I started using RxJava in combination with SqlBrite and I am having some issues with the zip operator.
Let's say I have 2 classes, User and Tweet.
public class User {
public long id;
public List<Tweet> users;
...
}
public class Tweet {
public long id;
public User poster;
...
}
And their respective SQLite tables:
user
---------
id INTEGER PRIMARY KEY
tweet
---------
id INTEGER PRIMARY KEY
poster_id INTEGER
FOREIGN KEY(poster_id) REFERENCES user(id)
and their respective SqlBrite DAO
UserDao:
public class UserDao {
private final BriteDatabase briteDb;
private final TweetDAO tweetDao;
...
public Observable<List<User>> getUsersWithTheirTweets() {
Observable<User> usersObs = briteDb.createQuery("user", "SELECT * FROM user")
.map(new Func1<SqlBrite.Query, List<User>>() {
@Override
public List<User> call(SqlBrite.Query query) {
Cursor cursor = query.run();
List<User> result = new ArrayList<>(cursor.getCount());
while (cursor.moveToNext()) {
User user = UserTable.parseCursor(cursor);
result.add(user);
}
cursor.close();
return result;
}
})
// transform Observable<List<User>> into Observable<User>
.flatMap(new Func1<List<User>, Observable<User>>() {
@Override
public Observable<User> call(List<User> users) {
return Observable.from(users);
}
});
// combine each user with his tweets
return Observable.zip(usersObs, usersObs.flatMap(new Func1<User, Observable<List<Tweet>>>() {
@Override
public Observable<List<Tweet>> call(User user) {
return tweetDao.getTweetsByUser(user);
}
}), new Func2<User, List<Tweet>, User>() {
@Override
public User call(User user, List<Tweet> tweets) {
user.tweets = tweets;
return user;
}
}).toList();
}
}
TweetDAO:
public class TweetDAO {
private final BriteDatabase briteDb;
...
public Observable<List<Tweet>> getTweetsForUser(final User user) {
briteDb.createQuery("tweet", "SELECT * FROM tweet WHERE poster_id = ?", Long.toString(user.id))
.map(new Func1<SqlBrite.Query, List<User>>() {
@Override
public List<Tweet> call(SqlBrite.Query query) {
Cursor cursor = query.run();
List<Tweet> result = new ArrayList<>(cursor.getCount());
while (cursor.moveToNext()) {
Tweet tweet = TweetTable.parseCursor(cursor);
tweet.user = user;
result.add(tweet);
}
cursor.close();
return result;
}
})
}
}
As you can see in UserDao, I tried to use the a combination of zip and flatMap operators to populate the list of Tweet for each User.
First question: is there a better way to do that?
Second question: that zip operator seems to hang forever and never completes... I see that tweetDao.getTweetsByUser(user) is called but the Func of the zip is never called... Would anyone have an idea why?
Third question: is there a better way of transforming an Observable<List<T>> to an Observable<T> other than using a combination of flatMap and from ?
Damn, that RxJava is powerful but the learning curve is pretty steep...