed so let's just do three Caleigh was like

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the example in the beginning.

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And so let's go ahead and move on.

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We have a replay subject that can replay up to three elements for future subscribers.

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So let's go ahead and we're going to create what's called an account manager and I'm just calling it

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the account manager because it is subscribing to the account subject and it's doing stuff with the items

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that are pushed into our subject.

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So go ahead and call account manager or var account manager and we're going to go ahead and subscribe

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to our account subject.

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So go ahead and call account subject.

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Subscribe and on next as you may have suspected we're going to go ahead and print out the transaction.

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OK.

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So go ahead and print and we're going to say transaction amount and we'll use stringing cancellation

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here to print out every element as we go through our sequence.

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OK the dollar signs zero of course is the temporary variable that's going to house that value.

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And this way we can print out every transaction amount that we pass it.

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So let's go ahead and let's go see how this works.

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If I call account subject on next and I pass it in a double let's say that we had a transaction for

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$12 and 49 cents.

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Watch what happens.

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It prints out saying transaction amount.

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Oh you know what we should actually put a dollar sign there so it looks like an actual transaction.

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Let's see if that fixed it should have it's just a dollar sign in our print statement.

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OK.

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Transaction amount 12:49 really cool account subject Dodd on next.

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Let's pass in another value maybe we maybe we bought a new Xbox let's say two ninety nine for our next

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value transaction amount to ninety nine.

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Account subject.

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On next let's give it another value.

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How about $15 K and it prints out $15.

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Now watch what happens when I print out another one account subject on next and let's give it $80 watch

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what happens we get a fourth transaction amount.

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Now you were probably thinking Wait we're supposed to have a buffer size of three.

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It's not supposed to be able to allow me to do more than three.

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Well I'll do a fifth one here and we'll see what's going on.

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Let's do.

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Seventy four dollars and twelve cents.

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We get a fifth print.

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Now you might be thinking what is going on.

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That's because a replay subject actually works just like a behavior subject.

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But the first subscription receives everything.

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So every on next call that we pass in the first subscription is going to get all of those.

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But watch what happens when I create a new subscription.

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I'm going to type var last three transactions and we're going to call account subject and we're going

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to subscribe on next just like so now what I'm going to do is I'm going to print.

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Well you know what.

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Let's say let's do the same thing as above transaction amount.

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OK.

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And we're going to pass in transaction amount zero dollars.

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Now the thing that you might be noticing.

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Oh you know what I need to make a space between these because it's hard to see so I'm going to print

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out a line break there.

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OK cool.

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So that's easy to see the two.

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So I'm also going to add maybe account manager and then I'll add last three just to show you the difference.

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So the account manager basically is capturing everything just like a behavior subject.

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And the reason I'm saying behavior subject and not publish subject is because I can add elements to

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my account subject before I actually subscribe.

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And they'll still show up.

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Watch this if I passed in ten dollars and 15 cents.

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You'll see it will show up in my initial transaction and I did that before I subscribed.

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Just like a behavior subject can show the most recent element.

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I can also pass in one before I subscribe.

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Now the account manager is getting every single thing that I've done.

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But look what the last three transactions is capturing $15 80 dollars and 74 12.

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Those are the last three elements that have subscribed or that have been pushed into my subscription.

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Let's add another one.

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How about $100 even.

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You'll notice my last three transactions will update so I get the last three.

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That's super cool.

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So this would be really helpful for kind of like showing a recent history in an app or if you think

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about Twitter on the left you'll have like a suggested followers to follow and it shows you three followers.

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What I could do is I could have that set up to something where it downloads three followers that are

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ones I might like and it'll show.

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Or maybe a list of followers and it'll show the most recent three.

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And then if I remove one it would show me the next recent three and then so on and so forth that's how

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replay subjects can be really helpful.

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It's pretty cool as you can see we are basically able to pass in values to our first subscription which

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acts like a behavior subject.

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We're able to pass in or we receive the last three elements pushed in to our replay subject and that's

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all dependent on the buffer size.

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Watch what happens if I update it to five when it prints out you'll notice we now have five recent transactions.

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OK that's pretty cool.

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So this is how replay subjects work.

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It's not a very complex idea but it's really cool and really robust there's lots of cool ways that this

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could be used in a real application.

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So I hope that you've enjoyed learning about replay subjects.

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And we're going to move on to the next video where are you going to learn about variables not the normal

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swift variables but variables in our ex-wifes they're a little bit different.

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Great job with this video and I'll see in the next one.