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Now we're going to talk a little
bit about using the extend method.

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We touched briefly
before on class methods.

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Class methods are used when a method
needs to have functionality, but

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it may not be tied to
a specific instance of a class.

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It's much easier to understand if we
actually look at some code to see how

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it works.

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Let's go ahead and
see how that works now using WorkSpaces.

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Okay so I have a WorkSpace right
here with another very simple class.

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And this is just a customer class.

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It has one attr_accessor for the name.

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We initialize the class with the name and
set that instance variable.

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And then the to_s method
just shows the name in brackets.

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So now, what we're going to do
is create a module that will

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track different customers.

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And what I mean by that is, instead of
keeping track of every single instance of

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a customer, we want to be able to
call Customer.create with a name.

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And then that will be trackable
on the customer class.

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So here's how we're going to do it.

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Let's go ahead and
create a module called Tracking.

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Now, the methods that we're going
to be writing in here are all

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going to be appearing like a mix in.

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However, since we're going to
be using the extend key word,

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they will appear as class methods.

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So first, let's go ahead and

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create something which is
a method called instances.

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And what this is gonna do
is return an empty array

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the first time that we access it.

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Now this array,
once we extend the class with this module,

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will be available on the class.

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Now that sounds a little bit strange, so
let's just go ahead and see how it works.

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If we extend this with the Tracking
module, this isn't going to work yet.

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Let's just see Customer.instances, and

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this should return an array,
we'll just call inspect.

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And actually, let's go ahead and

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say Customer.instances and
use a format string.

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And then put the output from that.

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So now if we run extend.rb,
we can see we've got this empty array.

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Now this array exists only
in the Customer class.

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And we get to it by accessing this method.

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So what this is going to do
is conditionally assign this

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instances class variable at
this point to an empty array.

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That will also return the array
that we just created or

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it will just return the array
if it's already been created.

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So now let's go ahead and
implement this create method on Customer.

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And we're going to pass it one
argument which is the name.

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Now we know any class that we
mix in this tracking module into

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is going to have a name.

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So what we're gonna do is say object,
is a new name.

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Now what this is going to do
is call the initialize method

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of the class that this was mixed into.

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And return that and
assign it to this object method.

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Then, we'll append that.

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To the instances array and
return that variable.

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So the reason we call this object
is because we don't know exactly

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what kind of object is going to
come in to this tracking module.

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All we know is that we're passing
the initialized method, the argument.

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We append it to the array,
and then we return it.

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So now, let's go ahead and
see how that works.

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So, we'll just say Customer.create,

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and then we will create
a customer called Jason.

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And we'll do the same
thing again with Kenneth.

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And then let's go ahead and
look at the instances.

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Gonna clear my screen, and
run this one more time.

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And you'll notice we've got no
customer instances here, and

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then we properly create Jason and Kenneth.

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And now this instances array
contains two different objects.

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So this isn't super useful, but let's say
we wanted to find a certain customer.

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What we could do is create this
method called find with the name.

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And since it's an array,
we can just call instances.find.

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And return the first name that matches
whatever it is we pass in to this

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find method.

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So let's see how that works.

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We'll go ahead and try and find Jason.

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Gonna clear my screen and run this again.

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And you'll see that we get back Jason.

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And lets go ahead and try and
find a name that doesn't exist.

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And see what gets returned there,
and nothing gets returned.

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So by calling extend and
passing in the module name,

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instead of being mixed in
to instances of the class,

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extend mixes in to the class
methods of the class.

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It can be a little bit confusing.

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Go ahead and
try it on your own now using WorkSpaces.