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As I mentioned at the end
of the previous video,

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something is amiss in our
comic books controller class.

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No worries, though.

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In this video, we'll dissect the issue and
propose a solution.

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Here's our ComicsBooksController class.

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Our controllers Detail action
method is currently responsible for

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creating the comic book model
instance that we pass to our view.

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Ideally, our controller wouldn't be
concerned with that aspect of our app.

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Why?

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Let's take a look at a scenario.

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Imagine that we had another method in
another class that needed a comic book

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model instance for
Amazing Spider Man, number 700.

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With our current implementation, we'd need
to repeat this code in the second method.

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Doing this would violate
a design principle called DRY or

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Don't Repeat Yourself.

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So, how do we avoid repeating ourselves?

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We move our code that creates a comic book
model instance into its own method named

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GetComicBook.

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Now, any method in our website that needs
an instance of that comic book model,

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can call the GetComicBook method.

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Eventually, we're going to need a method

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that will return an array of comic
books for our comic books list page.

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We can put that method in a class
along with the GetComicBook method.

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We can think of this class as
a central location, or repository, for

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storing and
managing our comic book model instances.

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Let's name this class
ComicBook Repository.

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By organizing our data
access code in this way,

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we're implementing
the Repository Design Pattern.

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Creating a design where we don't
repeat ourselves allows for

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our classes to naturally
be more tightly focused.

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Each class should have a single
responsibility, or concern, and

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not overlap with any other
classes on our website.

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This is known as the separation
of concerns design principle.

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Separating concerns also helps make
our website easier to maintain.

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With our improved design, if we wanted how
our comic book model instances are created

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or retrieved, we'd only need to change
the comic book repository class.

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The MVC design pattern
is also an example of

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the Separation of
Concerns Design Principle.

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The model is concerned about our
website's data and business logic.

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The view is concerned about providing
a visual representation of the model.

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And the controller is concerned
about processing requests and

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preparing responses.

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Separating our website's concerns
using the MVC design pattern

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makes our code easier to understand and
more maintainable.

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Software design principles
represent a set of guidelines

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that help us avoid running into
problems with our code in the long run.

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Across all software development platforms
and languages, these principles have

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originated from lots of developers solving
the same problems over and over again.

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There are other design principles,

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aside from the ones that we've
mentioned in this video.

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See the teacher's notes for
links to additional resources.

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In the next video, let's add the comic
book repository class to our project.