Android -room persistent library - DAO calls are async, therefore how to get callback?

If you want to do your query synchronously and not receive notifications of updates on the dataset, just don't wrap you return value in a LiveData object. Check out the sample code from Google.

Take a look at loadProductSync() here

There is a way to turn off async and allow synchronous access.

when building the database you can use :allowMainThreadQueries()

and for in memory use: Room.inMemoryDatabaseBuilder()

Although its not recommended. So in the end i can use a in memory database and main thread access if i wanted super fast access. i guess it depends how big my data is and in this case is very small.

but if you did want to use a callback.... using rxJava here is one i made for a list of countries i wanted to store in a database:

public Observable<CountryModel> queryCountryInfoFor(final String isoCode) {
    return Observable.fromCallable(new Callable<CountryModel>() {
        @Override
        public CountryModel call() throws Exception {
            return db.countriesDao().getCountry(isoCode);
        }
    }).subscribeOn(Schedulers.io())
       
 .observeOn(AndroidSchedulers.mainThread());

}

you can then easily add a subscriber to this function to get the callback with Rxjava.

As Bohsen suggested use livedata for query synchronously. But in some special case, we want to do some asynchronous operation based on logic. In below example case, I need to fetch some child comments for the parent comments. It is already available in DB, but need to fetch based on its parent_id in recyclerview adapter. To do this I used return concept of AsyncTask to get back the result. (Return in Kotlin)

Repositor Class

fun getChildDiscussions(parentId: Int): List<DiscussionEntity>? {
        return GetChildDiscussionAsyncTask(discussionDao).execute(parentId).get()
    }

private class GetChildDiscussionAsyncTask constructor(private val discussionDao: DiscussionDao?): AsyncTask<Int, Void, List<DiscussionEntity>?>() {
        override fun doInBackground(vararg params: Int?): List<DiscussionEntity>? {
            return discussionDao?.getChildDiscussionList(params[0]!!)
        }
    }

Dao Class

@Query("SELECT * FROM discussion_table WHERE parent_id = :parentId")
fun getChildDiscussionList(parentId: Int): List<DiscussionEntity>?