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Hey, welcome back.

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We're gonna take a short break from
code to talk about the popular software

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design pattern called
model-view-presenter, or MVP, for short.

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Now don't worry if this video
seems a little unrelated at first

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to the app that we're working on.

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We don't need to know all the intricacies

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of the model-view-presenter
design pattern.

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But the more we know about it and

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keep it in mind,
the better software developers we will be.

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We don't necessarily need to follow it for
smaller apps like this.

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But good programming patterns and

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practices are invaluable on larger
projects or when working with teams.

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By the way,
in a previous version of this course,

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we taught this as
the model-view-controller design pattern.

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The two are often used interchangeably,
but there are subtle differences.

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And the model-view-presenter
pattern more closely aligns

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with how Android apps are architected.

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So first off,
what is a software design pattern?

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When writing code,

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you'll always end up facing problems
that are common to 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.

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And while you weren't explicitly
aware of it, we've been working with

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the model-view-presenter
design pattern all along.

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Let's start with a very generic example.

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Think of a slideshow presentation
you have given at work or

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in a class using a tool like PowerPoint,
Google Slides, or Keynote.

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In this case, you are the presenter.

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Your job is to control what is on each
slide and when they are displayed.

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These slides are being presented on
a large screen, which is the view.

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This is how the audience
views your material.

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Lastly, the data used to create
your slides is the data model.

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Maybe your presentation is about earnings,
or the results of experiments, or

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maybe your favorite episodes of Dr. Who.

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Whatever it is, you collected data and

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formatted it in some way
to be viewed on the screen.

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This pattern is the same for
Android apps and games.

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The model is the data
that you use in the app,

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the view is how it is actually
displayed on the screen, and

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the presenter manipulates the data and
objects in some way.

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So next,
how does this apply to our current app?

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Think for a moment about how this
interactive story is set up.

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On the phone, itself, is the screen
that the user interacts with.

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The layouts we create for our activities
are what are shown on the screen, so

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let's call those the views.

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The layout is a presentational aspect
of the app that the user views and

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interacts with.

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Now what piece of our app presents
these layouts, these views?

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What part of our code manages how and
when things are shown?

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And what should happen
when things are tapped?

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That's right, the activities.

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These are the presenters in
the MVP pattern in Android.

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As a side note,

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you will sometimes hear the presenter
referred to as the controller.

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Since it controls both the view
on the screen and the data model.

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We'll stick with presenter, though,
as that's more widely used for Android.

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If you're interested in reading more about
the exact differences between MVP and MVC,

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check the teacher's notes for some links.

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The presenter's also responsible for
interacting with the data model behind

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the scenes, just like how you create and
display slides in a presentation.

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Presenter is what adds, changes,
or deletes data in the model

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based on what the user is doing
on the screen, or the view.

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So that leaves us with the model.

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All the data in our app,

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as well as the logic around that data,
goes into model objects.

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It helps to make the connection by
thinking of the phrase, data model.

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Now what kind of data
are we about to work with?

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That's right, in this case,
it's our story.

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The data can really come from anywhere.

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We'll build it right into our app here,
but it could be data from someplace

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on the web or even elsewhere on the phone,
like from a gallery of photos.

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Wherever we get our data from,
the model object's represented, and

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our presenter gets it ready for display.

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Presenter may even adapt the data,
like when we take a long time step and

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convert it to a shortened representation
that may differ from region to region.

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When we follow this MVP pattern,

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the model does not 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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This setup is the basic architecture for
most Android apps.

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The core Android code that we use to
develop apps has been designed in

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such a way that we're kind of
forced to follow this pattern.

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The model is whatever data
we need to work with.

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The views are the actual
user interface of an app.

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The layouts of buttons and
other things are the XML layout files.

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And the presenter is the activity where
we do stuff behind the scenes and

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update the view or the data model.

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As you go further with Android
development, you may even come across

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variations of these patterns
that have the same basic ideas,

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but differ in some of the details for
very specific reasons.

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We'll eventually talk more about
those in later courses and workshops.

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But understanding the basic MVP
architecture is some key groundwork.

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All right, so that tells us a bit about
what MVP is and how it's used in our apps.

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But we're still missing the why behind it.

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Don't worry, though,
that's coming up next in the next video.