LiveData observer fired twice, even with viewLifecycleOwner

Question

I'm struggling with a LiveData observer which is firing twice. In my fragment I'm observing a LiveData as below, using viewLifeCycleOwner as LifeCycleOwner

private lateinit var retailViewModel: RetailsViewModel

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        retailViewModel =  ViewModelProviders.of(this).get(RetailsViewModel::class.java)
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {

  retailViewModel.retailLiveData.observe(viewLifecycleOwner, Observer {
    // updating UI here, but firing twice!
  }

  retailViewModel.getRetailById(retail.id)
}

And this is my view model:

class RetailsViewModel(override val service: MyFoodyApiService = MyFoodyApiService.service) :
    BaseViewModel(service) {

    var retailLiveData: MutableLiveData<Retail> = MutableLiveData()

    fun getRetailById(id: Int) {
        scope.launch {
            try {
                val response =
                    service.getRetailById(authString, id).await()
                when (response.isSuccessful) {
                    true -> {
                        response.body()?.let { payload ->
                            retailLiveData.postValue(payload.data)
                        } ?: run {
                            errorLiveData.postValue("An error occurred: ${response.message()}")
                        }
                    }
                    false -> errorLiveData.postValue("An error occurred: ${response.message()}")
                }
            } catch (e: Exception) {
                noConnectionLiveData.postValue(true)
            }
        }
    }

}

When I run the fragment for the first time, everything works fine, however when I go to its DetailFragment and come back, retailLiveData Observer callback is fired twice. According to this article this was a known problem solved with the introduction of viewLifeCycleOwner who should be helpful to remove active observers once fragment's view is destroyed, however it seems not helping in my case.

Maybe `retailViewModel.retailLiveData` is updated twice? Maybe this is expected. — EpicPandaForce, Aug 09 '19 at 16:11
Maybe this is related to the presenter/viewmodel logic, for example getting data once from local cache and then after updating the cache from remote source it is fired again, right? — CodeRanger, Aug 09 '19 at 16:15
@EpicPandaForce code updated with the call witch triggers the live data. I call that getRetailById only right after the Observer. — Nicola Gallazzi, Aug 09 '19 at 16:21
@CodeRanger updated with my viewmodel, no local caching provided yet — Nicola Gallazzi, Aug 09 '19 at 16:24
Where do you call `retailViewModel.getRetailById(retail.id)`? — esentsov, Aug 09 '19 at 16:43
I mean, yeah, you're subscribing (receiving previously obtained value), and you are triggering a new change by calling `getRetailById` which fetches new data, so it will be called twice. What is unexpected here? — EpicPandaForce, Aug 09 '19 at 17:00
@EpicPandaForce so I am supposed to call getRetailById in my OnCreate method, isn't it? — Nicola Gallazzi, Aug 09 '19 at 17:31
maybe considering onAttach() for your view model operations is a good fix. — CodeRanger, Aug 10 '19 at 10:45

esentsov · Accepted Answer · 2019-08-09T16:59:53.727

6

This happens because view model retains value when you open another fragment, but the fragment's view is destroyed. When you get back to the fragment, view is recreated and you subscribe to retailLiveData, which still holds the previous value and notifies your observer as soon as fragment moves to started state. But you are calling retailViewModel.getRetailById(retail.id) in onViewCreated, so after awhile the value is updated and observer is notified again.

One possible solution is to call getRetailById() from view model's init method, the result will be cached for view model lifetime then.

edited Aug 09 '19 at 16:59

answered Aug 09 '19 at 16:53

esentsov

6,372
21
28

Thanks @esentov this could be a solution, however if I call getRetailById() before adding subscribers (I'm thinking about the first run of the fragment) am I going to receive the callback in the same way? – Nicola Gallazzi Aug 09 '19 at 17:28
You'll receive callback only once, because `retailLiveData` does not have initial value, so the observer will not be notified when it's added. It will be notified only when value is actually set to live data. – esentsov Aug 09 '19 at 17:32

LiveData observer fired twice, even with viewLifecycleOwner

1 Answers1