android – 无法创建ViewModel类的实例

我正在尝试使用Android架构组件编写示例应用程序,但即使在尝试了几天之后我也无法使用它.它给了我以上例外.

生命周期所有者： –

public class MainActivity extends LifecycleActivity {

    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = findViewById(R.id.tv_user);
        Postviewmodel viewmodel = viewmodelProviders.of(this).get(Postviewmodel.class);
        viewmodel.loadPosts();
        viewmodel.getPost().observe(this, new Observer<Post>() {
            @Override
            public void onChanged(@Nullable Post post) {
                if(post != null) {
                    textView.setText(post.toString());
                }
            }
        });
    }
}

视图模型： –

public class Postviewmodel extends viewmodel {
    private MediatorLiveData<Post> post;
    private PostRepository postRepo;

    Postviewmodel() {
        post = new MediatorLiveData<>();
        postRepo = new PostRepository();
    }

    public LiveData<Post> loadPosts() {
        post.addSource(postRepo.getPost(),
                post -> this.post.setValue(post)
        );
        return post;
    }

    @NonNull
    public LiveData<Post> getPost() {
        return post;
    }
}

解决方法:

让你的构造函数公开.