Android asks for not to query on main thread, but asyc queries are delaying

Question

As the title says, android needs queries out of main thread since it will trhow java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time otherwise. So I managed to make async queries as many tutorials explain, but it doesn't make so much sense (so far) as I could achieve.

public class NewDetalleDiarioActivity extends AppCompatActivity {

    @Override
    protected void onStart() {
        super.onStart();
        db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database").build();
        findPeriodo();
        findDiario();
    }

    private void findPeriodo() {
        periodo = Diarios.getPeriodo(db);
        if (periodo == null) {
            Intent intent = new Intent(NewDetalleDiarioActivity.this, NewPeriodoActivity.class);
            startActivity(intent);
        }
    }

PROBLEM/ERROR:

If periodo is null, another activity is started, otherwise this one continues its thread. The problem is that, when I debug it (which slows proceses, of course) periodo returns an instance from the database, but when I run the code without debugging, periodo is null.

public class Diarios {
    public static Periodo getPeriodo(AppDatabase db) {
        return Factory.getIntPeriodo().getPeriodo(db);
    }
}

.

public class Factory {
    private static IntPeriodo intPeriodo;
    public static IntPeriodo getIntPeriodo() {
        return (intPeriodo == null) ? intPeriodo = new BusPeriodo() : intPeriodo;
    }
}

.

public class BusPeriodo implements IntPeriodo {
    // I don't think it's necessary to post the interface...
    @Override
    public Periodo getPeriodo(final AppDatabase db) {
        final Periodo[] periodo = new Periodo[1];
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() { //the async query that is driving me mad.
                periodo[0] = db.periodoDao().getPeriodo(new Date());
            }
        });
        return periodo[0];
    }
}

What's the proper way to make select queries without getting them delayed? The select query is indeed working, I don't think is necessary to post it (because it is returning an unique result when I debug), but it returns null when I run the code without debugging!! Please help.

That's because `Runnable#run()` that you passed to `AsyncTask.execute(...)` will be run in the background thread and its execution is indeterministic. Before the background thread finished, you already return `periodo[0]` which might have not been set — cartesify, May 02 '20 at 04:45
You can try to follow something similar like this https://stackoverflow.com/questions/44167111/android-room-simple-select-query-cannot-access-database-on-the-main-thread — cartesify, May 02 '20 at 05:01

Gonzalo Dambra · Accepted Answer · 2020-05-03T19:23:33.913

SOLUTION:

As @user7041125 suggested, but instead I made a new class with an interface to call methods back to the activity, like this:

public class PeriodoBridge extends AsyncTask<Void, Void, Periodo> implements IntPeriodoBridge {

    private WeakReference<Activity> weakActivity;
    private IntPeriodoBridge caller; //implement this interface in the activity which needs to query
    private AppDatabase db;
    private Periodo periodo;

    public PeriodoBridge(Activity activity, IntPeriodoBridge caller, AppDatabase db) {
        weakActivity = new WeakReference<>(activity);
        this.caller = caller; // assign activity instance to the local interface instance
        this.db = db;
        executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    }

    @Override
    protected Periodo doInBackground(Void... voids) {
        periodo = Diarios.getPeriodo(db);
        return periodo;
    }

    @Override
    protected void onPostExecute(Periodo periodo) {
        Activity activity = weakActivity.get();
        if (activity == null) {
            return;
        }
        if (periodo == null) {
            Intent intent = new Intent(activity, NewPeriodoActivity.class);
            activity.startActivity(intent);
        } else {
            setPeriodo(periodo);
        }
    }

    @Override  //this is an interface method (IntPeriodoBridge)
    public void setPeriodo(Periodo periodo) {
        caller.setPeriodo(periodo);  //I can set the query result back to the activity class with this
    }

Call the init method of this class. The activity implements IntPeriodoBridge and in that way I can set the query result object to the activity class.

Android asks for not to query on main thread, but asyc queries are delaying

1 Answers1