Android LiveData at Kotlin using Retrofit and coroutines
This article is about using Android Components ViewModel, LifeCycle and LiveData. These components allow you to not worry about the life cycle of the Activity.
Also considered an example of the use of modern Coroutines in conjunction with the repository on the Retrofit.
Retrofit coroutines extension
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kotlin-coroutines-retrofit
Retrofit extension on Kotlin. These are just two files. I just added them to the project. You can connect them via Dependency to Gradle. There are usage examples on Github.
Also connect Adapter addCallAdapterFactory (CoroutineCallAdapterFactory ()) .
ServerAPI and Repository are in the same file
REST API
Implementing the REST API on Kotlin. It does not have any specific changes.
Livedata
Next, consider the Repository. This is the main service for getting livedata. Initialize LiveData with the load state: Resource.loading (null) . Next, wait for the end of the awaitResult () request . This call should be in the Coroutin async (UI)
At the end of the request, we can handle the result. If all is well, the result will be saved in mutableLiveData.value = Resource.success (result.value) The important point is that the link to the new instance should be, otherwise observer LiveData will not work. see: new Resource <> (SUCCESS, data, null);
Wrapper data
Wrapper - Resource <T> is used for error handling and state transfer in Fragment.
It store three states:
The data itself:
Viewmodel
StoresViewModel requests data from the repository and stores in the internal variable stores
ViewModelProviders
To transfer parameters to the ViewModel, we will extend the standard ViewModelProviders
For example, for passing to the LoginViewModel you need two parameters (Login, Password). To send a token to StoresViewModel, use one (Token)
Fragment
Getting StoresViewModel:
Using Observer for change data:
PS
To store Token and use it throughout the application, I applied the library / extension from Fabio Collini. The application is well described in his article. The link is on the page in Github or lower in this article.
prefs-delegates by Fabio Collini
Gradle
Links
All in one example
Android Architecture Components samples
LiveData Overview
Async code using Kotlin Coroutines
Multithreading and Kotlin
Also considered an example of the use of modern Coroutines in conjunction with the repository on the Retrofit.
fun main(args: Array<String>): Unit = runBlocking { // Wait (suspend) for Result val result: Result<User> = api.getUser("username").awaitResult() // Check result type when (result) { //Successful HTTP result is Result.Ok -> saveToDb(result.value) // Any HTTP error is Result.Error -> log("HTTP error with code ${result.error.code()}", result.error) // Exception while request invocation is Result.Exception -> log("Something broken", e) } } Retrofit coroutines extension
')
kotlin-coroutines-retrofit
Retrofit extension on Kotlin. These are just two files. I just added them to the project. You can connect them via Dependency to Gradle. There are usage examples on Github.
Also connect Adapter addCallAdapterFactory (CoroutineCallAdapterFactory ()) .
ServerAPI and Repository are in the same file
REST API
Implementing the REST API on Kotlin. It does not have any specific changes.
ServerAPI
import android.arch.lifecycle.MutableLiveData import android.util.Log import com.jakewharton.retrofit2.adapter.kotlin.coroutines.experimental.CoroutineCallAdapterFactory import kotlinx.coroutines.experimental.android.UI import kotlinx.coroutines.experimental.async import retrofit2.Retrofit import retrofit2.converter.gson.GsonConverterFactory import okhttp3.OkHttpClient import okhttp3.logging.HttpLoggingInterceptor import retrofit2.Call import retrofit2.http.* import ru.gildor.coroutines.retrofit.Result import ru.gildor.coroutines.retrofit.awaitResult object ServerAPI { var API_BASE_URL: String = getNetworkHost(); var httpClient = OkHttpClient.Builder().addInterceptor( HttpLoggingInterceptor().apply { level = if (BuildConfig.DEBUG) HttpLoggingInterceptor.Level.BODY else HttpLoggingInterceptor.Level.NONE }) var builder: Retrofit.Builder = Retrofit.Builder() .baseUrl(API_BASE_URL) .addCallAdapterFactory(CoroutineCallAdapterFactory()) .addConverterFactory(GsonConverterFactory.create()) var retrofit = builder .client(httpClient.build()) .build() var netService = retrofit.create<NetService>( NetService::class.java!!) interface NetService { @GET("api/stores") fun getStoreAll(@Header("Authorization") bearer: String): Call<Array<Store>> } } Livedata
Next, consider the Repository. This is the main service for getting livedata. Initialize LiveData with the load state: Resource.loading (null) . Next, wait for the end of the awaitResult () request . This call should be in the Coroutin async (UI)
At the end of the request, we can handle the result. If all is well, the result will be saved in mutableLiveData.value = Resource.success (result.value) The important point is that the link to the new instance should be, otherwise observer LiveData will not work. see: new Resource <> (SUCCESS, data, null);
Repository
class Repository { fun getStores(token: String) : MutableLiveData<Resource<Array<Store>>>{ val mutableLiveData = MutableLiveData<Resource<Array<Store>>>() mutableLiveData.value = Resource.loading(null) val req = PostsAPI.netService.getStoreAll(token) try { async(UI) { val result = req.awaitResult() // Check result type when (result) { //Successful HTTP result is Result.Ok -> { mutableLiveData.value = Resource.success(result.value) } // Any HTTP error is Result.Error -> { mutableLiveData.value = Resource.error("Http Error!", null) } // Exception while request invocation is Result.Exception -> Log.d(TAG, result.exception.message) } } } catch (e: Exception) { Log.d(TAG, e.toString()) } return mutableLiveData } } Wrapper data
Wrapper - Resource <T> is used for error handling and state transfer in Fragment.
