如何通过 ViewModel 从 Fragment 观察 Repository LiveData

Question

I am having a hard time figuring out how I can connect my Repository and ViewModel 's live data in-case of @GET request and observe them in the fragment.

I don't have this problem when the request type is @POST because I can use Transformation.switchMap on the body and whenever the body changes repository's function gets invoked and emits value to the response live data something like this

val matchSetsDetail: LiveData<Resource<MatchDetailBean>> = Transformations.switchMap(matchIdLiveData) { matchId ->
        val body = MatchSetRequest(matchId)
        repository.getMatchSet(body)
    }

but in case of @GET request, I have several query parameter that my View supplies

I have this retrofit API call in repository class and the code looks like this

class Repository {
    fun checkInCheckOutUser(apiKey: String, userId: Int, status: String, latitude: Double, longitude: Double, checkedOn: Long): LiveData<Resource<BaseResponse>> = liveData {
            emit(Resource.Loading())
            try {
                val response: Response<BaseResponse> = ApiClient.coachApi.checkInCheckOutUser(apiKey, userId, status, latitude, longitude, checkedOn)
                if (response.isSuccessful && response.body() != null) {
                    if (response.body()!!.isValidKey && response.body()!!.success) {
                        emit(Resource.Success(response.body()!!))
                    } else {
                        emit(Resource.Failure(response.body()!!.message))
                    }
                } else {
                    emit(Resource.Failure())
                }
            } catch (e: Exception) {
                emit(Resource.Failure())
            }
        }
}

and ViewModel

class CheckInMapViewModel : ViewModel() {
    val checkInResponse: LiveData<Resource<BaseResponse>> = MutableLiveData()

        fun checkInCheckOut(apiKey: String, userId: Int, status: String, latitude: Double, longitude: Double, checkedOn: Long): LiveData<Resource<BaseResponse>> {
            return repository.checkInCheckOutUser(apiKey,userId,status,latitude,longitude,checkedOn)
        }
    }

The main problem is I want to observe checkInResponse the same way I am observing in case of @POST request but don't know how to pass observe repository LiveData as I did with my post request above using Transformations.switchMap . Can anyone help me with this case?

Edit - Here is my retrofit service class as asked

interface CoachApi {
    @POST(Urls.CHECK_IN_CHECK_OUT_URL)
    suspend fun checkInCheckOutUser(
        @Query("apiKey") apiKey: String,
        @Query("userId") userId: Int,
        @Query("status") status: String,
        @Query("latitude") latitude: Double,
        @Query("longitude") longitude: Double,
        @Query("checkedOn") checkedOn: Long
    ): Response<SelfCheckResponse>

    @POST(Urls.SELF_CHECK_STATUS)
    suspend fun getCheckInStatus(
        @Query("apiKey") apiKey: String,
        @Query("userId") userId: Int
    ): Response<SelfCheckStatusResponse>
}

Answer 1

The Transformations.switchMap() just utilizes MediatorLiveData . Since your use-case is a bit different, you could just directly implement it yourself.

class CheckInMapViewModel : ViewModel() {
    private val _checkInResponse = MediatorLiveData<Resource<BaseResponse>>
    val checkInResponse: LiveData<Resource<BaseResponse>> = _checkInResponse

    fun checkInCheckOut(apiKey: String, userId: Int, status: String, latitude: Double, longitude: Double, checkedOn: Long) {
        val data = repository.checkInCheckOutUser(apiKey,userId,status,latitude,longitude,checkedOn)
        _checkInResponse.addSource(data) {
            if (it is Resource.Success || it is Resource.Failure)
                _checkInResponse.removeSource(data)
            _checkInResponse.value = it
        }
    }
}

This code is assuming that data only emits one terminal element Resource.Success or Resource.Failure and cleans up the source with it.

Answer 2

Your desirable approach is possible using an intermediate LiveData which holds the request params, named queryLiveData . When the checkInCheckOut function is called, we set a new value for it that causes a change in checkInResponse . Then the change will be transformed into the result of repository.checkInCheckOutUser using switchMap .

CheckInMapViewModel:

class CheckInMapViewModel : ViewModel() {

    private val queryLiveData = MutableLiveData<CheckInCheckOutParam?>()

    init {
        queryLiveData.postValue(null)
    }

    val checkInResponse: LiveData<Resource<BaseResponse>> =
        queryLiveData.switchMap { query ->
            if(query == null) {
                AbsentLiveData.create()
            } else {
                repository.checkInCheckOutUser(
                    query.apiKey,
                    query.userId,
                    query.status,
                    query.latitude,
                    query.longitude,
                    query.checkedOn
                )
            }
        }

    fun checkInCheckOut(
        apiKey: String,
        userId: Int,
        status: String,
        latitude: Double,
        longitude: Double,
        checkedOn: Long
    ) {
        queryLiveData.postValue(
            CheckInCheckOutParam(apiKey, userId, status, latitude, longitude, checkedOn)
        )
    }

    private data class CheckInCheckOutParam(
        val apiKey: String,
        val userId: Int,
        val status: String,
        val latitude: Double,
        val longitude: Double,
        val checkedOn: Long
    )
}

AbsentLiveData:

/**
 * A LiveData class that has `null` value.
 */
class AbsentLiveData<T : Any?> private constructor(resource: Resource<T>) :
    LiveData<Resource<T>>() {

    init {
        // use post instead of set since this can be created on any thread
        postValue(resource)
    }

    companion object {

        fun <T> create(): LiveData<Resource<T>> {
            return AbsentLiveData(Resource.empty())
        }
    }
}

Answer 3

Try this:

class CheckInMapViewModel : ViewModel() {
    private val _checkInResponse: MediatorLiveData<Resource<BaseResponse>> = MediatorLiveData()
    val checkInResponse: LiveData<Resource<BaseResponse>>
    get() = _checkInResponse

    init {
        _checkInResponse.addSource(checkInCheckOut()) {
            _checkInResponse.value = it
        }
    }

    fun checkInCheckOut(apiKey: String, userId: Int, status: String, latitude: Double, longitude: Double, checkedOn: Long): LiveData<Resource<BaseResponse>> {
        return repository.checkInCheckOutUser(apiKey,userId,status,latitude,longitude,checkedOn)
    }
}