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Hey everybody this is Caleb with devs Lopes dotcom and welcome to this video on flat map flat map is

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a function in arke swift that we can use to basically pass in and observable we call flat map on the

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observable.

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And then what we can do is we can cast each element of that observable into its own observable.

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And then with that we take all of those observable and merge them into a brand new observable of whatever

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element we decide to pull out of our original observable and we're going to show you how this works.

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It's a little bit different than map but we're going to show you how it works now so go ahead and pull

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open your X playground go ahead and.

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Right click on map click new playground page and then go ahead and select all delete and paste in your

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four lines that you need if you need to go back to map to get that that's fine.

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We need our swift playground Support Foundation and needs indefinite execution.

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So this should be called flat map like so.

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And what we're going to do is we're first going to create a Dispose bag by typing let dispose bag equals

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dispose bag.

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And as you can tell it's not showing up yet which shows me.

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Look at that.

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Now it's good.

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Looks like it works just fine.

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And now we have our dispose bag which we'll use to deallocate things when we need to.

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And what we're going to do to show how this works is I'm going to actually create a structure called

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game like a video game and we're going to give it a property of price.

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But instead of using a normal swift type we're going to go ahead and use the arc swift variable type.

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So let's go ahead and do that.

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Now let's call struct game and we're going to give it one property price.

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Now like I said this is going to be of type variable and we're going to go ahead and set it to be a

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variable of type double just like that.

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And now we can create an instance of games and set their price.

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Pretty cool.

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So let's go ahead and do that.

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We're going to type let overwatch and we're going to set that to be equal to game.

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But watch what happens since we're using a struct.

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We can just call price and give it a price.

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OK.

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Now it asks for a variable of type variable.

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So we're going to give it that variable and we're going to pass it a value OK just like so.

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Of fifty nine ninety nine.

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OK that's the price for overwatch then.

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We're going to create another game instance here.

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And this is a fun game for Playstation 4.

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And I think X-Box Maybe but I know for for sure.

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Star wall if you haven't played it you should go check it out.

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It's of type game and it has a price as well of type variable.

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OK and we're going to set it's variable property to be 1999.

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I'm just guessing on the price for that one.

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But now we have two instances of game and we've set a variable property with the price for that item.

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Now what we're going to do is we're going to create a publish subject so that we can add these games

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into the published subject and eventually we're going to print out the game's price but I'm going to

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show you how we can take one observable like a published subject and we can call flat map on it so that

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we can pass in a another observable and we can map out the values of that.

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So go ahead and type let.

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Game subject equals published subject of type game.

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OK.

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Just like that.

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And now we have a published and now we have a published subject of type game.

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So next what we need to do is we need to call our games subject and what we want to do is we want to

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extract the variable value of fifty nine ninety nine and 1999 and we want to print that to the con..

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So to do that we can type games subject subscribe and I can go ahead and type on next whoops and then

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what I can do is I can print a dollar sign zero.

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But of course nothing is going to happen because we haven't pushed in any games yet.

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So let's go do that really quick.

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Go ahead and type games subject on next and pass in overwatch and watch what happens when we pass it

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in.

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It will print out whatever subject we just passed in.

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Right.

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OK let's do that again with game subject on next.

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And we'll pass in Star wall ok really cool

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so there we go we get a print it says this is of type game with a price with a type of variable and

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a type of double k.

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But that's not what we wanted we wanted to extract the price of the game and that comes from the variable

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k.

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So what we can do instead of just subscribing and trying to print out the game itself.

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You could type game up price and you could print it out right.

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There you go.

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But the issue the variable here is of type variable.

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So the price is of type variable What can we do to get the actual value out we could type datt value.

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And when we do that we get the actual value of the game and that's cool and all but what if we wanted

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to convert this game subject of type publish subject into an observable type price.

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And what if we wanted to monitor that observable and change the price later on we could use an observable

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to do that.

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So what we're going to do instead of just subscribing right now is actually going to get rid of all

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this what we're going to do is we're going to call it a flat map.

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OK.

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Like so and we're going to put some brackets there and we're going to use flat map to basically cast

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the price into its own type of observable.

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Okay.

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And what we can do is using flat map we're going to cycle through all of the elements in our published

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subject.

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So for instance overwatch and star wall and we can use the dollar sign zero to access the game.

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Right.

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That is game that is of type game.

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Now we can pull out the price of type variable but we want to use this variable as an observable case

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so we can monitor its its value and change it and react to it accordingly so we can type Dopp as observable.

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Now the price is not only a variable it is an observable.

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So that means that we can dive into it and we can subscribe it.

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That means we can dive into it.

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We can subscribe to it and not only that but we can monitor the values and we can print them out.

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So now that the price is observable.

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Watch what I can do.

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I can call datt subscribe K I can subscribe to this observable of type price and on next K when the

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value comes in I can print dollar sign zero and that will print out the price because its now observable.

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Do you see that.

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So what I did was I passed in all of these game instances into the game subject and then I used a flat

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map to basically take the to basically take the game and I put it here.

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I pulled out its price and I set that variable to be observable then since it's observable I subscribe

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to it and when I print out the observable it print it prints out the actual value that the variable

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is which is of type double.

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Thats the coolest thing.

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So I need to do some work here though because a flat map should be down here.

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This should be tabbed and subscribe should be beneath and theres one more thing we got to do.

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We have to add this to our dispose bag.

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So go ahead and just add it to the Dispose bag there.

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Press Enter to return it down and you know let's make this pretty let's say Game price and let's encapsulate

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that there at a dollar sign.

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And there we go.

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So when it cycles through we turned the price variables into a sequence of observables and watch what

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happens.

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If I wanted to change the price of overwatch if I were to type overwatch price value and set it to be

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how about 14 ninety nine let's say it's on sale watch what happens.

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Star Wars price are over watches price drops down.

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So that's a pretty pretty good deal.

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So as you can see I can update the observable just like that.

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That's pretty neat.

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So this is flat map it's very similar to map but instead of simply passing all the values through and

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performing an operation on those values returning them as an array what we're doing instead is we are

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taking an observable like a published subject.

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We're passing it in and cycling through each individual element or in our case these two games then

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we're converting them into observables.

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And what we're doing with those is we're then passing all those observables into one single observable

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that can be cycled through and printed out.

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So it's a little bit different.

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You'll probably use map more than you will use flat map.

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But I wanted to show you how to use flat map properly.

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So this is flat map if you're confused.

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Make sure you go read the apple documentation there's a lot more information on flat map and some of

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the other use cases you could use.

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But in Arek Swift it's going to be useful for converting one type of observable into another type of

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observable by cycling through the elements and casting them as observable.

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So great job with this video.

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Let's head over to the next one where we're going to talk about filter.

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One of my favorite arcs with functions.