It store three states:
public enum Status { SUCCESS, ERROR, LOADING } The data itself:
@Nullable public final T data; Resource <T>
This data is generic.
// A generic class that contains data and status about loading this data. public class Resource<T> { @NonNull public final Status status; @Nullable public final T data; @Nullable public final String message; private Resource(@NonNull Status status, @Nullable T data, @Nullable String message) { this.status = status; this.data = data; this.message = message; } public static <T> Resource<T> success(@NonNull T data) { return new Resource<>(Status.SUCCESS, data, null); } public static <T> Resource<T> error(String msg, @Nullable T data) { return new Resource<>(Status.ERROR, data, msg); } public static <T> Resource<T> loading(@Nullable T data) { return new Resource<>(Status.LOADING, data, null); } public enum Status { SUCCESS, ERROR, LOADING } } Viewmodel
StoresViewModel requests data from the repository and stores in the internal variable stores
val api = Repository() stores = api.getStores(token) Viewmodel
class StoresViewModel (context: Context, token: String) : ViewModel() { val stores: MutableLiveData<Resource<Array<Store>>> init { val api = Repository() stores = api.getStores(token) } } ViewModelProviders
To transfer parameters to the ViewModel, we will extend the standard ViewModelProviders
For example, for passing to the LoginViewModel you need two parameters (Login, Password). To send a token to StoresViewModel, use one (Token)
AppViewModelFactory
class AppViewModelFactory(private val contect: Context, vararg params: Any) : ViewModelProvider.NewInstanceFactory() { private val mParams: Array<out Any> init { mParams = params } override fun <T : ViewModel> create(modelClass: Class<T>): T { return if (modelClass == LoginViewModel::class.java) { LoginViewModel(contect, mParams[0] as String, mParams[1] as String) as T } else if (modelClass == StoresViewModel::class.java) { StoresViewModel(contect, mParams[0] as String) as T } else { super.create(modelClass) } } } Fragment
Getting StoresViewModel:
viewModel = ViewModelProviders.of(this, AppViewModelFactory(requireActivity(), tokenHolder.token)).get(StoresViewModel::class.java) Using Observer for change data:
// Observe data on the ViewModel, exposed as a LiveData viewModel.stores.observe(this, Observer<Resource<Array<Store>>> { storesResource -> Fragment
override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { val view = inflater.inflate(R.layout.stores_fragment, container, false) val tokenHolder = TokenHolder(PreferenceManager.getDefaultSharedPreferences(requireActivity())) viewModel = ViewModelProviders.of(this, AppViewModelFactory(requireActivity(), tokenHolder.token)).get(StoresViewModel::class.java) recyclerView = view.findViewById<RecyclerView>(R.id.store_list).apply { setHasFixedSize(true) } return view } override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) // Observe data on the ViewModel, exposed as a LiveData viewModel.stores.observe(this, Observer<Resource<Array<Store>>> { storesResource -> val stores = storesResource?.data stores?.let { viewAdapter = StoresAdapter(stores!!) recyclerView.adapter = viewAdapter } if (storesResource?.status == Resource.LOADING){ log("Loading...") } if (storesResource?.status == Resource.ERROR){ log("Error : " + storesResource?.message) } }) } PS
To store Token and use it throughout the application, I applied the library / extension from Fabio Collini. The application is well described in his article. The link is on the page in Github or lower in this article.
prefs-delegates by Fabio Collini
class TokenHolder(prefs: SharedPreferences) { var token by prefs.string() private set var count by prefs.int() private set fun saveToken(newToken: String) { token = newToken count++ } } Gradle
implementation 'android.arch.lifecycle:extensions:1.1.1' implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:0.30.0" implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:0.30.0" implementation "com.squareup.retrofit2:retrofit:2.4.0" implementation "com.squareup.retrofit2:converter-gson:2.4.0" implementation "com.jakewharton.retrofit:retrofit2-kotlin-coroutines-experimental-adapter:1.0.0" // If you use Kotlin 1.2 or 1.3 // compile 'ru.gildor.coroutines:kotlin-coroutines-retrofit:0.13.0' // compile 'ru.gildor.coroutines:kotlin-coroutines-retrofit:0.13.0-eap13' Links
All in one example
Android Architecture Components samples
LiveData Overview
Async code using Kotlin Coroutines
Multithreading and Kotlin
Source: https://habr.com/ru/post/427475/
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