Room Database Query

I'm new to Room and i'm trying to query my database to get a row from it. I attempted doing so by querying it with the primary key which is id but the problem is i don't know how to return the target object from the repository.

Room

query

database

row

querying

primary key

object

repository

This is the Dao

Dao

@Query("SELECT * FROM targets WHERE id = :id LIMIT 1")
Targets findTargetById(int id);


THis is the Repository class

public Targets findTarget (int id)
new findTargetByIDAsyncTask(mTargetsDao).execute(id);



private static class findTargetByIDAsyncTask extends AsyncTask<Integer, Void, Targets>

private TargetsDao mAsyncTaskDao;

findTargetByIDAsyncTask(TargetsDao dao)
mAsyncTaskDao = dao;



@Override
protected Targets doInBackground(Integer... integers)

return mAsyncTaskDao.findTargetById(integers[0]);


@Override
protected void onPostExecute(Targets targets)
super.onPostExecute(targets);




2 Answers
2

You have two ways to return a result.

The first way is to call AsyncTask.get() method, but it will still hold a MainThread what leads to ANR if a task will longer than 5 seconds:

AsyncTask.get()

public Targets findTarget (int id)
return new findTargetByIDAsyncTask(mTargetsDao).execute(id).get();



The second way is more complicated but it will not hold the MainThread. You should add a Callback class:

public interface Callback
void onSuccess(Targets targets);



Each method of your repository will look like that:

public void findTarget (Callback callback, int id)
new findTargetByIDAsyncTask(mTargetsDao, callback).execute(id);



And AsynTask will look like that:

private static class FindTargetByIDAsyncTask extends AsyncTask<Integer, Void, Targets>

private final TargetsDao mAsyncTaskDao;
private final Callback callback;

FindTargetByIDAsyncTask(TargetsDao dao, Callback callback)
mAsyncTaskDao = dao;
this.callback = callback;



@Override
protected Targets doInBackground(Integer... integers)
return mAsyncTaskDao.findTargetById(integers[0]);


@Override
protected void onPostExecute(Targets targets)
callback.onSuccess(targets);




The point is to get the data/object from a background thread. You can use Android's AsyncTask or a ExecutorService. A simple example is if you want to get a String of a user name the method will be:

AsyncTask

ExecutorService

private String getName() ExecutionException e)
e.printStackTrace();

return name;



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