Having trouble in observing a liveData list in my viewModel?

Question

I am displaying markers from my database on my map, But i have a filter for the user from where he can adjust the time and the markers will show in that time. I want to observe the list that is updated each time when the user changes the time filter. He selects a start time and end time and I query from the database to get a list with that condition .. I want to observe this list? How can I do that?

my ViewModel:

      public MediatorLiveData<List<LocationTracking>> fetchData(LiveData<List<LocationTracking>> source){
    if (oldSource == null){
        oldSource = source;
    } else {
        list.removeSource(oldSource);
    }
    list.addSource(source, locationTrackings -> {
        list.postValue(locationTrackings);
    });
    return list;
}

And in my activity I'm doing this:

My callback isn't called for on change when a new value is inserted in my database. Any help would be highly appreciated.

When the user wants to see whole list without time filter.

mViewModel.fetchData(mViewModel.getFilteredLocationList(
                        mDataRange.get(0).getTimeStamp(),
                        mDataRange.get(1).getTimeStamp()
                )).observe(this,mFilteredListObserver);

When the user selects a time Filter.

mViewModel.fetchData(mViewModel.getFilteredLocationList(
                data.getLongExtra(LocationsFilterActivity.START_TIME, 0),
                data.getLongExtra(LocationsFilterActivity.END_TIME, 0)
        )).observe(this,mFilteredListObserver);

Are you getting the markers from database when coming to the screen first time ? — Muhammad Awais, Oct 23 '19 at 07:48
@MuhammadAwais No, I get them only on the click of a button or when the user sets the time range . — Muhammad Abdullah, Oct 23 '19 at 08:35
@MuhammadAbdullah have you solved your issue? Please let me know if my answer helps. — Wess, Nov 11 '19 at 09:06

Wess · Answer 1 · 2019-11-08T06:25:19.317

I've struggled with a similar problem. Once you understand the solution it's quite simple. You need to do something like this in your ViewModel:

private MutableLiveData<DataRange> dataRange = new MutableLiveData<DataRange>();
public LiveData<List<LocationTracking>> list = Transformations.switchMap(dataRange, 
newDataRangeValue -> getFilteredLocationList(
        newDataRangeValue.get(0).getTimeStamp(),
        newDataRangeValue.get(1).getTimeStamp())
);

public void onDataRangeChanged(DataRange dataRange) {
    this.dataRange.setValue(dataRange);
}

In your Activity observe the list. The switchMap() method will listen for any changes on dataRange so if you do mViewModel.onDataRangeChanged(newDataRange), getFilteredLocationList() will be called again and list will be updated in Activity.

I highly recommend reading this great article.

Having trouble in observing a liveData list in my viewModel?

1 Answers1