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We've just added functionality
to our save and delete buttons.

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But that doesn't do us much good if
main activity won't show the pizzas.

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Just like how we created a view model for
our creator activity,

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we should also have a viewModel for
main activity.

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This viewModel will store which pizzas
should be displayed in main activity,

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let's create a new class named
MainViewModel, inside of our main package.

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Then let's make it extend from viewModel,
And

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use Alt + Enter to add
the call to the constructor.

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Inside this class we'll want to
maintain a list of pizzas, and

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we'll want that list of pizzas to
be based on data in our database.

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By using something called live data,

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room gives us a way to tie
together our UI and our database.

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So if something changes in our database
that change will be automatically

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reflected in the UI without
us having to do anything.

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Inside the class, let's copy and paste
in the code from the teacher's notes and

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then walk through it.

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Use Alt + Enter to import the pizza
class and the liveData class.

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We start by creating a private
variable named pizzas,

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which is a liveData containing a list
of pizzas, and we set it equal to null.

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When we call the getPizzas function,
if our pizzas object is null,

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then we populate it with
pizzas from the data base.

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Otherwise, we'll just
return our pizzas object.

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This way, we're only loading from
the data base when we need to, we should

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also fix this error by making pizzaDao
get all function return alive data.

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Let's use Cmd or
Ctrl + B to jump to the declaration, and

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then add live data around
the list of pizzas.

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Perfect, and that's really all you
have to do to use a live data.

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Okay, now to make a use of our live data
object, let's head over to MainActivity,

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And add some space at
the bottom of onCreate.

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Then to retrieve our live data, we'll
first need to retrieve our view model,

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then call the getAll method.

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To retrieve the view model, let's do
the same thing we did in creator activity.

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Start by calling viewModelProviders
with a capital V,

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.of and passing in this for our activity.

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Then on the next line, let's add .get and

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pass in the Java class of our ViewModel,

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MainViewModel::class.java.

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Now that we've got the viewModel on the
next line, let's add a call to getPizzas,

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To retrieve our live data
containing a list of pizzas.

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Once we've got the live data,

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we can get notified when anything
changes by adding an observer.

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On the next line, let's call
the .observe function on our live data.

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Then we need to pass in a lifecycle
owner and an observer object.

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Luckily, our activity is a lifecycle
owner, so for the first parameter,

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let's just pass in this.

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Then for the observer parameter,
let's pass in a new observer

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and use Enter to select
the option with the brackets.

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Now whenever the underline
pizza data changes,

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it'll call the code
between these brackets,

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inside it will be a list of
pizzas that might be null.

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So first let's make sure that
the list of pizzas is not null.

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So if it is != null, and
so long as it isn't null,

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we'll want to update our adapter
to use the new pizza list.

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An easy way to do this is to just clear
the adaptor and then repopulate it.

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Inside the if statement let's start
by clearing our adaptors list.

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So my adaptor.list.clear.

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Then let's add our new pizza
list to the adaptor's list.

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MyAdapter.list.addAll, And
we'll pass an it.

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And finally let's notify the adapter
that the data has been changed.

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MyAdapter.norifydatasetchanged, great.

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Now if we run the app we should be
able to save and delete pizzas, and

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those pizzas should show
up in main activity.

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Or we'll get an error.

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It looks like Room can't
verify the data integrity.

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It looks like we've changed schema but
forgot to update the version number.

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And it suggests we can fix this
by increasing the version number.

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Okay, let's head in to
our PizzaDatabase class,

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And change the version from 1 to 2.

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Then let's run the app again,
And we got another error.

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This time it says a migration from
1 to 2 was required but not found.

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Please provide the necessary
migration path.

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In Room whenever you change
something about your database,

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you need to tell Android how to handle
that change by specifying a migration.

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If you'd like to learn
more about migrations,

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check out the teacher's notes below.

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But for us rather than handle a migration,
when we make a change,

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we'd like the database to
just start from scratch.

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To do this, In the app class,

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Where we declare our database,

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let's move the call to .build
down a couple of lines and

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in the middle add a call to
.fallbackToDestructiveMigration.

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Then if we run the app one more time,
we should be good to go.

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So let's try creating a pizza.

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Let's add some toppings, Give it a name,

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Make sure it stays across rotation,
all right.

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And let's save it, and there it is our
pizza is being shown in main activity.

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And if we click on it, well it looks like
we haven't handled that part quite yet.

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In the next video, we'll see what's
going on when we try to select a pizza.