Android Room LiveData select query parameters

Question

I decided to teach myself Java and Android by doing a simple database app. I already implemented some functionality the "lazy" way - all selects are done on the main thread.

Now I want to use LiveData for selects. I've read the simplistic training guide on android developers and implemented a more complex solution from codelabs guide, with LiveData and RecyclerView. Inserts, updates, deletes, and selects for whole tables work flawlessly, however I have no idea how to pass parameters to selects.

Example: I have an activity with scrollable list of all records and I want to apply some filters to the list (search). From what I understand the actual select method from DAO is called only once (when ViewModel is created), so how do I update query with new parameters?

Other example: I have other activity that displays all columns of a record (for viewing and editing). How do I pass id to query to select a single row?

My database code is more less the same as in this codelab

Edit: I finally did it like that: every time I want to update query parameters I call select from DAO (through ViewModel and Repo) and add a new observer to that new list. This solution doesn't seem optimal but I guess it works...

Carnes

you can have multiple select statements with Dao – Demeter

I thought of that, but let's say I want a text field in which user can type any search string – Carnes

so ? whats the issue here ?? though these doesnt require a select statement. – Demeter

you have your list from select all statement...then from that whole list filter the list which the user searches...and show the searched list – Demeter

hmmm... so basically filter using java not sql... maybe I'll try. Thanks for the idea – Carnes

yes..there are a lot of demos.. you can do whichever way you feel like.. :) it depends upon the function – Demeter

Answer 1

I think your solution is a Transformations.switchMap. The simplest example how it may works in the case from codelab

public class WordViewModel extends AndroidViewModel {
    private WordRepository mRepository;
    private LiveData<List<Word>> mAllWords;
    private LiveData<List<Word>> searchByLiveData;
    private MutableLiveData<String> filterLiveData = new MutableLiveData<>();

    public WordViewModel (Application application) {
        super(application);
        mRepository = new WordRepository(application);
        mAllWords = mRepository.getAllWords();
        searchByLiveData = Transformations.switchMap(filterLiveData, 
                                                     v -> mRepository.searchBy(v));
    }

    LiveData<List<Word>> getSearchBy() { return searchByLiveData; }
    void setFilter(String filter) { filterLiveData.setValue(filter); }
    LiveData<List<Word>> getAllWords() { return mAllWords; }
    public void insert(Word word) { mRepository.insert(word); }
}

So, when you set filter value, it will change value of searchByLiveData

I hope this helps you.

Halvah

Finally got some time to try your solution and it works wonderfully! Thank you so much! That's exactly what I was looking for. – Carnes

In repository.searchBy(v) it must be mRepository? – Anaplasty

@Anaplasty yes, i fix it. regarding the error, show me the code and perhaps i will be able to help you – Halvah

in the line void setFilter(String filter) { filterLiveData.value = filter; } the ".value" does not exist in filterLiveData – Anaplasty

@Anaplasty setValue, sure – Halvah

@Halvah it was helpful for me where I had to fetch data from Room based on various sort options selected by the user. Earlier, I was doing it in a very unusual way. Thanks! – Woundwort

Thank you very much! This has resolved a fair bit of headache for me :) – Jacqualinejacquard

Your code helped a lot. But there is a problem in the line : private LiveData<String> filterColor = new MutableLiveData<String>();instead use private MutableLiveData<String> filterColor = new MutableLiveData<String>(); because the LiveData's setValue() has a protected access while MutableLiveData's setValue() has a public access. – Aphorize

have save mylife – Verla