Convert LiveData to MutableLiveData

Question

Apparently, Room is not able to handle MutableLiveData and we have to stick to LiveData as it returns the following error:

error: Not sure how to convert a Cursor to this method's return type

I created a "custom" MutableLiveData in my DB helper this way:

class ProfileRepository @Inject internal constructor(private val profileDao: ProfileDao): ProfileRepo{

    override fun insertProfile(profile: Profile){
        profileDao.insertProfile(profile)
    }

    val mutableLiveData by lazy { MutableProfileLiveData() }
    override fun loadMutableProfileLiveData(): MutableLiveData<Profile> = mutableLiveData

    inner class MutableProfileLiveData: MutableLiveData<Profile>(){

        override fun postValue(value: Profile?) {
            value?.let { insertProfile(it) }
            super.postValue(value)
        }

        override fun setValue(value: Profile?) {
            value?.let { insertProfile(it) }
            super.setValue(value)
        }

        override fun getValue(): Profile? {
            return profileDao.loadProfileLiveData().getValue()
        }
    }
}

This way, I get the updates from DB and can save the Profile object but I cannot modify attributes.

For example: mutableLiveData.value = Profile() would work. mutableLiveData.value.userName = "name" would call getValue() instead postValue() and wouldn't work.

Did anyone find a solution for this?

Solution 1

Call me crazy but AFAIK there is zero reason to use a MutableLiveData for the object that you received from the DAO.

The idea is that you can expose an object via LiveData<List<T>>

@Dao
public interface ProfileDao {
    @Query("SELECT * FROM PROFILE")
    LiveData<List<Profile>> getProfiles();
}

Now you can observe them:

profilesLiveData.observe(this, (profiles) -> {
    if(profiles == null) return;

    // you now have access to profiles, can even save them to the side and stuff
    this.profiles = profiles;
});

So if you want to make this live data "emit a new data and modify it", then you need to insert the profile into the database. The write will re-evaluate this query and it will be emitted once the new profile value is written to db.

dao.insert(profile); // this will make LiveData emit again

So there is no reason to use getValue/setValue, just write to your db.

Solution 2

If you really need to, then you can use the mediator trick.

In your ViewModel

 val sourceProduct: LiveData<Product>() = repository.productFromDao()
 val product = MutableLiveData<Product>()
    
 val mediator = MediatorLiveData<Unit>()

 init {
      mediator.addSource(sourceProduct, { product.value = it })
 }

In fragment/activity

observe(mediator, {})
observe(product, { /* handle product */ })

Solution 3

Since Room doesn't support MutableLiveData and has support for LiveData only, your approach of creating a wrapper is the best approach I can think of. It will be complicated for Google to support MutableLiveDatasince the setValue and postValue methods are public. Where as for LiveData they are protected which gives more control.