在另一个活动中加载ViewModel实例

Question

When app start I need to download data and populate all viewModels to avoid slowdowns during the app usage. First activity is:

public class MainActivity extends AppCompatActivity {
private MainViewModel mainViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);

   }
}

I'm thinking to create in the activity above the instance of the other activity's viewModel and pass it through the intent. I'm not sure if is it right to create the instance of a viewModel not in its consistent activity.

Answer 1

The ViewModel class is designed to store and manage UI-related data in a lifecycle conscious way.

If you want to share any data between your viewModels, the best idea would be creating a singleton data source and provide it inside of viewModels, such as SharedPreferences , sqlite datase, etc.

If you really need to use a viewModel as your data source you can create a singletone instance of it by implementing a custom ViewModelFactory which has been already discussed in this github issue :

you can use factory to make viewmodel and this factor will return single object of view model.. As:

class ViewModelFactory() : ViewModelProvider.Factory {

    override fun create(modelClass: Class): T {
        if (modelClass.isAssignableFrom(UserProfileViewModel::class.java)) {
            val key = "UserProfileViewModel"
            if(hashMapViewModel.containsKey(key)) {
                return getViewModel(key) as T
            } else {
                addViewModel(key, UserProfileViewModel())
            return getViewModel(key) as T
        }
    }
    throw IllegalArgumentException("Unknown ViewModel class")
    }

    companion object {
        val hashMapViewModel = HashMap<String, ViewModel>()

        fun addViewModel(key: String, viewModel: ViewModel) {
            hashMapViewModel.put(key, viewModel)
        }

        fun getViewModel(key: String): ViewModel? {
            return hashMapViewModel[key]
        }
    }
}

In Activity:

viewModelFactory = Injection.provideViewModelFactory(this)

// Initialize Product View Model
userViewModel = ViewModelProviders.of(this,viewModelFactory).get(UserProfileViewModel::class.java)

This will provide only single object of UserProfileViewModel which you can share between Activities.