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In this video we'll start creating a function to help us set up the game. We'll also learn about how we can use extension functions to write less code!
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[MUSIC]
0:00
We finally finished creating all of the
pieces that make up a game of solitaire.
0:03
And now it's time that we start putting
these pieces together to create a fully
0:09
functional game.
0:12
The first step in any game of
Solitaire is to set up the play area.
0:14
We need to make sure that we're starting
the game with an empty waste pile,
0:18
as well as empty foundation piles.
0:21
Then we need to shuffle
the deck [SOUND] and
0:24
deal out the cards into
seven new tableau piles.
0:26
Inside our GameModel class, let's
create a new function named resetGame.
0:28
This function will be responsible for
resetting our game to the starting state.
0:34
We'll call it once when
the app is launched and
0:38
we'll call it another time each
time we want to start a new game.
0:41
Inside this function,
0:44
the first thing we're going to do is
reset the waste pile to be empty.
0:45
So let's type wastePile.clear and
0:49
then move on to clearing
out our foundation piles.
0:52
To clear out our foundation piles,
0:55
all we need to do is call our
reset function on each pile.
0:57
Sounds like we need to use another for
loop.
1:01
Let's add a line below where we clear
our waste pile and then type for (pile
1:03
in foundationPiles) and then brackets.
1:08
To loop over all of our foundation
piles then inside the for loop,
1:15
let's call the reset function on
each of the piles, pile.reset.
1:20
And now we've reset all of our
foundation piles as well, but
1:26
there's actually a better
way we can do this.
1:29
So let's delete that and
1:32
instead let's type
foundationPiles.forEach.
1:35
And then inside the brackets,
let's type it,
1:40
which represents a foundation pile .reset.
1:44
And there we go.
1:50
It's functionally the same code but
now we're doing it all on one line.
1:51
Before we get any further,
let's take a minute to talk about this for
1:56
each function and
how it works behind the scenes.
1:59
First off, we need to realize that for
each is a function that takes an action.
2:02
Also known as a lambda expression,
which remember,
2:07
is just an anonymous function
with different syntax.
2:11
To make that a little more clear,
2:14
I'm going to add parentheses
around the brackets.
2:16
And since there's no error,
2:20
it's clear that this is really
just the first parameter.
2:21
In Kotlin,
when a function only has one parameter and
2:25
that parameter is an action, we're
allowed to leave off the parentheses.
2:29
So I'll undo adding them.
2:33
And now I'd like to talk
about extension functions.
2:37
You probably haven't noticed but
2:40
we've used a few extension functions
already like first and last.
2:42
An extension function is a way for
2:46
us to add a function to a class
without modifying that class.
2:48
Check it out.
2:52
Let's say we wanted to have
a function on the string class
2:53
to tell us if the underlying string
was longer than five characters.
2:56
And we wanted to name this function,
isLongerThan5.
3:01
This would be pretty impossible with Java.
3:04
But with Kotlin, it's super easy.
3:06
And just for fun,
let's do this as a package level function.
3:09
So let's add some space below
our GameModel class and
3:13
then type fun_String dot,
the function name,
3:16
isLongerThan5.
3:21
Then let's add our parentheses.
3:26
Make it return a Boolean and
then add our brackets.
3:28
If you want to add a function
to an already existing class,
3:32
all you need to do is add the name of
the function after the name of the class.
3:36
It's super easy and super useful.
3:42
Back inside the function,
let's make it return true
3:45
if the string is longer than five
characters, and false otherwise.
3:48
So let's type return.
3:52
And then to access the underlying string,
we just use the this keyword.
3:55
So we'd like to return this.length > 5.
4:00
Now that we've created our first extension
function, let's see how we can use it.
4:05
Let's jump over to our app file and
4:10
then let's see if Treehouse is
longer than five characters.
4:13
First, let's create a variable for
our Treehouse string.
4:17
val treehouse = Treehouse.
4:20
And then let's print out whether or not
our string is longer than five characters.
4:27
sout Tab or just the println function,
and then inside this,
4:32
let's print treehouse.isLongerThan5.
4:38
Then let's quickly run this by first
changing our run configuration to
4:42
AppKT and then hitting the Run button.
4:47
And there we go,
treehouse is longer than five characters.
4:50
Now that we've got a better
understanding of extension functions,
4:55
let's go ahead undo all of that.
4:58
But before we continue on with our
resetGame function, let's use Cmd or
5:07
Ctrl+B on the forEach method
to go to its declaration.
5:12
And here we can see that forEach is an
extension function on the array class, and
5:18
it takes on a function
mapping from type T,
5:23
which in our case is a foundation pile,
to Unit.
5:26
And inside the function,
it just loops over all the elements and
5:31
this array and
performs the action on each element.
5:35
Pretty sweet, right?
5:40
In the next video,
5:42
we'll pick up where we left off with
our GameModel's resetGame function.
5:43
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