Database Access with Room and RxJava

Room Persistency Library

Since version 1.5.2 we no longer use any cursors or raw db queries but instead Room.

Room in a nutshell

• Provides abstraction layer over SQLite
• More efficient way of retrieving data from database ⇒ less code ⇒ fewer bugs
• Tables: Each class annotated with @Entity
• Data Access Objects (DAOs): Responsible for defining the methods that access the database (Annotations: @Query @Update @Delete)
• Database: Class annotated with @Database, lists all entities contained in the database, and the DAOs which access them.

RxJava in a nutshell

• RxJava is an implementation of http://reactivex.io/ for Java, which provides an API for asynchronous programming with observable streams.
• We use RxJava in combination with Room to get so called Flowables. A Flowable is an Observable that emits items whenever the data is changed in the database (and oc initially)
• Also with rxjava no work is done on the ui thread, only the code that is within the subscribe method.

How to use room in an Activity?

A: Depends on whether we already introduced RxJava for this or not.

• Without RxJava and if there is a corresponding manager:

protected void onStart() {
        super.onStart();

        // Gets all news from database
        nm = new NewsManager(this);
        List<News> news = nm.getAllFromDb(this);
        ...
}

• Without RxJava and without corresponding manager:

protected void onStart() {
        super.onStart();

        calendarDao = TcaDb.getInstance(context).calendarDao();
        calendarItems = calendarDao.getAllByDateNotCancelled(Utils.getDateString(new Date()));
        ...
}

• With RxJava:

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        KinoLocalRepository.db = TcaDb.getInstance(this);
        kinoViewModel = new KinoViewModel(KinoLocalRepository.INSTANCE, KinoRemoteRepository.INSTANCE, disposable);
        ...

}
 @Override
    protected void onStart() {
        super.onStart();
        //access:
        kinoViewModel.getAllKinos()
                     .doOnError(throwable -> setContentView(R.layout.layout_error))
                     .subscribe(kinos -> {
                           //Do what you want to do with the data
                     });
}

Helpful packages/classes

Location of DAOs, Migrations, and the Database:

• https://github.com/TCA-Team/TumCampusApp/tree/master/app/src/main/java/de/tum/in/tumcampusapp/database

Location of available viewmodels (or RxJava integrations)

• https://github.com/TCA-Team/TumCampusApp/tree/master/app/src/main/java/de/tum/in/tumcampusapp/viewmodel

Corresponding repositories for RxJava ViewModels

• https://github.com/TCA-Team/TumCampusApp/tree/master/app/src/main/java/de/tum/in/tumcampusapp/repository