how to change LiveData observable query parameter with room database in android?

Question

I am using live data with room database and my activity observes live data provided from room database.

@Query("SELECT * FROM BUS WHERE BUS_CATEGORY = :busCategory")
LiveData<List<Bus>> getLiveBuses( String busCategory);

ViewModels gets LiveData via Dao(Data Access Object) and activity observes this live data.

Now it works fine. But when busCategory changes i can't modify this live data to get buses for newly selected busCategory.

So how can i observe this same liveData where query parameters is changeable?

when changing your `busCategory` you need to call that `viewmodel` method again, to fetch new data with new `busCategory`. — Muhammad Awais, Oct 09 '19 at 11:18
@MuhammadAwais then it observes for both bus category, in onChanged method it also gets value for previous busCategory — Bashir Fardoush, Oct 09 '19 at 11:26
For the second method you don't need to call `observe` method. As it is already observing in your view. Once the data is changed your `onChanged()` method will be called with new data. — Muhammad Awais, Oct 09 '19 at 11:27
I have the same problem. Need to change the query param. After days of tests and searches, I did not come with a clear answer. Hope someone will illuminate us. — Dan Alboteanu, Oct 30 '19 at 19:36

score 21 · Accepted Answer · edited Oct 30 '19 at 20:00

I suggest you to to use viewModel. I did the query and observe changes using MutableLiveData. First step

val mutableBusCategory: MutableLiveData<String> = MutableLiveData()

Setter for mutablelivedata

fun searchByCategory(param: String) {
    mutableBusCategory.value = param
}

observable to observe the change

val busObservable: LiveData<Bus> = Transformations.switchMap(mutableBusCategory) { param->
    repository.getLiveBuses(param)
}

and final step to observe the live data

 busObservable.observe(this, Observer {
        //your logic for list})

and to trigger mutablelivedata

searchByCategory(//categoryName)

I have more than 1 param. How can I use this approach? – The Bat Jan 11 '21 at 12:32 — The Bat, Jan 11 '21 at 12:32

score 0 · Answer 2 · answered Oct 09 '19 at 11:15

0

I don't think this is a reasonable expectation. It would make more sense to fire off a new query and subscribe to that.

answered Oct 09 '19 at 11:15

ayoola adedeji

i can remove observer and set new observer and call for new livedata. But how to stop current live data instance ? so that room will not emit data for current livedata instance. – Bashir Fardoush Oct 09 '19 at 11:32
Assuming you only want one response you can unsubscribe to it after getting that response? – ayoola adedeji Oct 10 '19 at 12:03

2 Answers2