How does LiveData know when data is changed in Room Database?

Question

I am learning Mvvm pattern in android, and I don't understand one thing. How Live Data knows when data has changed in Room Database? I have this code:

Fragment:

 newUserViewModel.getListItemById(itemId).observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User user) {
            tv.setText(user.getName());
        }
    });

View model:

 public LiveData<User> getListItemById(String itemId){       
    return repository.getListItem(itemId);
}

Repository:

 public LiveData<User> getListItem(String itemId){
    return userDao.getUSerByID(itemId);
}

DAO:

@Query("SELECT * FROM User WHERE itemId = :itemId")
LiveData<User> getUSerByID(String itemId);// When this query gets executed and how live Data knows that our Table is changed?

let's say we inserted new User in Database. When is @Query("SELECT * FROM User WHERE itemId = :itemId") gets executed when there is new data in our database?) and How LiveData knows that we have new User in table and callback Observer owner that data has changed?

Answer 1

After diving in the Android Room code, I found out some things:

1. Room annotation processor generates code from Room annotations (@Query, @Insert...) using javapoet library

2. Depending on the result type of the query (QueryMethodProcessor), it uses a "binder" or another one. In the case of LiveData, it uses LiveDataQueryResultBinder.

3. LiveDataQueryResultBinder generates a LiveData class that contains a field _observer of type InvalidationTracker.Observer, responsible of listen to database changes.

Then, basically, when there is any change in the database, LiveData is invalidated and client (your repository) is notified.