如何在 Java 中使用 androidx.lifecycle.ViewModelProvider

Question

I'm recently learning about the architectural components and was following the old tutorial where they used the old method:

mainActivityViewModel =
    new ViewModelProvider(this).get(MainActivityViewModel.class);

But in the documentation for the ViewModelProvider, the only constructors available are ViewModelProvider(ViewModelStoreOwner, Factory) & ViewModelProvider(ViewModelStore, Factory) .

So I did something like this but I'm not sure what to do in the overridden method and it currently returns null that crashes the program.

public class MainActivity extends AppCompatActivity {
    private NoteViewModel noteViewModel;

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

        noteViewModel = new ViewModelProvider(this, new ViewModelProvider.Factory() {
            @NonNull
            @Override
            public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
                return null;
            }
        }).get(NoteViewModel.class);

        noteViewModel.getAllNotes().observe(this, new Observer<List<NoteEntity>>() {
            @Override
            public void onChanged(List<NoteEntity> noteEntities) {
                Toast.makeText(MainActivity.this,"Changed",Toast.LENGTH_LONG).show();
            }
        });
    }
}

Is my approach correct? I'm absolutely lost right now. What am I supposed to return from the overridden method?

Answer 1

Use this

noteViewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(NoteViewModel.class;

We use custom factory, when we pass param to the constructor of ViewModel (apart from Application param).

and gradle dependency in case,

def lifecycle_version = "2.2.0"

    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    //
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"

Answer 2

First you need to initialize viewmodel in activity class like :

noteViewModel = ViewModelProvider(this).get(NoteViewModel.class);

In NoteViewModel.java

You need to define livedata variable for storing updated data provided by model and update post to view model

NoteViewModel.java file look like :

public class NoteViewModel extends AndroidViewModel {
    AppRepository appRepository;
    MediatorLiveData<List<NoteEntity>> mediatorData;
   
    public NoteViewModel (@NonNull Application application) {
        super(application);
        mediatorData=new MediatorLiveData<>();
        appRepository=new AppRepository((MyApplication).apiService, application.retrofit);

    }

    public MediatorLiveData<List<NoteEntity>> getMediatorLiveData() {
        return mediatorVideoData;
    }

   

    public void getNoteEntry()
    {
        try {

            mediatorData.addSource(appRepository.getNoteEntry(), new Observer<List<NoteEntity>>() {
                @Override
                public void onChanged(@Nullable List<NoteEntity> data) {
                    mediatorData.postValue(data);
                }
            });
        }catch (Exception e)
        {

        }
    }

   
}

In onCreate() of Mainactivity register observer like and call the API from view model like

 noteViewModel.getMediatorLiveData().observe(this, new Observer<List<NoteEntity>>() {
            @Override
            public void onChanged(List<NoteEntity> noteEntities) {
                Toast.makeText(MainActivity.this,"Changed",Toast.LENGTH_LONG).show();
            }
        });

  noteViewModel.getNoteEntry();

AppRepository.java file look like

class AppRepository() {
ApiService apiService;
Retrofit retrofit;
public AppRepository(ApiService apiService ,Retrofit retrofit){
 this.apiService = apiService;
 this.retrofit = retrofit;
}

    public MediatorLiveData<List<NoteEntity>>  getNotes() {

        MediatorLiveData<List<NoteEntity>> data = new MediatorLiveData<List<NoteEntity>>()
       
        apiService.getNotes()
            .enqueue(new Callback<List<NoteEntity>> {

                @Override 
                void onResponse(
                    Call<List<NoteEntity>> call,
                    Response<List<NoteEntity>> response
                ) {
                    if (response.isSuccessful()) {
                        if(response.body()!=null){
                        data.postValue(response.body()); //successfull data
                         }else{
                         data.postValue(null); //error
                         }
                        
                    } else {
                        data.postValue(null); //error
                    }
                }
                 @Override
                fun onFailure(
                    Call<List<NoteEntity>> call,
                    Throwable t
                ) {
                     data.postValue(null); //error 
                }
            })

        return data;
    }
}

Answer 3

Kotlin Code.

In build.gradle add

implementation "androidx.activity:activity-ktx:1.1.0"

then

val noteViewModel by viewModels<NoteViewModel>()

Answer 4

Figured it out after 4 hours.

noteViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(NoteViewModel.class);

Also these dependencies should be added to the gradle file.

    def room_version = "2.2.5"
    def lifecycle_version = "2.2.0"

    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
// LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"

    implementation "androidx.room:room-runtime:$room_version"
    annotationProcessor "androidx.room:room-compiler:$room_version"

Not sure if the error was caused because I missed the annotation dependency. But everything works now.