android mvvm框架搭建请给我写一份代码

好的，以下是一个基于Android MVVM框架搭建的简单示例代码：

Model层（数据源）：

public class UserRepository {
    private UserApi userApi;

    public UserRepository() {
        userApi = RetrofitClient.getInstance().create(UserApi.class);
    }

    public LiveData<User> getUser(String userId) {
        final MutableLiveData<User> data = new MutableLiveData<>();
        userApi.getUser(userId).enqueue(new Callback<UserResponse>() {
            @Override
            public void onResponse(Call<UserResponse> call, Response<UserResponse> response) {
                if (response.isSuccessful()) {
                    data.setValue(response.body().getData());
                }
            }

            @Override
            public void onFailure(Call<UserResponse> call, Throwable t) {
                data.setValue(null);
            }
        });
        return data;
    }
}

ViewModel层（数据处理）：

public class UserViewModel extends ViewModel {
    private UserRepository userRepository;
    private MutableLiveData<User> userLiveData = new MutableLiveData<>();

    public UserViewModel() {
        userRepository = new UserRepository();
    }

    public LiveData<User> getUser(String userId) {
        userLiveData = userRepository.getUser(userId);
        return userLiveData;
    }
}

View层（界面展示）：

public class UserActivity extends AppCompatActivity {
    private UserViewModel userViewModel;
    private TextView tvUserId;
    private TextView tvUserName;

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

        userViewModel = ViewModelProviders.of(this).get(UserViewModel.class);

        tvUserId = findViewById(R.id.tv_user_id);
        tvUserName = findViewById(R.id.tv_user_name);

        String userId = getIntent().getStringExtra("userId");
        userViewModel.getUser(userId).observe(this, new Observer<User>() {
            @Override
            public void onChanged(User user) {
                if (user != null) {
                    tvUserId.setText(user.getId());
                    tvUserName.setText(user.getName());
                } else {
                    Toast.makeText(UserActivity.this, "获取用户信息失败", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

以上代码实现了从后台获取User对象并展示到界面上的功能，具体使用时可以根据实际需求调整。