ChangeListener is not being called on RealmObject

[Murtowski]

classpath "io.realm:realm-gradle-plugin:3.5.0"

I'm converting objects froming from realm to LiveData (lastly introduced) using extension method

fun<T: RealmObject> T.asLiveData(realm: Realm) = LiveDataRealmObject(this, realm)

Belo is how I convert RealmObject from Realm to set them to LiveData
LiveDataRealmObject::class

class LiveDataRealmObject<T : RealmObject>(private val result: T, realm: Realm) : LiveData<T>() {
    
    private val listener = RealmChangeListener<T> { t ->
        if (t != null && t.isValid)
            realm.executeTransaction {
                value = realm.copyFromRealm(t)
            }

    }

    override fun onActive() {
        result.addChangeListener(listener)
    }

    override fun onInactive() {
        result.removeChangeListener(listener)
    }
}

I retrieve results like this.

fun getCarDetails(): LiveData<CarDetails> {
        return (mRealm.where(CarDetails::class.java).equalTo(CarDetails.ID, 1).findFirstAsync()).asLiveData(mRealm)
    }

And save new one, either override like this

fun saveCarDetails(carDetails: CarDetails){
        mRealm.executeTransactionAsync { realm ->
            carDetails.id = 1 // keep only one copy of the object
            realm?.copyToRealmOrUpdate(carDetails)
// listener isn't trigged
        }
    }

However listener of LiveDataRealmObject is not called - it is called just one after first results came.
This works if I'would operate on RealmResults by quering .findAllAsync()

[Zhuinden]

it is called just one after first results came.

This is expected behavior, see documentation:

Similar to findFirst() but runs asynchronously on a worker thread. An listener should be registered to the returned RealmObject to get the notification when query completes. The registered listener will also be triggered if there are changes made to the queried RealmObject. If the RealmObject is deleted, the listener will be called one last time and then stop. The query will not be re-run.

And if the object did not exist, then the query will return with isValid() == false and will not be re-run.

You can also see #4360 (comment) and #4352 (comment)

If you need to observe whether a RealmObject exists, then you'll need to obtain RealmResults<T> with findAllAsync()

[Murtowski]

That means copyOrUpdate remove object if it want to updateIt?

[Zhuinden]

No, but it means that if the object didn't exist, then findFirstAsync() won't return it for you in the change listener after it was added.

[Murtowski]

ok I understand

[dalinaum]

Hello. @GitHubMurt

As @Zhuinden said, this is expected behavior.

I am closing this issue. If you have another idea, feedback, we can discuss it in another issue. Thanks.