ViewModelProviders is not working inside my Fragment

问题

This is what I'm trying to do.

• Set an ArrayList of object inside a Fragment
• Get that array from an observer within the FragmentActivity container (the activity that hosts all the fragments)

So, What I have done is the following.

First I created the SharedViewModel from where I will set and get the data from :

SharedViewModel.kt

class SharedViewModel: ViewModel() {

    var personArrayObj:MutableLiveData<ArrayList<Person>> = MutableLiveData()

    fun setPersonArray(personArray:ArrayList<Person>){
        personArrayObj.value = personArray
    }

    val getPersonArray:LiveData<ArrayList<Person>>
    get() = personArrayObj

}

Now , I instantiate this ViewModel inside the MainActivity that hosts all the fragments creation, so , I can get the ArrayData from wherever Fragment its set :

MainActivity.kt

class MainActivity: FragmentActivity(){

private lateinit var viewModel:SharedViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        viewModel = ViewModelProviders.of(this).get(SharedViewModel::class.java)
        viewModel.getPersonArray.observe(this, Observer { it:ArrayList<Person>
            Log.d("Array:",""+it)
        })
    }

}

Then the last thing I need to do is to set the value of that Array from whatever Fragment I want, and then the host Activity will be triggered if the value has changed, so, to do this, I simple do a viewModel.setValue(personArray) at my desired Fragment .

The Problem

When I try to instantiate the SharedViewModel inside any fragment I get this problem :

I tried to assert !! for the null, use let too, but is not working and I wonder from where it comes that FragmentActivity required

If I use viewLifecycleOwner instead of activity it says to me that it cant be called either with Fragment or FragmentActivity ?

Any clues?

Thanks .

回答1:

Fragments offer a helper method for retrieving a non-null Activity - requireActivity(). Use that instead of activity:

viewModel = ViewModelProviders.of(requireActivity()).get(SharedViewModel::class.java)

As an alternative, you can include the fragment-ktx dependency in your app and use the by activityViewModels() Kotlin property extension instead of using ViewModelProviders.of() at all:

val viewModel: SharedViewModel by activityViewModels()

回答2:

Basically, issue is that inside your fragment class activity context would be nullable and ViewModelPorviders require Non-null context to work with.

One solution would be making your ViewModel object nullable (because your activity is nullable) inside your fragment like something below:

var viewModel: SharedViewModel? = null

then during initialization,

viewModel = activity?.let { ViewModelProviders.of(it).get(SharedViewModel::class.java) }

回答3:

Ok, I was not seeing it

I was making an instance of the viewModel inside my Fragment, and another one, inside my MainActivity, so doing this, we prevent the observer to work.

To solve this, I just made a simple method that returns the instance that I made inside my MainActivity

 fun getViewModelInstance():SharedViewModel = viewModel

Then I just call that instance from any Fragment to work with it

(activity as MainActivity).getViewModelInstance().setPersonArray(personArray)

回答4:

Put the thing in onActivityCreated() instead of onCreateView()... That will solve the issue of getActivity() nullable... Because according to the lifecycle of a fragment, onActivityCreated() is after onCreareView()... And the activity is created on the call of onActivityCreated()... So, the getActivity() will return the activity just created

来源：https://stackoverflow.com/questions/56811012/viewmodelproviders-is-not-working-inside-my-fragment