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Being Included

5:42 with Jason Seifer

One thing that’s really neat about the include method when using a module is that it gives us a way to hook in to that behavior and do something when a module is included in a class. We can use this to further extend the functionality in our modules. For example, when a module is included in a class, we could use that to run methods, write methods, or include other modules.

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First, we modify our Fetcher module to have an included class method. This method defines an attribute on the class it is included in called fetch_count.

module Fetcher
  def self.included(klass)
    attr_accessor :fetch_count
  end

The fetch method is then modified to say how many times the fetch count has occurred. The ||= operator is called the conditional assignment operator. It conditionally sets the value of @fetch_count to 0 if we haven't accessed it yet.

  def fetch(item)
    @fetch_count ||= 0
    @fetch_count += 1
    puts "[#{@name}, Fetch count: #{@fetch_count}] I'll bring that #{item} right back!"
  end
end


class Dog
  include Fetcher
  def initialize(name)
    @name = name
  end
end

dog = Dog.new("Fido")
dog.fetch("ball")
dog.fetch("toy")

Now, when we create a new Dog instance and run the fetch method, we get the following output:

[Spot, Fetch count: 1] I'll bring that ball right back!       
[Spot, Fetch count: 2] I'll bring that toy right back!

This fetch_count variable is specific to each Dog instance:

dog2 = Dog.new("Spot")
dog2.fetch("ball")
dog2.fetch("toy")

So running that produces the following output:

[Fido, Fetch count: 1] I'll bring that ball right back!       
[Fido, Fetch count: 2] I'll bring that toy right back!
[Spot, Fetch count: 1] I'll bring that ball right back!       
[Spot, Fetch count: 2] I'll bring that toy right back!

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One thing that's really neat about the include method when using a module 0:00

is that it gives us a way to hook into that behavior and 0:04

do something when a module is included. 0:08

We can use this to further extend the functionality in our modules, for 0:11

example, when a module is included in a class we can use that to run methods, 0:16

write methods, or even include other modules. 0:22

Let's see how included works now using workspaces. 0:26

Okay, so here I am in a workspace and I've created a new file called included.rb. 0:32

And the content of included.rb, 0:38

are just from the fetcher.rb that we just had, and 0:41

all it includes is the fetcher module, the dog class, 0:46

and an initialization of a new dog instance right here called Fido. 0:51

And he is going to fetch a ball and a toy. 0:57

Let's just go ahead and run it and see what happens. 1:01

Okay, so let's go ahead and write the included method. 1:05

Now we write the included method by calling included and the word ends. 1:11

So it's just another method and once we give it the specific name called included. 1:20

It automatically runs for us. 1:26

However, this is going to be a little bit different then every method we've seen so 1:28

far because It is a class method. 1:31

So just like in the math module where every item that we ran 1:36

was a class method, with square root, cosine, and all that. 1:42

Same thing happens with the included method. 1:47

This runs on the class. 1:52

And it runs automatically when a module is included in a class. 1:54

So let me save the file and lets see what happens when we run it. 1:59

It's gonna give us an error and we'll see how to fix it. 2:01

So it says wrong number of arguments one through zero there's an argument error. 2:05

That means this method requires an argument. 2:10

Now the argument that goes into the included method 2:16

is the class in which the module is included into. 2:19

So in this case, when we include the Fetcher module in the dog class, 2:25

the dog class gets sent in as the argument to the included method. 2:30

Which means we have to account for that. 2:35

Now normally, we could name it something like class. 2:38

But that's a reserved word in Ruby. 2:41

So, conventionally, we can call it klass or base or something like that. 2:44

But let's just stick with klass right now, even though it's misspelled. 2:48

And for right now we'll just print something to the screen saying, 2:55

klass has been included. 2:58

So I'm gonna save that and clear the screen on the bottom and run this again. 3:00

And we'll see dog has been included. 3:07

Okay, so this comes down here, and 3:11

as soon as this method is included, this code gets run. 3:14

Now because this method is run in the context of a class method, 3:19

we can call other class methods, inside of this method. 3:24

So that sound a little strange but you know how we write 3:29

arrt_accessor when we want to have some sort of attribute, like right here, 3:33

attr_accessor :fetch_count. 3:36

That's actually a class method. 3:41

And since it is, we can move it out of there, and into the fetcher module. 3:44

Now, we have access to this variable called fetch_count. 3:53

So let's go ahead and print it out when we fetch an item. 3:59

And what we'll do first is say fetch_count, 4:08

we're gonna set that conditionally equal to 0. 4:12

And then increment it by one. 4:17

So what's gonna happen is as soon as this fetch method is called after 4:19

being mixed into whatever class, if fetch_count has not been accessed yet 4:24

we set it to zero and then increment it by one and then run this. 4:30

And say okay we're gonna display the count, 4:35

and we'll bring the item right back. 4:37

So now I'm gonna clear my screen, and we are going to run this again. 4:39

And we can see we have one and two which is the fetch count. 4:42

That's not very descriptive, so let's go ahead and write the name right here. 4:47

Name, Fetch count. 4:52

Okay, so now Fido will display his name each time we call this fetch method. 4:56

So all right, we have Fido, he's got a fetch count of one. 5:04

And then the second time it's run it got a fetch count of two. 5:07

And this is going to be specific to each instance, so we could create another one, 5:13

say dog2.fetch ball, dog2.fetch toy. 5:17

And if we were to save this the attr_accessor is created individually for 5:24

each dog object. 5:29

So now we see Fido has a fetch count of two and Spot has a fetch count of two. 5:32

And again, if we had another class, 5:36

we could just mix in this fetcher module to that as well. 5:38

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