Android Room - 汇总值并将其放入 TextView

Question

也许这有点愚蠢，但我只是坚持这个：

我有我的房间数据库 + 实体 + Daos。

实体有一个名为“成本”的列 - 所以基本上我想总结所有成本（当然，在跟踪是否有新输入时，请检查总和是否已更改 - 如果是，当然更改值/textview)，然后简单地将带有 .toString() 的值放入 TextView。

我已经有一个带有 Adapter / ViewModel / Repository 类的 RecyclerView+CardView（他们只将输入的名称放入 CardView）

我只是不知道将成本放在哪里 - 我认为我不需要新的 ViewModel/Repository 但我应该将我的 Dao 的 sumAllCost() 方法放在哪里

public class SubRepository {

private SubDAO mSubDao;
private LiveData<List<Sub>> mAllSubs;

SubRepository(Application application){
    SubDatabase db = SubDatabase.getInstance(application);
    mSubDao = db.getsubDAO();
    mAllSubs = mSubDao.getAllSubs();
}

public LiveData<List<Sub>> getAllSubs(){
    return mAllSubs;
}

public void insert (Sub sub){
    new insertAsyncTask(mSubDao).execute(sub);
}

private static class insertAsyncTask extends AsyncTask<Sub, Void, Void> {
    private SubDAO mAsyncTaskDao;

    insertAsyncTask (SubDAO dao){
        mAsyncTaskDao = dao;
    }

    @Override
    protected Void doInBackground(final Sub... params){
        mAsyncTaskDao.addSub(params[0]);
        return null;
    }
}
}

---

public class SubViewModel extends AndroidViewModel {

private SubRepository mRepository;

private LiveData<List<Sub>> mallSubs;

public SubViewModel (Application application) {
    super (application);
    mRepository = new SubRepository(application);
    mallSubs = mRepository.getAllSubs();
}

public LiveData<List<Sub>> getAllSubs(){
    return mallSubs;
}

public void insert (Sub sub){
    mRepository.insert(sub);
}}

---

class HomeSubAdapter extends RecyclerView.Adapter<HomeSubAdapter.ViewHolder> {

List<Sub> subList;
Context context;
private final LayoutInflater mInflater;

public static FragmentManager fragmentManager;

HomeSubAdapter(Context context) {
    mInflater = LayoutInflater.from(context);
}


//Just wanna show the name so only one tv
public class ViewHolder extends  RecyclerView.ViewHolder {
    public TextView main_cv_dummy;
    public ViewHolder(View itemView) {
        super(itemView);
        main_cv_dummy = itemView.findViewById(R.id.main_cv_dummy);


    }
}





@Override
public HomeSubAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
    View subview = mInflater.inflate(R.layout.snippet_main_cardview, parent, false);
    return new ViewHolder(subview);
}

@Override
public void onBindViewHolder (HomeSubAdapter.ViewHolder holder, final int position) {

    if (subList!= null){
        final Sub current = subList.get(position);
        holder.main_cv_dummy.setText(current.getSubName());

        holder.main_cv_dummy.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //Fragment currentFrag = ShowInfoFragment.newInstance(current);
                //fragmentManager.beginTransaction().replace(R.id.compLayout, currentFrag).addToBackStack(null).commit();
                Log.d("Clicking: ", current.getSubName() + " was clicked - Listener worked");
            }
        });
    }
    else {
        holder.main_cv_dummy.setText("Create a new Sub!");
    }

}

public void setSubs(List<Sub> subs){
    this.subList = subs;
    notifyDataSetChanged();
}






@Override
public int getItemCount(){
    if(subList!=null){
        return subList.size();
    } else
    {
        return 0;
    }
}

---

带有 onCreate 的 Activity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);
    Log.d(TAG, "onCreate: starting");
    middle_vf = (ViewFlipper)findViewById(R.id.middle_vf);

    tv_sum = (TextView) findViewById(R.id.tv_sumCosts);




    mSubViewModel = ViewModelProviders.of(this).get(SubViewModel.class);

    mSubViewModel.getAllSubs().observe(this, new Observer<List<Sub>>() {
        @Override
        public void onChanged(@Nullable List<Sub> subs) {
            mAdapter.setSubs(subs);

        }
    });


    fragmentManager = getSupportFragmentManager();

    //Whenever screen is called - call for recycler view
    mRecyclerView = (RecyclerView) findViewById(R.id.main_rv);
    mLayoutManager = new LinearLayoutManager(this);
    mRecyclerView.setLayoutManager(mLayoutManager);




    //give dummy-subs to Adapter to Recycler
    mAdapter = new HomeSubAdapter(this);
    mRecyclerView.setAdapter(mAdapter);


    setupBottomNavigationView();
    setupFAB();

}

---

我真的不确定这整个 SUM 的事情

//Get all costs
@Query("SELECT SUM(costs) FROM subscriptions")
void getAllCosts();

//Get all subs
@Query("SELECT * FROM subscriptions")
LiveData<List<Sub>> getAllSubs();

---

也许有人可以帮忙 - 我现在觉得很愚蠢对不起！我已经多次搜索合适的解决方案，但我找不到对我有帮助（或我理解）的东西

Answer 1

您应该为合成列提供别名（即使没有它也可以工作，但一般来说，访问域聚合函数结果的列名更容易）：

SELECT SUM(costs) AS value FROM subscriptions

当一个查询期望返回一些不能使用void（无）的东西时，那应该是实际返回的数据类型；对于货币价值和LiveData，请尝试LiveData<BigDecimal>。 documentation 也有一个相关的例子，可能会更详细地解释它。