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The way we've modeled our app so

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far follows a very important design
pattern in iOS development.

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When writing code, there are always common
problems faced by all developers.

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For example, one of those problems is how
to architect our apps so

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that our components are largely reusable.

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Solutions to these common problems are
known as design patterns, and while you

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weren't explicitly aware of it, we've been
implementing a design pattern all along.

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A common way of structuring code in your
apps is using the Model, View,

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Controller design pattern or MVC.

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In the MVC pattern, objects or

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code in your app assume one of three
roles, a Model, View or a Controller.

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Let's start with the model.

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All the data in our app as well as the
logic around that data,

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whether you are manipulating the data in
some way or

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using that data to compute new values, all
that code goes in a model object.

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Your data can come from anywhere.

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In our app, the fact book class that we
just created is the data model, and

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it consists of facts stored in an array.

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The data could also come from some other
place, like the web.

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Wherever we get our data from, the model
collects this data,

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performs some logic on it, and gets it
ready for use.

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When we follow the MVC pattern the model
does not

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communicate with the view in any way.

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This way the model object can be separated
from the view and

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reused across other views without having
to repeat or

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re-implement some of the same
functionality.

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The views in the MVC pattern consist of
everything that a user can see.

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A views purpose is to draw to the screen
and

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to display data from the model object so
that the user can see it.

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At, in our application,

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the views are represented by the
Main.storyboard file and

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consist of our main view along with the
labels and the button that we've added.

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The views are supposed to visually
represent the model, but

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can't communicate with the model.

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And the model is supposed to handle the
manipulation of this data for

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the view, but can't communicate with the
view directly.

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This is where the controller comes in.

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The controller acts as the intermediary
and

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contains all the logic to help facilitate
communication between the two.

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In our app the view control class, view
controller is the controller.

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So when we move the data model out of the
view controller we were

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explicitly creating the model class to
follow the MVC pattern.

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So why do we follow the MVC pattern?

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Well, for starters Apple really wants us
to.

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The MVC design pattern is considered a
core competency when developing for

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Apple's platforms.

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But more importantly following the pattern
will build good programming practices,

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that become invaluable as you work with
teams and on larger projects.

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Following the MVC pattern in our project
also builds on

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another good programming practice.

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The single responsibility principle.

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Good object oriented design principles
states,

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that every class should have a single
responsibility.

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And by structuring our code as models,
views and

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controllers we keep app responsibility
narrowly within each class.

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Now there's some ambiguity when
implementing the MVC pattern, and

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developers may do things slightly
differently.

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Adhering to this pattern is best learned
through practice.

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And as we learn in this project and
onwards, we will talk about

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the decisions of where we implement things
and how it relates to this pattern.