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

Question

From what i have read Room doesn’t allow you to issue database queries on the main thread (as can cause delays on the main thread)). so imagine i am trying to

Answer 1

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.

Answer 2

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>?

Answer 3

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