Update (or create) flow inside itself and emit this, Room, Flow, MVVM

Question

I have been creating an SMS app. I have a list of conversations stored in Room database as ConversationEntity.

This is my query:

@Query("SELECT * FROM conversation_entity ORDER BY timestamp DESC")
fun getAllConversations(): Flow<List<ConversationEntity>>

I would like to observe (collect) data from this query inside my repository class, but I have to map it to a List<Conversation>. I know how to collect this data, I known to map List<ConversationEntity> into List<Conversation>. But I have no idea how should I emit list of conversations?

I tried things like emit second flow from the first one, or use MutableStateFlow and set date by .value

I do not understand your question. Your query already gives you a flow of a list of `ConversationEntity`. So what is the problem here? Does it not work like this? Do you want to do mapping on the flow or on the list? — ChristianB, Jan 08 '21 at 14:18
@ChristianB I think he wants to call `.collect{ }` from `getAllConversations` and map the list to another list while creating another flow. — Andrew, Jan 08 '21 at 22:56

score 1 · Answer 1 · answered Jan 08 '21 at 23:05

1

I hope I understood your question correctly. If you want to collect getAllConversations() and then emit another flow from it while mapping it to another flow, you can simply call the function.collect { } and then emit it.

Since you did not provide much codebase, here is an example:

val myTestFlow: Flow<Int> = flowOf(1,2,3,4,5,6,7,8)

fun myNewFlow(): Flow<String> = flow  {
    myTestFlow.collect { myInt ->
        emit(myInt.toString())
    }
}

answered Jan 08 '21 at 23:05

Andrew

4,264
1
21
65

What you do is a plain mapping. You do not need to collect and re-emit. You could just do `flow.map { it.toString() }`, but I do not know if that is what @rogalz wants. – ChristianB Jan 09 '21 at 10:49

ChristianB · Accepted Answer · 2021-01-09T12:14:17.087

I am still confused what you mean, since you said you know how to collect a Flow and how to map List to List. So anyway let me give it a try:

class DAO {
  @Query("SELECT * FROM conversation_entity ORDER BY timestamp DESC")
  fun getAllConversations(): Flow<List<ConversationEntity>>
}

class Repository(private val dao: Dao) {
  fun getConversations(): Flow<List<Converstaion>> {
    // this maps every emitted element of the flow
    return dao.getAllConversations.map { list: List<ConversationEntity> ->
      // and this maps every element in the list
      list.map { conversationEntity ->
        conversationEntity.mapToConversation()
      }
    }
  }
}

class ConversationMapper {
  // Maps ConversationEntity to Conversation
  fun ConversationEntity.mapToConversation(): Conversation {
    // I have no idea of the fields, so you have to implement this mapping function yourself.
    return Converation(...)
  }
}

And that's it. Here is how you can use it in your ViewModel:

class YourViewModel : ViewModel(private val repository: Repository) {
  val converstationLiveData: LiveData = repository.getConversations().toLiveData()
}

Hope that helps you. But if this is still not what you meant, then please update your question accordingly.

Update (or create) flow inside itself and emit this, Room, Flow, MVVM

2 Answers2