如何修复 'Cannot create an instance of ViewModel class'？

Question

我尝试了 MVVM 架构，我实现了所有必需的 classes 和方法。在 MainActivity 中创建 ViewModel class 的对象时，出现此错误 java.lang.RuntimeException: Cannot create an instance of class com.prathameshmore.getnotes.viewmodel.NoteViewModel。

我尝试使用 YouTube 教程中的这个示例。我做了所有正确的实施。我尝试制作 ViewModel class 和构造函数 public，但应用程序仍然在运行时崩溃。

MainActivity.java

public class MainActivity extends AppCompatActivity {

private NoteViewModel noteViewModel;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
    noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
        @Override
        public void onChanged(List<Note> notes) {
            Toast.makeText(MainActivity.this, "Updated", Toast.LENGTH_SHORT).show();
        }
    });

}
}

NoteViewModel.java

public class NoteViewModel extends AndroidViewModel {

private NoteRepository noteRepository;
private LiveData<List<Note>> allNotes;

public NoteViewModel(@NonNull Application application) {
    super(application);
    noteRepository = new NoteRepository(application);
    allNotes = noteRepository.getAllNotes();
}

public void insert(Note note) {
    noteRepository.insert(note);
}

public void update(Note note) {
    noteRepository.update(note);
}

public void delete(Note note) {
    noteRepository.delete(note);

}

public void deleteAllNotes() {
    noteRepository.deleteAllNotes();
}

public LiveData<List<Note>> getAllNotes() {
    return allNotes;
}

}

NoteRepository.java

public class NoteRepository {

private NoteDao noteDao;
private LiveData<List<Note>> allNotes;

public NoteRepository(Application application) {
    NoteDatabase database = NoteDatabase.getInstance(application);
    noteDao = database.noteDao();
    allNotes = noteDao.getAllNotes();
}

public void insert(Note note){
    new InsertNoteAsyncTask(noteDao).execute(note);
}

public void delete(Note note) {
    new DeleteNoteAsyncTask(noteDao).execute(note);
}

public void update(Note note) {
    new UpdateNoteAsyncTask(noteDao).execute(note);
}

public void deleteAllNotes() {
    new DeleteAllNotesAsyncTask(noteDao).execute();
}

public LiveData<List<Note>> getAllNotes() {
    return allNotes;
}

private static class InsertNoteAsyncTask extends AsyncTask<Note, Void, Void> {

    private NoteDao noteDao;

    private InsertNoteAsyncTask(NoteDao noteDao) {
        this.noteDao = noteDao;
    }

    @Override
    protected Void doInBackground(Note...notes) {
        noteDao.insert(notes[0]);
        return null;
    }
}

private static class UpdateNoteAsyncTask extends AsyncTask<Note, Void, Void> {

    private NoteDao noteDao;

    private UpdateNoteAsyncTask(NoteDao noteDao) {
        this.noteDao = noteDao;
    }

    @Override
    protected Void doInBackground(Note...notes) {
        noteDao.update(notes[0]);
        return null;
    }
}

private static class DeleteNoteAsyncTask extends AsyncTask<Note, Void, Void> {

    private NoteDao noteDao;

    private DeleteNoteAsyncTask(NoteDao noteDao) {
        this.noteDao = noteDao;
    }

    @Override
    protected Void doInBackground(Note...notes) {
        noteDao.delete(notes[0]);
        return null;
    }
}

private static class DeleteAllNotesAsyncTask extends AsyncTask<Void, Void, Void> {

    private NoteDao noteDao;

    private DeleteAllNotesAsyncTask(NoteDao noteDao) {
        this.noteDao = noteDao;
    }

    @Override
    protected Void doInBackground(Void...voids) {
        noteDao.deleteAllNotes();
        return null;
    }
}


