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6
Room Architecture Written by Subhrajyoti Sen

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In the previous section, you learned all the basics behind data storage on Android. You learned how to work with permissions, shared preferences, content providers, JetPack DataStore and SQLite.

SQLite is a fast, lightweight local database natively supported by Android that allows you to store large amounts of data in a structured way. The only downside of SQLite is that its syntax isn’t very intuitive since the way to interact with it can be very different from platform to platform.

Therefore, in this chapter, you’ll learn about one of the most popular libraries that helps you simplify your interaction with SQLite: Room.

Along the way, you’ll also learn:

  • How Object Relational Mappers work.
  • About Room’s integration with Google’s architecture components
  • The basics behind entities, DAOs and Room databases.
  • The advantages and disadvantages of Room.
  • The app you are going to build in the rest of this section.

Surely, you’re ready to dive in!

Object Relational Mappers

Before using Room properly in your projects, you first need to learn what Room is.

Room is a type of data persistence library commonly known as Object Relational Mapper or ORM. ORMs are tools that allow you to retrieve, delete, save and update the contents of a relational database using the programming language of your choice.

ORMs are implemented as libraries or frameworks that provide an additional layer of abstraction called the data layer, allowing you to better interact with your database using a syntax similar to the object-oriented world.

To better understand how ORMs work, imagine that you have a Movie class with three properties: An id, a name and a release_date.

The Movie class with its properties.
The Movie class with its properties.

The diagram above is a class diagram that represents the Movie class. Like most object-oriented languages, each of these properties has a specific data type such as Int, String or Date.

With the help of an ORM, you can easily use the Movie class to create a new table in your database. In an ORM, classes represent a table inside your database and each property represents a column. For example, the ORM will translate the Movie class into a table like this:

Empty table with three columns: id, name and release_date.
Empty table with three columns: id, name and release_date.

Each column would also have the data type that best represents the original data type of the original property. For example, a String would be translated as a varchar and an Integer as an Int.

The way to create new records inside the tables differs from each implementation. For instance, some ORMs automatically create new entries each time a new class instance is created. Other ORMs such as Room use Data Access Objects or DAOs to query your tables.

The following is a simple example of how you would use a DAO in Room to create new Movie records in the previously mentioned table:

movieDao.insert(Movie(1, "Harry Potter", "10-11-05"))
movieDao.insert(Movie(2, "The Simpsons", "03-10-02"))
movieDao.insert(Movie(3, "Avengers", "08-01-10"))

And your table would look like this:

The table filled with data and three columns: id, name and release_date.
The table filled with data and three columns: id, name and release_date.

Easy, right?

Note: Room can autogenerate the primary key, in this case, ID. You will learn how to do that in a later chapter.

Now, check out how Room and Google’s android architecture components work and interact with each other.

Room and Android Architecture Components

Over the years, Android developers have adopted different practices in developing the architecture of their apps. Some programmers preferred to use an MVVM architecture with SugarORM, while others used MVP with ObjectBox and Firebase. This led to confusion since there was no recommended or official way of doing things.

Database

On a device, the data is stored on a local SQLite database. Room provides an additional layer on top of the usual SQLite APIs that avoids creating a lot of boilerplate code using the SQLiteOpenHelper class.

private const val SQL_CREATE_ENTRIES =
        "CREATE TABLE question (" +
                "question_id INTEGER PRIMARY KEY," +
                "text TEXT"

private const val SQL_DELETE_ENTRIES = "DROP TABLE IF EXISTS question"

class QuizDbHelper(context: Context) : SQLiteOpenHelper(context, DB_NAME, null, DATABASE_VERSION) {
    companion object {
        const val DATABASE_VERSION = 1
        const val DB_NAME = "question_database.db"
    }
    override fun onCreate(db: SQLiteDatabase) {
        db.execSQL(SQL_CREATE_ENTRIES)
    }
    override fun onUpgrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
        db.execSQL(SQL_DELETE_ENTRIES)
        onCreate(db)
    }
    override fun onDowngrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
        onUpgrade(db, oldVersion, newVersion)
    }
}
@Database(entities = [(Question::class)], version = 1)
abstract class QuestionDatabase : RoomDatabase() {
  abstract fun questionsDao(): QuestionDao
}

Entities

Entities in Room correspond to tables in your database and are usually defined in Kotlin as data classes. Take a look at the following Entity example:

@Entity(tableName = "question") //1
data class Question(
    @PrimaryKey //2
    @ColumnInfo(name = "question_id") //3
    var questionId: Int,
    @ColumnInfo(name = "text")
    val text: String)

DAOs

DAO stands for Data Access Object. DAOs are interfaces or abstract classes that Room uses to interact with your database using annotated functions. Your DAOs will typically implement one or more CRUD operations (create, read, update and delete operations) on a particular Entity.

@Dao //1
interface QuestionDao {
    
  @Insert(onConflict = OnConflictStrategy.REPLACE) //2
  fun insert(question: Question)

  @Query("DELETE FROM question") //3
  fun clearQuestions()

  @Query("SELECT * FROM question ORDER BY question_id") //4
  fun getAllQuestions(): LiveData<List<Question>>

}

Repository

This class acts as a bridge between your data sources and your app. The repository class handles the interaction with your Room database and other backend endpoints such as web services and Open APIs. Repositories make it easy to abstract the data and network layers away from the rest of the app. This abstraction makes it easy to add new data sources without affecting the rest of the app.

ViewModel

Just like the Repository acts as a bridge between your data sources and your app, the ViewModels act as a bridge between your Repository and your user interface. The ViewModel communicates the data coming from your Repository to your Views and has the advantage of surviving configuration changes since it’s lifecycle-aware.

LiveData

LiveData is a data holder class that implements the Observer pattern. This means it can hold information and be observed for changes. Your views such as Fragments or Activities observe LiveData objects returned from your ViewModels and update the relevant widgets as needed. The unique thing about LiveData is that it emits changes only if the observer is in an active state. Hence, if your Activity is in the backstack or the background, LiveData will not emit any changes.

A diagram describing the interaction between Android Architecture components.
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Advantages of using Room

Room uses a local SQLite database to store your data. Therefore, some of the advantages and disadvantages of SQLite apply to Room as well.

Frequently asked Room questions

Are ORMs really necessary? Can’t I just use plain old SQLite?

Your app

This chapter has been full of theory and concepts. You’re probably wondering when you are actually going to start writing some code.

The main screen of DroidQuiz.
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Key points

  • Room is an ORM developed by Google as a part of Android Architecture Components to simplify the interaction with your SQLite database and reduce boilerplate code.
  • Entities in Room correspond to tables in your database.
  • DAO stands for Data Access Object.
  • The Repository class handles the interaction with your Room database and other backend endpoints.
  • The ViewModel communicates the data coming from your repository to your views and has the advantage of surviving configuration changes.
  • LiveData is a data holder class that can hold information and be observed for changes.
  • ORMs provide an additional layer of abstraction that allows you to interact with your relational database with an Object-Oriented Language syntax.

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