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Now that we've got the UI working,
let's get working on the database.

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The first thing we'll need to do is add
a couple imports to our build.gradle file.

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I've already got them on the clipboard,
but

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they're the same ones we used
in the pizza keeper app.

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Then we need to sync the project, and
we can close the build.gradle file.

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Next, I'll add a data package, for
organizing all of our database files.

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And I'll start with the bottom,
by creating entity to represent a note.

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We call this class Note,
then we need to make it a data class.

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And instead of the brackets, we'll use the
parenthesis, and then we'll give it an id.

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And actually first let's give
it the @Entity the annotation.

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Then let's give it an auto
generating primary key,

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so this will be @Primary key,

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autoGenerate = true, and
we'll call it val id.

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And it can be an integer
which might be null, and

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then we will have the text of the ID,
which is a string.

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Let's see what we've got here.

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This doesn't equal an int, there we go.

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I'll wind this up.

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Awesome, so
a really simple table for our note.

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It's got an id which we're going to
auto generate by passing in null,

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and we'll store the text as a string.

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Then we'll need a data access
object to access our note.

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So let's make a new interface.

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This would be NoteDao, and
let's give it the @Dao annotation And

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inside let's write a get all function.

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So that'll be query select star from,
it'll be note.

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And the function will be called getAll.

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And you will want this

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to return a LiveData

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of a list of notes.

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And then in order to have the OK button
work, we need an insert function.

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So we insert and
fun insert takes in a note, and that's it.

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Then to tie it in altogether
we need a database class.

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So lets create MyDatabase.

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That will be a class and
its actually an abstract class,

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and we wanna use @Database,
then we need to add the entities.

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So, let's do this as an array of
K classes, So, Kotlin classes,

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meaning we do Note::class
instead of class.java.

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And then for the version, set it to 1.

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And this needs to extend
from RoomDatabase,

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And the constructor invocation,
then we need to add an abstract

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function called noteDao,
that returns a NoteDao.

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Right, have to add the parentheses for
our functions.

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Okay, and that should do it for
the database.

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So now let's add the view model.

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So let's add here a new view model class
for our main activity, MainViewModel.

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This is a class.

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So I'll hide the project pane for
a second, and

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make this extend from
the view model class.

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And we'll need to add the parentheses,
and then let's get to our view model.

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Let's start by declaring
a private variable, Named notes,

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which is a live data of a list
of notes of our note class.

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So, just like we we're
returning in the noteDao,

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we'll give this the ability to be null,
and initialize it to null,

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and then we'll have a function
to populate this notes variable.

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So, let's do function, get notes, and

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this returns a LiveData
of a list of notes.

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Though note, this one cannot be null.

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And inside this function,
if notes is null,

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then we'll retrieve the notes.

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And otherwise we will return,

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The notes and assert that they're true,
or rather assert that they're not null.

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So inside here we'll need to add code to
retrieve our notes from the database.

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To get access to the database,

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let's do the same thing we
did in the Pizza Keeper app.

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Let's create a new app class,
Which extends from application.

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And then let's override, so
Control O The onCreate method.

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And let's add our lateinit var db.

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MyDatabase, so
we are adding our database variable again,

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scoped to the entire project,
and we're using a lateinit.

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So inside onCreate,
before the call to supper.onCreate,

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let's set db = Room, which we'll
need to import .databaseBuilder.

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And then provide a context, application
context, the class of the database.

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So MyDatabase::class.java,
and the name of the database,

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so, MyDatabase, and
then we need to build it.

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And since this comes before
the call to super.onCreate,

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we'll have access to our database
from anywhere within the app.

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Like in our main view model.

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So if we call getNotes and
notes from our view model is null,

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let's go ahead and
retrieve those notes from the database.

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Let's set notes = db.otesDoa.getall.

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Perfect.

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Our view model is all set up ready to go.

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Let's head back into main activity,
and I'll hide the Project pane.

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And below where we set
our OnClickListener,

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let's add a couple lines, and
get access to our viewModel.

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So, val viewModel = ViewModelProviders,

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With an S, .of, and pass in this for

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the activity, and then, .get, and

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pass in MainViewModel :: class.java.

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Then to get a live data of our list of
notes we can call viewModel.getNotes.

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And then to observe t we just
need to add a call to observe,

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pass in a lifecycle owner like
the activity And then pass in an observer.

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We can create a new one by just typing in,
Observer, with a capital O, and

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accepting Autocomplete.

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Inside the observer, we are dealing
with a list of notes that might be null.

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So let's start by first making
sure that list is not null.

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If, it, not equals null.

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Once we make sure that our list of notes
isn't null, let's go into the adapter and

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clear the list, and
then add back in each of the elements.

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So, we'll do adapter.list.clear,

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and then adapter.list.addAll.

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And we'll need to pass in the string
portion of all of our notes.

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So here we have a list of notes, so
let's take this and let's call .map.

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And this will take our list of notes and
map it to something else.

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So lets map it to it, so in here we
have a note, let's take that .text.

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And now we've taken our list of notes and
transformed into a list of strings,

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where each item in the list just
contains the text of the note.

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So once we've done that
we just need to call

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adaptor.notifyDataSetChange to
make the changes happen.

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And now that we're updating UI,
based on our viewModel,

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all that's left is that when we click the
button, instead of updating the adaptor,

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we should save to the database.

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So let's delete these two adapter lines
and call DB, or rather a new thread,

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and inside that thread we'll
call db.noteDao.insert.

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And we'll have to insert a new note,
so up here let's create a new note.

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And for the ID we'll pass in null,
and for the text,

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we will give it, A value of note.

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So we can't reuse this variable, so let's
rename this one actually to noteText.

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So this is used, noteText,
noteText is now blank,

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and that's all that's used for
so let's pass noteText in there.

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And we can input the note class, great.

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Then we've got a note that we'll
be inserting into our database.

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And all right,
let's see what it looks like.

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And we're getting some error about a line
we need to add to the build.gradle file.

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Not sure what that's about,
but we can do that.

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And the app synced, so hopefully that took
care of that issue, let's run the app.

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And see what I did wrong.

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It looks like our db variable was never
initialized, and I think I know why.

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I didn't add the application
to the manifest, and

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it warns me of that right here.

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So we come into the manifest,
we type android name and app.

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Now when we run the app,
we will at least get a different error.

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All right we've got text
let's type hello world and

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hit OK, and it's in there.

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And if we rotate the app it's still there,
it's still there.

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I'll send another message like,
This is pretty cool,

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and one exclamation point, and
then we'll say OK, got that in there.

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Still doing the rotation,
let's leave the app using the back button,

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cuz remember that will cause a full
destruction of the activity, and

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then let's reopen it Might be faster just
to hit the Run button in Android Studio.

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And awesome, there it is.