是否可以制作两个 LiveData 的一个 LiveData? [英] Is it possible to make one LiveData of two LiveDatas?

问题描述

我有两个 DAO、两个存储库和两个 POJO.有什么方法可以创建两个的一个 Livedata?我需要它来为 Recyclerview 制作单个列表.POJO 是类似的对象.

I have two DAOs, two Repositories and two POJOs. There is some way to create one Livedata of two? I need it to make single list for Recyclerview. POJOs are similar objects.

费用存储库:

public class ExpenseRepository {

private ExpenseDao expenseDao;
private LiveData<List<Expense>> allExpenses;

public ExpenseRepository(Application application) {
    ExpenseIncomeDatabase database = ExpenseIncomeDatabase.getInstance(application);
    expenseDao = database.expenseDao();
    allExpenses = expenseDao.getExpensesByDay();
}

public LiveData<List<Expense>> getAllExpensesByDay() {
    return allExpenses;
}

收入库:

public class IncomeRepository {

private IncomeDao incomeDao;
private LiveData<List<Income>> allIncomes;

public IncomeRepository(Application application) {
    ExpenseIncomeDatabase database = ExpenseIncomeDatabase.getInstance(application);
    incomeDao = database.incomeDao();
    allIncomes = incomeDao.getIncomesByDay();
}

public LiveData<List<Income>> getAllIncomesByDay() {
    return allIncomes;
}

费用道:

@Dao
public interface ExpenseDao {

@Query("SELECT * FROM expense_table ORDER BY day") 
LiveData<List<Expense>> getExpensesByDay();

收入道:

@Dao
public interface IncomeDao {

@Query("SELECT * FROM income_table ORDER BY day") 
LiveData<List<Income>> getIncomesByDay();

每日视图模型:

public class DailyFragmentViewModel extends AndroidViewModel {

private ExpenseRepository expenseRepository;
private IncomeRepository incomeRepository;
private LiveData<Pair<List<Expense>, List<Income>>> combined;
private ExpenseDao expenseDao;
private IncomeDao incomeDao;

public DailyFragmentViewModel(@NonNull Application application) {
    super(application);
    expenseRepository = new ExpenseRepository(application);
    incomeRepository = new IncomeRepository(application);
    combined = new DailyCombinedLiveData(expenseDao.getExpensesByDay(), incomeDao.getIncomesByDay());
}

public LiveData<Pair<List<Expense>, List<Income>>> getExpensesAndIncomes() {
    return combined;
}

推荐答案

在 kotlin 中创建了这个扩展函数

Created this extension function in kotlin

fun <A, B> LiveData<A>.zipWith(stream: LiveData<B>): LiveData<Pair<A, B>> {
 val result = MediatorLiveData<Pair<A, B>>()
  result.addSource(this) { a ->
    if (a != null && stream.value != null) {
        result.value = Pair(a, stream.value!!)
    }
  }
 result.addSource(stream) { b ->
    if (b != null && this.value != null) {
        result.value = Pair(this.value!!, b)
    }
 }
 return result
}

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