Room Dao LiveData as return type causing compile time error

Question

I am using Room and implemented Dao that returns LiveData. It was working fine with below dependency added.

implementation "androidx.room:room-runtime:2.1.0-alpha04" kapt "androidx.room:room-compiler:2.1.0-alpha04" 

But when I added new Room coroutine dependency as mentioned below.

implementation "androidx.room:room-runtime:2.1.0-alpha04" implementation "androidx.room:room-coroutines:2.1.0-alpha04" kapt "androidx.room:room-compiler:2.1.0-alpha04" 

Below is code which compiles

@Dao interface AccountDao{      @Query("SELECT * FROM account_master")     suspend fun getAllAccounts(): List<Account> } 

Below is the code which gives error.

@Dao interface AccountDao{      @Query("SELECT * FROM account_master")     suspend fun getAllAccounts(): LiveData<List<Account>> } 

started to receive error.

PlayGround/app/build/tmp/kapt3/stubs/debug/com/playground/www/x/datasource/dao/AccountDao.java:11: error: Not sure how to convert a Cursor to this method's return type (androidx.lifecycle.LiveData<java.util.List<com.playground.www.x.datasource.entity.Account>>). public abstract java.lang.Object getAllAccounts(@org.jetbrains.annotations.NotNull() 

Any one facing similar issue?

Answer 1

I think the solution here is actually to just return the LiveData without using Coroutines. LiveData works out of the box, there's no reason to use Coroutines when returning LiveData.

When using LiveData it already handles it on a background thread. When NOT using LiveData then in that case you can use Coroutines (and maybe eventually Coroutines Channels) or RxJava2.

See this codelab for an example: https://codelabs.developers.google.com/codelabs/android-room-with-a-view-kotlin. Here they need a background thread for inserts but not for the returned LiveData.

Note: there seems to be a mistake in the actual codelab where the DAO is not returning a LiveData. I've corrected that in the sample below.

@Dao interface WordDao {      @Query("SELECT * from word_table ORDER BY word ASC")     fun getAllWords(): LiveData<List<Word>>      @Insert     suspend fun insert(word: Word)      @Query("DELETE FROM word_table")     fun deleteAll() }  class WordRepository(private val wordDao: WordDao) {      val allWords: LiveData<List<Word>> = wordDao.getAllWords()      @WorkerThread     suspend fun insert(word: Word) {         wordDao.insert(word)     } } 

Answer 2

Remove the suspend function. LiveData is already asynchronous. No need for a suspend function.

@Dao interface AccountDao{      @Query("SELECT * FROM account_master")     fun getAllAccounts(): LiveData<List<Account>> }