是否可以将两个 LiveData 做成一个 LiveData？

Question

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 是相似的对象。

费用库：

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;
}

ExpenseDao:

@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();

DailyViewModel：

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;
}

Answer 1

我猜你想把它们结合起来，是吗？您将需要一个 MediatorLiveData，但是说您现在需要 Object 的人是错误的。你需要的是 MediatorLiveData<Pair<List<Expense>, List<Income>>>.

public class CombinedLiveData extends MediatorLiveData<Pair<List<Expense>, List<Income>>> {
    private List<Expense> expenses = Collections.emptyList();
    private List<Income> incomes = Collections.emptyList();

    public CombinedLiveData(LiveData<List<Expense>> ld1, LiveData<List<Income>> ld2) {
        setValue(Pair.create(expenses, incomes));

        addSource(ld1, expenses -> { 
             if(expenses != null) {
                this.expenses = expenses;
             } 
             setValue(Pair.create(expenses, incomes)); 
        });

        addSource(ld2, incomes -> { 
            if(incomes != null) {
                this.incomes = incomes;
            } 
            setValue(Pair.create(expenses, incomes));
        });
    }
}

您可以使它成为通用的，它是 combineLatest 的实现，用于使用 2-arity (Pair) 的元组的两个 LiveData。

编辑： 像这样：

public class CombinedLiveData2<A, B> extends MediatorLiveData<Pair<A, B>> {
    private A a;
    private B b;

    public CombinedLiveData2(LiveData<A> ld1, LiveData<B> ld2) {
        setValue(Pair.create(a, b));

        addSource(ld1, a -> { 
             if(a != null) {
                this.a = a;
             } 
             setValue(Pair.create(a, b)); 
        });

        addSource(ld2, b -> { 
            if(b != null) {
                this.b = b;
            } 
            setValue(Pair.create(a, b));
        });
    }
}

请注意，在这种情况下我无法将 Collections.emptyList() 设置为 A 和 B 的初始值，您将需要检查 nulls当您访问该对中的数据时。

编辑： 您可以使用库 https://github.com/Zhuinden/livedata-combinetuple-kt (Kotlin) or https://github.com/Zhuinden/livedata-combineutil-java/ (Java) 做同样的事情。

Answer 2

而不是让 class 添加 2 个实时数据，另一个 class 添加 3 个实时数据，等等。我们可以使用更抽象的方式，我们可以添加尽可能多的实时数据我们想要。

import androidx.lifecycle.LiveData
import androidx.lifecycle.MediatorLiveData

/**
 * CombinedLiveData is a helper class to combine results from multiple LiveData sources.
 * @param liveDatas Variable number of LiveData arguments.
 * @param combine   Function reference that will be used to combine all LiveData data.
 * @param R         The type of data returned after combining all LiveData data.
 * Usage:
 * CombinedLiveData<SomeType>(
 *     getLiveData1(),
 *     getLiveData2(),
 *     ... ,
 *     getLiveDataN()
 * ) { datas: List<Any?> ->
 *     // Use datas[0], datas[1], ..., datas[N] to return a SomeType value
 * }
 */
class CombinedLiveData<R>(vararg liveDatas: LiveData<*>,
                          private val combine: (datas: List<Any?>) -> R) : MediatorLiveData<R>() {

    private val datas: MutableList<Any?> = MutableList(liveDatas.size) { null }

    init {
        for(i in liveDatas.indices){
            super.addSource(liveDatas[i]) {
                datas[i] = it
                value = combine(datas)
            }
        }
    }
}

Answer 3

在 kotlin 中创建了这个扩展函数

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
}

是否可以将两个 LiveData 做成一个 LiveData？

Is it possible to make one LiveData of two LiveDatas?

android

android-room

android-livedata