MediatorLiveData or switchMap transformation with multiple parameters
Solution 1:
Source : https://plus.google.com/+MichielPijnackerHordijk/posts/QGXF9gRomVi
To have multiple triggers for switchMap()
, you need to use a custom MediatorLiveData
to observe the combination of the LiveData objects -
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));
}
});
}
}
Then you can do this -
CustomLiveData trigger = new CustomLiveData(code, nbDays);
LiveData<DayPrices> dayPrices = Transformations.switchMap(trigger,
value -> dbManager.getDayPriceData(value.first, value.second));
If you use Kotlin and want to work with generics:
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 }
}
}
Then:
val dayPrices = Transformations.switchMap(DoubleTrigger(code, nbDays)) {
dbManager.getDayPriceData(it.first, it.second)
}
Solution 2:
Custom MediatorLiveData
as proposed by @jL4 works great and is probably the solution.
I just wanted to share the simplest solution that I think is to use an inner class to represent the composed filter values :
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;
}
}
}
Then in the activity/fragment :
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
});
}
Solution 3:
A simplification of jL4's answer, (and also in Kotlin in case it helps anybody)... no need to create a custom class for this:
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
}
}
Explanation
Any update in firstLiveData
or secondLiveData
will update the value of combinedValues
, and emit the two values as a pair (thanks to jL4 for this).
Calling liveData.value
can be null, so this solution makes the values in Pair nullable to avoid Null Pointer Exception.
So for the actual results/datasource call, the switch map is on the combinedValues
live data, and the 2 values are extracted from the Pair
and null checks are performed, so you can be sure of passing non-null values to your data source.