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Using coroutines in a right way

I am implementing the coroutine for first time. I am following MVP pattern for a simple login app. Here is my code flow –

The login button clicked will follow this direction –

LoginFragment -> LoginPresenter -> Repository -> APIRepository -> RetrofitInterface

The login response will follow this direction –

RetrofitInterface -> APIRepository -> Repository -> LoginPresenter -> LoginFragment

Here is the code –

RetrofitInterface.kt

@POST("login")
    fun loginAPI(@Body loginRequest: LoginRequest): Deferred<LoginResponse>?

Here is my Result.kt

sealed class Result<out T : Any> {

    class Success<out T : Any>(val data: T) : Result<T>()

    class Error(val exception: Throwable, val message: String = exception.localizedMessage) : Result<Nothing>()
}

APIRepository.kt

override suspend fun loginAPICall(loginRequest: LoginRequest) : Result<LoginResponse>? {
        try {
            val loginResponse = apiInterface?.loginAPI(loginRequest)?.await()
            return Result.Success<LoginResponse>(loginResponse!!)
        } catch (e : HttpException) {
            return Result.Error(e)
        } catch (e : Throwable) {
            return Result.Error(e)
        }
    }

Repository.kt

override suspend fun loginUser(loginRequest: LoginRequest): Result<LoginResponse> {
        if (isInternetPresent(context)) {
            val result = apiRepositoryInterface?.loginAPICall(loginRequest)
            if (result is Result.Success<LoginResponse>) {
                val loginData = result.data
                cache?.storeData(loginData)
            }
            return result!!
        } else {
            return Result.Error(Exception())
        }
    }

How do I launch a coroutine now in my presenter? I need to execute this API call on a background thread and publish the results on UI thread?

Answer

You need to launch a coroutine in a Presenter using local scope and injected CoroutineContext to be able to change it, for example in Unit Tests:

class Presenter(
    private val repo: Repository,
    private val uiContext: CoroutineContext = Dispatchers.Main
) : CoroutineScope { // creating local scope

    private var job: Job = Job() // or SupervisorJob() - children of a supervisor job can fail independently of each other

    // To use Dispatchers.Main (CoroutineDispatcher - runs and schedules coroutines) 
    // in Android add dependency: implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.0.1'
    override val coroutineContext: CoroutineContext
        get() = uiContext + job

    fun detachView() {
        // cancel the job when view is detached
        job.cancel()
    }

    fun login(request: LoginRequest) = launch { // launching a coroutine
        val result = repo.loginUser(request) // calling 'loginUser' function will not block the Main Thread, it suspends the coroutine

        //use result, update UI
        when (result) {
            is Success<LoginResponse> -> { /* update UI when login success */ } 
            is Error -> { /* update UI when login error */ }
        }
    }
}
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