Question

我已经创建了视图模型和存储库，其中在构造函数中我将 application 上下文作为视图模型和存储库类中的参数传递，但它显示错误并且应用程序崩溃。

以下是我的堆栈跟踪：

2021-02-14 03:24:20.782 8516-8516/com.app.itunes E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.app.itunes, PID: 8516
java.lang.RuntimeException: Cannot create an instance of class com.app.itunes.ViewModels.HistoryViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
    at com.app.itunes.fragments.HistoryFragment.onCreateView(HistoryFragment.java:38)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
    at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)

Caused by: java.lang.InstantiationException: java.lang.Class<com.app.itunes.ViewModels.HistoryViewModel> has no zero argument constructor
    at java.lang.Class.newInstance(Native Method)

下面是我的代码：

HistoryViewModel.java

public class HistoryViewModel extends ViewModel {

 public HistoryRepository historyRepository;
 public LiveData<List<Tracks>> allTracks;

 public HistoryViewModel(Application application){

    historyRepository = new HistoryRepository(application);
 }

 public LiveData<List<Tracks>> allTracks(){
     return historyRepository.myTracks();
  }
}

HistoryRepository.java

public class HistoryRepository {

public TrackDao trackDao;
public LiveData<List<Tracks>> allTracks;
public TrackDb db;

public HistoryRepository(Application application){

    db = TrackDb.getInstance(application);
    trackDao = db.trackDao();
    allTracks = trackDao.getAllTracks();
}

public LiveData<List<Tracks>> myTracks(){

    return allTracks;
 }

}

HistoryFragment.java

private void loadHistory(){

       historyViewModel = new ViewModelProvider(getActivity()).get(HistoryViewModel.class);

       fragmentHistoryBinding.historyRecycler.setHasFixedSize(true);
       fragmentHistoryBinding.historyRecycler.setLayoutManager(new GridLayoutManager(getActivity(),2,GridLayoutManager.VERTICAL, false));

       historyViewModel.allTracks().observe(getActivity(), new Observer<List<Tracks>>() {
           @Override
           public void onChanged(List<Tracks> tracks) {

               if(tracks.size() > 0){
                   HistoryAdapter adapter = new HistoryAdapter();
                   adapter.setTrackList(tracks);
               }
               else{

               }
           }
       });
}

我可以尝试什么来解决这个问题？

Answer 1

如果您需要一个可以访问 ViewModel 的 Jetpack Context — 并且如果您没有使用 Dagger/Hilt 或 Koin 等依赖倒置系统 — 您可以使用 AndroidViewModel。 ViewModelProviders 和相关类知道如何“开箱即用”使用 ViewModel 和 AndroidViewModel。

否则，具有带参数的构造函数的 ViewModel 需要您创建自定义 ViewModelProvider.Factory 实现并在请求您的视图模型实例时使用它。

无法创建 ViewModel 的实例

1 个答案: