多参数MediatorLiveData或switchMap变换

Question

我在我的ViewModel中使用ViewModel，所以我在片段中观察到的LiveData集合会对code参数的变化做出反应。

这是非常有效的：

public class MyViewModel extends AndroidViewModel {

    private final LiveData<DayPrices> dayPrices;
    private final MutableLiveData<String> code = new MutableLiveData<>();
    // private final MutableLiveData<Integer> nbDays = new MutableLiveData<>();
    private final DBManager dbManager;

    public MyViewModel(Application application) {
        super(application);
        dbManager = new DBManager(application.getApplicationContext());
        dayPrices = Transformations.switchMap(
            code,
            value -> dbManager.getDayPriceData(value/*, nbDays*/)
        );
    }

    public LiveData<DayPrices> getDayPrices() {
        return dayPrices;
    }

    public void setCode(String code) {
        this.code.setValue(code);
    }

    /*public void setNbDays(int nbDays) {
        this.nbDays.setValue(nbDays);
    }*/

}

public class MyFragment extends Fragment {

    private MyViewModel myViewModel;

    myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
    myViewModel.setCode("SO");
    //myViewModel.setNbDays(30);
    myViewModel.getDayPrices().observe(MyFragment.this, dataList -> {
        // update UI with data from dataList
    });
}

问题

现在我需要另一个参数(在上面的代码中注释了nbDays)，这样我的LiveData对象就可以对两个参数的更改(code和nbDays)做出反应。

我如何链式转换？

一些阅读指出了MediatorLiveData，但它并没有解决我的问题(仍然需要调用带有两个参数的单个DB函数，我不需要合并2 liveDatas)。

所以我尝试了这一点，而不是switchMap，但是code和nbDays总是空的。

dayPrices.addSource(
    dbManager.getDayPriceData(code.getValue(), nbDays.getValue),
    apiResponse -> dayPrices.setValue(apiResponse)
);

一种解决方案是将一个对象作为单个参数传递给我，我确信这有一个简单的解决方案。

Answer 1

来源：https://plus.google.com/+MichielPijnackerHordijk/posts/QGXF9gRomVi

要为switchMap()拥有多个触发器，您需要使用自定义MediatorLiveData来观察LiveData对象的组合-

class CustomLiveData extends MediatorLiveData<Pair<String, Integer>> {
    public CustomLiveData(LiveData<String> code, LiveData<Integer> nbDays) {
        addSource(code, new Observer<String>() {
            public void onChanged(@Nullable String first) {
                setValue(Pair.create(first, nbDays.getValue()));
            }
        });
        addSource(nbDays, new Observer<Integer>() {
            public void onChanged(@Nullable Integer second) {
                setValue(Pair.create(code.getValue(), second));
            }
        });
    }
}

然后你就能做到-

CustomLiveData trigger = new CustomLiveData(code, nbDays);
LiveData<DayPrices> dayPrices = Transformations.switchMap(trigger, 
    value -> dbManager.getDayPriceData(value.first, value.second));

如果您使用Kotlin并希望使用泛型：

class DoubleTrigger<A, B>(a: LiveData<A>, b: LiveData<B>) : MediatorLiveData<Pair<A?, B?>>() {
    init {
        addSource(a) { value = it to b.value }
        addSource(b) { value = a.value to it }
    }
}

然后：

val dayPrices = Transformations.switchMap(DoubleTrigger(code, nbDays)) {
    dbManager.getDayPriceData(it.first, it.second)
}

Answer 2

@jL4 4建议的自定义MediatorLiveData工作得很好，很可能是解决方案。

我只想分享我认为最简单的解决方案，就是使用内部类来表示组合的过滤器值：

public class MyViewModel extends AndroidViewModel {

    private final LiveData<DayPrices> dayPrices;
    private final DBManager dbManager;
    private final MutableLiveData<DayPriceFilter> dayPriceFilter;

    public MyViewModel(Application application) {
        super(application);
        dbManager = new DBManager(application.getApplicationContext());
        dayPriceFilter = new MutableLiveData<>();
        dayPrices = Transformations.switchMap(dayPriceFilter, input -> dbManager.getDayPriceData(input.code, input.nbDays));
    }

    public LiveData<DayPrices> getDayPrices() {
        return dayPrices;
    }

    public void setDayPriceFilter(String code, int nbDays) {
        DayPriceFilter update = new DayPriceFilter(code, nbDays);
        if (Objects.equals(dayPriceFilter.getValue(), update)) {
            return;
        }
        dayPriceFilter.setValue(update);
    }

    static class DayPriceFilter {
        final String code;
        final int nbDays;

        DayPriceFilter(String code, int nbDays) {
            this.code = code == null ? null : code.trim();
            this.nbDays = nbDays;
        }
    }

}

然后在活动/片段中：

public class MyFragment extends Fragment {

    private MyViewModel myViewModel;

    myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
    myViewModel.setDayPriceFilter("SO", 365);
    myViewModel.getDayPrices().observe(MyFragment.this, dataList -> {
        // update UI with data from dataList
    });
}

Answer 3

简化了jL4的答案，(也在Kotlin中，以防对任何人有帮助).不需要为此创建自定义类：

class YourViewModel: ViewModel() {

    val firstLiveData: LiveData<String> // or whatever type
    val secondLiveData: LiveData<Int> // or whatever

    // the Pair values are nullable as getting "liveData.value" can be null
    val combinedValues = MediatorLiveData<Pair<String?, Int?>>().apply {
        addSource(firstLiveData) { 
           value = Pair(it, secondLiveData.value)
        }
        addSource(secondLiveData) { 
           value = Pair(firstLiveData.value, it)
        }
    }

    val results = Transformations.switchMap(combinedValues) { pair ->
      val firstValue = pair.first
      val secondValue = pair.second
      if (firstValue != null && secondValue != null) {
         yourDataSource.yourLiveDataCall(firstValue, secondValue)
      } else null
    }

}

解释

firstLiveData或secondLiveData中的任何更新都将更新combinedValues的值，并将这两个值作为一对发出(这要感谢jL4 )。

调用liveData.value可以为null，因此此解决方案使成对的值可为空，以避免空指针异常。

因此，对于实际的结果/数据源调用，开关映射位于combinedValues活动数据上，从Pair中提取两个值并执行空检查，这样就可以确保将非空值传递给数据源。