Calling Web Service with Android Architecture Component

Introduction

How does the architecture look like?

The picture above is taken from the official Android site. I would like to classify the above picture into two layers.

1. Repository: Talking to the local data source(database) and remote data source(web service)
2. Remote data source: Connecting to web service

1. ViewModel: Communicating to Repository then update the data to UI (fragment/activity) respectively
2. Fragment/Activity: All about the UI (set up and update)

The demo application fetches the data from https://swapi.co. The application will display a list of species in Star War.

Data Layer

interface Swapi {
    @GET("species/")
    fun getSpecies(): Call<SpeciesList>
}

class SpeciesRepositoryImpl(val apiService: Swapi) : SpeciesRepository {

    override fun getSpecies(successHandler: (List<Species>?) -> Unit, failureHandler: (Throwable?) -> Unit) {
        apiService.getSpecies().enqueue(object : Callback<SpeciesList> {
            override fun onResponse(call: Call<SpeciesList>?, response: Response<SpeciesList>?) {
                response?.body()?.let {
                    successHandler(it.speciesList)
                }
            }

            override fun onFailure(call: Call<SpeciesList>?, t: Throwable?) {
                failureHandler(t)
            }
        })
    }
}

1. (successHandler: (List<Species>?) -> Unit ) This function is called when the call to the web service is successful. It takes List<Species> which is the response data from web service as an argument. You will see later how ViewModelcan access this data
2. (failureHandler: (Throwable?) -> Unit) This function is invoked when the call to web service is failing. It also takes Throwable as an argument that will be manipulated in ViewModel.

Presentation Layer

class SpeciesListViewModel : ViewModel() {

    @Inject
    lateinit var repository: SpeciesRepository

    var isLoading = MutableLiveData<Boolean>()

    var apiError = MutableLiveData<Throwable>()

    var speciesResponse = MutableLiveData<List<Species>>()

    fun getSpecies() {
        isLoading.value = true
        repository.getSpecies(
                {
                    speciesResponse.value = it
                    isLoading.value = false
                },

                {
                    apiError.value = it
                    isLoading.value = false
                })
    }
    //ignore other code
}

1. isLoading: set to true when start calling web service, and change to false when the call is successful or failure.
2. apiError: if there is an error in calling web service the Throwable data will be set to this variable.
3. speciesResponse: when the data is successfully obtained from web service, the response data will be set to this variable.

1. The first argument is the function that will be triggered when calling the web service is successful. There will be response data returning from web service can be referred to them as it
2. The second argument function is triggered when calling to the web service is fail and the Throwable data can be referred to as it.

class SpeciesActivity : AppCompatActivity() {

    private val registry = LifecycleRegistry(this)
    private lateinit var viewModel: SpeciesListViewModel
    private lateinit var adapter: SpeciesListAdapter

    override fun getLifecycle(): LifecycleRegistry = registry

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_species)
        
        //set up ui
        viewModel = createViewModel()
        attachObserver()
        viewModel.getSpecies()
    }

    private fun attachObserver() {
        viewModel.isLoading.observe(this, Observer<Boolean> {
            it?.let { showLoadingDialog(it) }
        })
        viewModel.apiError.observe(this, Observer<Throwable> {
            it?.let { Snackbar.make(rvSpecies, it.localizedMessage, Snackbar.LENGTH_LONG).show() }
        })
        viewModel.speciesResponse.observe(this, Observer<List<Species>> {
            it?.let { adapter.notifyDataSetChanged() }
        })
    }

    private fun createViewModel(): SpeciesListViewModel =
            ViewModelProviders.of(this).get(SpeciesListViewModel::class.java).also {
                ComponentInjector.component.inject(it)
            }

    private fun showLoadingDialog(show: Boolean) {
        if (show) progressBar.visibility = View.VISIBLE else progressBar.visibility = View.GONE
    }
}

1. viewModel.isLoading: when change occurs to this variable showLoadingDialog method will be triggered to show or hide progressBar
2. viewModel.apiError: Snackbar with an error message will show if the change occurs to this variable.
3. viewModel.speciesResponse: The recycler adapter will be notified when data is set to this value

Conclusion