CODEWITHSUNDEEP
androidandroid-architecture-componentsandroid-viewmodelandroid-mvvm
Jacek Kwiecień
Jacek Kwiecień

Reputation: 12639

ViewModel crashing while trying to recreate without factory

How to recreate ViewModel that was created with params injected via Factory after instance of activity is recreated?

ViewModel has a constructor like this: 

class MyViewModel(
        val model: MyModel,
        val repository: MyRepository
) : ViewModel()

I instantiate it with factory:

ViewModelProviders
                    .of(this, MyViewModelFactory(
                            model = MyModel()
                            repository = MyRepository()))
                    .get(MyViewModel::class.java)

I tried to recover ViewModel like this, while savedInstanceState is not null (activity is recreated):

ViewModelProviders
                    .of(this)
                    .get(MyViewModel::class.java)

This causes a crash because no 0 arguments constructor is present inside MyViewModel class.

As, for passing factory each time: My problem with that is, that whatever I pass as a MyModel to ViewModel, and that comes from activity Intent, might change later, due to user interaction. That would mean, when recreating, the MyModel in the Intent is outdated to the model that is already stored in ViewModel and was tampered by user interaction.

Upvotes: 1

Views: 1328

Answers (3)

theapache64
theapache64

Reputation: 11754

This causes a crash because no 0 arguments constructor is present inside MyViewModel class.

It'll crash, as you're not passing any factory to construct the ViewModel.

How to recreate ViewModel that was created with params injected via Factory after instance of activity is recreated?

AFAIK, you don't have to manually recreate the ViewModel on savedInstanceState. You may use viewModel to store data that are used in the activity.So, on recreation of the activity, the ViewModelProvider will not create a new instance of the viewModel but will give you the old instance and the data held in the viewModel will not be cleared.So there's no need to worry about savedInstanceState.

TIP: If you want to manage the creation of the factory and improve the recreation process. you may check this article on ViewModel with Dagger

Upvotes: 2

Jacek Kwiecień
Jacek Kwiecień

Reputation: 12639

Probably the only answer is, that it cannot be done this way. If the factory was provided, it has to be provided at all time. I don't know mechanism underneath ViewModel recreation but these are not as smart as I hoped.

My current implementation looks like this:

    viewModel = ViewModelProviders
            .of(this, MyFactory(MyRepository()))
            .get(MyMViewModel::class.java)

    val binding = DataBindingUtil.setContentView<ActivityCreateFoodstuffBinding>(this, R.layout.my_activity)
    binding.viewModel = viewModel

    if (savedInstanceState == null) {
        val model = intent.getParcelableExtra<MyModel>("model")
        viewModel.model.set(model)
    }

I use one param constructor in ViewModel that always takes repository, but model I moved away and set it only when the activity is freshly created.

Upvotes: 0

Arka Prava Basu
Arka Prava Basu

Reputation: 2560

There are no 0 argument constructors in MyViewModel. When you try to get the instance of ViewModel without providing a Factory it will look for a 0 argument constructor.

You can use this regardless of whether savedInstanceState is null or not.

ViewModelProviders
                    .of(this, MyViewModelFactory(
                            model,repository))
                    .get(MyViewModel::class.java)

Rather how you create repository and model changes on the basis of savedInstanceState value, depending on your use case or implementation.

Upvotes: 0

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