Room Persistence Library

• Room Persistence is used for saving structured data locally and caching network data so apps can work offline.
• It provides an abstraction layer over SQLite, allowing fluent database access while harnessing the full power of SQLite.
• Benefits include compile-time verification of SQL queries, convenience annotations that minimize repetitive boilerplate code, and streamlined database migration paths.

Setup

• Add dependencies to your app's build.gradle file to use Room in your app.
• Example dependencies include room-runtime, room-compiler, room-ktx, room-rxjava2, room-rxjava3, room-guava, and room-testing.
• The room version is set to "2.6.1" in the build.gradle file.

Primary Components

• The primary components are the Room Database, Data Access Objects (DAOs), and Entities.
• The Room Database serves as the main access point for the underlying connection to your app's persisted data.
• Data Access Objects (DAOs) provide methods to query, update, insert, and delete data in the database.
• Entities represent tables in your app's database.
• The Rest of the App interacts with the Room Database through the DAOs to get and set data in the Entities.

Room Database

• The database class holds the database and serves as the main access point.
• An example database with one Entity (Dish) is shown using the @Database annotation.
• The abstract class AppDatabase extends RoomDatabase and declares an abstract method to access the DishDao

DAO

• Data access objects provide methods to query, update, insert, and delete data in the database.
• An example of DishDao uses the @Dao annotation.
• @Query annotation is used to select data from the database.
• LiveData is used to write asynchronous observable queries, automatically updating UI views when query results change.
• All queries should run async to prevent blocking the UI.
• @Insert annotation is used to insert data into the database.
• @Delete annotation is used to delete data from the database.

Entities

• Data entities represent tables in your app's database.
• An example of Dish Entity contains an id, name, type, and price.
• The @Entity annotation denotes the class as a database table.
• @PrimaryKey annotation specifies the primary key of the table, with autoGenerate = true automatically generating the primary key for new records.
• @ColumnInfo annotation allows specifying the column name in the database.

Usage

• Define a static field from the AppDatabase class inside the launcher activity.
• Initialize the field inside onCreate using Room.databaseBuilder.
• Call the observe method for queries wrapped with LiveData inside onCreate.
• The runOnUiThread method is used to update the UI after running the query.
• ExecutorService can run queries async that are not wrapped with LiveData.
• In the delete example, runOnUiThread is not used because getAllDishes is linked with an observable, automatically updating the UI.

Practicing Exercise - Restaurant Dishes Application

• Create a simple app to show all dishes inside a restaurant as a listview.
• Each dish should have a name, type, and price. The user can add a new dish through an action button.
• Each row in the listview has a remove button to delete a selected dish.