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Alright so the Tasks Contract Class is

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a topic for this video. Now when we used

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the Content Provider in the previous app,

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we didn't know what the real columns in

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that database were called. In fact, we

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didn't even know that the data was

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coming from a database. The Content

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Provider abstracts the underlying data,

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and prevents a standard interface that

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we used to access the data we want. Now

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if you remember we use constants to

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refer to the actual fields. So our

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Content Providers are going to allow the

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same thing, so we'll start by creating a

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Contract Class to define the names that

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are used to access our data. Now Google's

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Contacts Contract class uses an

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interface to provide the column name

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constants. You can find a lot of argument

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on-line about whether using an interface

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just to provide constant values is a

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good thing or not. In this case, I think

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using an interface in a Contract class

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is perfectly valid. The fact that we're

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talking about a Contract here, is an

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indication that an interface is a

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suitable way to implement this. An

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interface is a Contract and guarantees

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that it'll always provide the fields

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and methods that it publishes. However,

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Kotlin provides another way - Objects. So

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you can find more information on Object

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declarations in the Kotlin documentation.

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I'm just going to briefly bring that up -

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that's a good resource link to check out.

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But one important use is to implement

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the singleton pattern. If you only ever

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want to have a single instance of

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something, then an object in Kotlin may

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be a good way to do that.

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It does however have one limitation; you

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can't pass arguments to its constructor,

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but actually we don't need to do that

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here. If you put an object inside a class

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it becomes a companion object, which

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we've looked at earlier. Now that's also

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documented on this page, just after this

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section - you can see the discussion there

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about Companion Objects. Okay so I'm

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going to use objects for the Contract

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classes for our app. So I'm going to go

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back to IntelliJ, and what I'm going to

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do is create a new Kotlin class. So I'm

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going to go to our package, right click

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New

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Kotlin File/Class, and we're going to call this

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one TasksContract. Now creating an

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object is very similar to creating a

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class - we just used the word object

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instead. So I'm going to type object

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TasksContract, then left and right

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curly braces. Now we'll start with a

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string to store the table name. We'll be

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referring to our table in a few

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different places, so this is a good place

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to define the name of the table. Now

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remember that external apps don't have

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to know anything about how we're

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implementing the underlying storage,

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which means the name of our table should

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only be visible inside our package. So

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I'm going to use the internal access

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modifier to achieve that. So we're going to

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type internal const Val TABLE

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underscore NAME equals Tasks in double quote,

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double quotes rather. Now note that we

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can use the const keyword inside an

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object - something we couldn't do in a

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class. Alright so that's the table name.

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Next we need to figure out our column

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names. Now what we'll do is we'll put

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them inside another object, in the Tasks

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Contract object. Now this is effectively

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the same as a companion object but we

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don't need the companion. Now I'm going to

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type it anyway, and we'll see what

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Android Studio's link checker has to

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say about it, and I'll just put a comment

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down here, "Tasks fields". If we type companion

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object Columns left and right curly

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braces, we get an error there, and if we

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have a look at the error: "Modifier

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companion is not applicable inside

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object". And so we can delete the word

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companion near the keyword and just

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declare an object, and I'll add the

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constants for the field names inside

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this object. We'll start with const val ID

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in uppercase equals double quotes

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_id. Then on the next line,

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const val, in uppercase TASK underscore

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NAME equals name double quotes. Next line

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const val

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Tech, TASK rather underscore

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DESCRIPTION, in uppercase again, equals

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and in double quotes Description. And

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lastly, const val TASK_SORT

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underscore ORDER is equal to SortOrder,

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capital S, capital O there, in double

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quotes. And obviously you can see, I've

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capitalized the Name, Description and

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SortOrder on lines 14 through 16.

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So our columns object behaves almost

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exactly the same as if it was defined in

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its own column.kt file. The only

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difference is that we can't refer to it as

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directly as columns. We have to use tasks

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Contract.columns. Now one advantage of

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this, is that we can have several columns

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objects. Now we're going to need to

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define the columns for our Timings table,

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and that object would be referred to as

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timingsContract.columns. Alright, so

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this can, this columns object is

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just to provide the constants we'll be

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using to refer to the column name in our

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Tasks table. So we're going to add more

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code to this TasksContract object later,

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to provide everything that we'll need

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for our Content Provider, but that's

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enough for now, except there's one change

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that I want to make. And I want to make a

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change to our code so that we can have a

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look at what Base Columns is. So if we

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come down here and change the ID, I'm

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actually going to add the word Base

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Columns, BaseColumns._ID and

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I'm going to delete out the double

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quotes _id that we had

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there. So if we click on this now we can

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check out the source code, and you can

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see there that BaseColumns is an

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interface that defines two constants:

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underscore ID and underscore COUNT. Now I

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mentioned that Android makes use of a

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primary key called underscore ID, and

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this is where that name's defined. Now we

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could have left it hard-coded in our

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code, but if Google provide a constant to

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use, then we really should use that

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constant. So that way, if they change

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things in future our code will still

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work and won't break. Now I can't really

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imagine any reason why they might decide

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to rename this field - it could break too

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much existing code for one thing. So if

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you prefer to

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used the string literal underscore ID - I'll

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just close this down - in your code instead of

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BaseColumns._ID that

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we've got here, that's probably fine. But

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I've done it this way, because you'll see

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this BaseColumns interface being used

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like this in a lot of code on the

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Internet

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Alright so that's enough to be getting

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on with. In the next video we'll start

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looking at creating the database class.

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So I'll see you in the next video.