}

日志

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.prathameshmore.getnotes, PID: 28833 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.prathameshmore.getnotes/com.prathameshmore.getnotes.views.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.prathameshmore.getnotes.viewmodel.NoteViewModel at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2723) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2784) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1523) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:163) at android.app.ActivityThread.main(ActivityThread.java:6238) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) Caused by: java.lang.RuntimeException: Cannot create an instance of class com.prathameshmore.getnotes.viewmodel.NoteViewModel at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:208) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103) at com.prathameshmore.getnotes.views.MainActivity.onCreate(MainActivity.java:25) at android.app.Activity.performCreate(Activity.java:6868) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2676) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2784) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1523) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:163) at android.app.ActivityThread.main(ActivityThread.java:6238) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.Constructor.newInstance0(Native Method) at java.lang.reflect.Constructor.newInstance(Constructor.java:430) at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:200) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103) at com.prathameshmore.getnotes.views.MainActivity.onCreate(MainActivity.java:25) at android.app.Activity.performCreate(Activity.java:6868) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2676) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2784) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1523) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:163) at android.app.ActivityThread.main(ActivityThread.java:6238) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) Caused by: java.lang.RuntimeException: cannot find implementation for com.prathameshmore.getnotes.database.NoteDatabase. NoteDatabase_Impl does not exist at androidx.room.Room.getGeneratedImplementation(Room.java:94) at androidx.room.RoomDatabase$Builder.build(RoomDatabase.java:851) at com.prathameshmore.getnotes.database.NoteDatabase.getInstance(NoteDatabase.java:31) at com.prathameshmore.getnotes.repository.NoteRepository.(NoteRepository.java:20) at com.prathameshmore.getnotes.viewmodel.NoteViewModel.(NoteViewModel.java:21) at java.lang.reflect.Constructor.newInstance0(Native Method) at java.lang.reflect.Constructor.newInstance(Constructor.java:430) at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:200) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103) at com.prathameshmore.getnotes.views.MainActivity.onCreate(MainActivity.java:25) at android.app.Activity.performCreate(Activity.java:6868) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2676) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2784) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1523) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:163) at android.app.ActivityThread.main(ActivityThread.java:6238) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794) Application terminated.

Answer 1

您的ViewModel需要一个工厂class。

public class MyViewModelFactory implements ViewModelProvider.Factory {
    private Application mApplication;
    private String mParam;


    public MyViewModelFactory(Application application, String param) {
        mApplication = application;
        mParam = param;
    }


    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        return (T) new MyViewModel(mApplication, mParam);
    }
}

并且在实例化视图模型时，您可以这样做：

MyViewModel myViewModel = ViewModelProviders.of(this, new MyViewModelFactory(this.getApplication(), "my awesome param")).get(MyViewModel.class);

Answer 2

这会很好：

1.Go 到您的 build.gradle(Module:app) 并将其添加到依赖项中：

`implementation "android.arch.lifecycle:extensions:$lifecycle_version"`

确保在你的依赖项中有 def lifecycle_version = "2.2.0"。
所以它应该是这样的：

dependencies {

    def lifecycle_version = "2.2.0"  //make sure to have this 

    //Some implementations ...  

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'


    implementation "android.arch.lifecycle:extensions:$lifecycle_version" // make sure to have this too.

}

2.Go 到您的 Activity（在您的情况下是 MainActivity）并输入以下行：

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

所以你的 MainActivity.java 应该是这样的：

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, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(NoteViewModel.class);
    noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
        @Override
        public void onChanged(List<Note> notes) {
            Toast.makeText(MainActivity.this, "Update, Enjoy coding :)", Toast.LENGTH_SHORT).show();
        }
    });

}
}

3.Run 你的应用。

Answer 3

考虑到 CBS 对 2.3.0-alpha01 更新的回答 lifecycle-extensions 工件不再发布。

Lifecycle Documentation

依赖关系

def lifecycle_version = "2.2.0"
implementation "androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
kapt "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

implementation "androidx.fragment:fragment-ktx:1.2.4"

AndroidViewModel Class

class TestVM (app : android.app.Application) : AndroidViewModel(app){
    //Code
}

在片段中

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    val viewModel = ViewModelProvider(this).get(TestVM::class.java)
}

Answer 4

放这个

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

而不是

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

如何修复 'Cannot create an instance of ViewModel class'？

How to fix 'Cannot create an instance of ViewModel class'?

java

android

mvvm

android-jetpack