SafeCoroutines

Introduction

The project uses the ext functions of CoroutinesUtils.kt. In this file, analogues of the functions launch and withContext

launch(Dispatchers.IO) -> launchIO
launch(Dispatchers.Main) -> launchMain
withContext(Dispatchers.IO) -> withIO
withContext(Dispatchers.Main) -> withMain

Calls to the standard library are recommended to be prohibited by custom rules Detekt:

Install

Just add dependency

    allprojects {
        repositories {
            // ...
            maven { url 'https://jitpack.io' }
        }
    }

    dependencies {
        implementation 'com.github.Evleaps:SafeCoroutines:$version'
    }

Why is it necessary to use these extensions?

Because the standard approach does not oblige us to handle errors, moreover, error handling in Kotlin is difficult because of its non-obviousness. The try-catch block won't catch the error in IO and the application doesn't care will fall, and the use of CoroutineExceptionHandler on the whole class leads to the fact that an error in one curutin can break all other coroutines in the scope. To avoid this, you must either create different scopes, or different instances of CoroutineExceptionHandler and this large number of approaches do not guarantees a unified approach to their processing

Problems

The contract does not oblige to process the fact of an error in the coroutine, which means that someone can neglect processing in vain
try-catch does not catch all errors and this leads to crashes
It is necessary to pass the parameters of the stream and the error handler to the constructor, this complicates readability code
There is no single approach to organizing asynchronous work
The error will come in a random thread, therefore, UI changes will lead to a crash
Using a single CoroutineExceptionHandler for the entire class or in MainClass will abort operations in this scope

Solution

We use our own extensions which internally create a coroutineExceptionHandler - therefore, any coroutine must handle the fact of an error, even if it does not need to be reacted to
Extensions are similar to subscribeBy - Familiar syntax from RxJava is clearer
No need to pass anything to the constructor - cleaner and simpler code
A unified approach - minimization of errors due to incorrect processing or its absence
In onError, the error will always come in the Main thread - ** so you can update the UI **
Each error is isolated - means we will not interrupt an important operation running in parallel in same scope

Samples

Before | Old

class MyViewModel {

    private val myExceptionHandler = CoroutineExceptionHandler { _, throwable ->
        withContext(Dispatchers.Main) { // because you cannot update the UI in the IO thread
            view.showError()
        }
    }

    fun foo() {
        launch(Dispatchers.IO, myExceptionHandler) {
            val campaign = marketingCampaignsInteractor
                .getMarketingCampaign(MarketingCampaignType.FAVORITES_CASHBACK.codeName)
                .awaitSingleOrNull()

            if (campaign is FavoritesCashbackCampaign) {
                val percent = loyaltyProgramInteractor.getFavoritesCashbackPercentage()
                withContext(Dispatchers.Main) { // because you cannot update the UI in the IO thread
                    view.setLikeCashbackPercent(percent)
                }
            }
        }
    }
}

After | new

class MyViewModel {

    fun foo() {
        launchIO(
            safeAction = {
                val campaign = marketingCampaignsInteractor
                    .getMarketingCampaign(MarketingCampaignType.FAVORITES_CASHBACK.codeName)
                    .awaitSingleOrNull()

                if (campaign is FavoritesCashbackCampaign) {
                    val percent = loyaltyProgramInteractor.getFavoritesCashbackPercentage()
                    withMain { // because you cannot update the UI in the IO thread
                        view.setLikeCashbackPercent(percent)
                    }
                }
            },
            onError = {
                // no need to do withMain as the error is always in the Main thread
                view.showError()
            }
        )
    }
}

Detekt custom rules

In order for all team members to use a unified approach to creating coroutines and error handling, I strongly advise you to include these rules in the list of Detekt custom rules.

If your project have not Detekt:

Detekt is a static analysis tool that can automate your routine work as a code reviewer.
GitHub
Site
How implement custom rules (Russian, please, use google translate)

Just insert the 2 rules you saw above in DetektCustomRuleSetProvider and the rules will work for you.

Testing

If you need to test coroutines with stream redefinition, as we did for Rx, then there is no need for coroutines in such a file, because we can use kotlinx-coroutines-test which overrides threads for testing.

I mainly based on information from this article on Medium .

How it works?

PS: Firstly need to add dependency

    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:$versions.version'

Test

@Before
fun setUp() {
    Dispatchers.setMain(StandardTestDispatcher())
}

@After
fun tearDown() {
    Dispatchers.resetMain() // reset the main dispatcher to the original Main dispatcher
}

// tests

Test samples

ViewModel or something like that

class LikeViewModel : CoroutineScope {

    // without clear because just for test
    private val viewModelContext: Job = SupervisorJob()
    override val coroutineContext = viewModelContext + Dispatchers.Main

    fun runSmthInIO(): Job {
        return launchIO(
            safeAction = {
                delay(3_000L)
                onSuccess()
            },
            onError = ::onError
        )
    }

    fun runSmthInMain(): Job {
        return launchMain(
            safeAction = {
                delay(4_000L)
                onSuccess()
            },
            onError = ::onError
        )
    }

    @VisibleForTesting
    fun onSuccess() {}
    private fun onError(t: Throwable) {}

    private var isFlagEnabled = false

    suspend fun checkThatFlagTrue(): Boolean {
        withIO {
            delay(5_000)
            isFlagEnabled = true
        }
        return isFlagEnabled
    }
}

Tests

More test in this repo:Link

More information about coroutine testing you can read hire.

@ExperimentalCoroutinesApi::class
class LikeViewModelTest {

    @Before
    fun setUp() {
        Dispatchers.setMain(StandardTestDispatcher())
    }

    @After
    fun tearDown() {
        Dispatchers.resetMain() // reset the main dispatcher to the original Main dispatcher
    }

    @Test
    fun `WHEN call fun in IO THEN onSuccess called`() = runTest {
        val viewModel: LikeViewModel = spy(LikeViewModel())

        val executionTime = measureTimeMillis {
            viewModel.runSmthInIO().join()
            verify(viewModel, times(1)).onSuccess()
        }

        print("runSmthInIO: Execution Time: $executionTime")
    }

    @Test
    fun `WHEN call fun in Main THEN onSuccess called`() = runTest {
        val viewModel: LikeViewModel = spy(LikeViewModel())

        val executionTime = measureTimeMillis {
            viewModel.runSmthInMain().join()
            verify(viewModel, times(1)).onSuccess()
        }

        print("Execution Time: $executionTime")
    }

    @Test
    fun `WHEN checkThatFlagTrue called THEN it must return true`() = runTest {
        val mainViewModel = LikeViewModel()

        val executionTime = measureTimeMillis {
            val isTrue = mainViewModel.checkThatFlagTrue()
            assertTrue(isTrue)
        }

        print("Execution Time: $executionTime")
    }
}

Don't want to use the experimental API

If you don't want to use the experimental api from kotlinx-coroutines-test you can use the standard approach of overriding the Dispatcher and injection.

inline fun CoroutineScope.launchSafe(
   crossinline safeAction: suspend () -> Unit,
   crossinline onError: (Throwable) -> Unit,
   dispatcher: CoroutineDispatcher, // <-- you must provide some Dispatcher manually
   errorDispatcher: CoroutineDispatcher = Dispatchers.Main
): Job {
   val exceptionHandler = CoroutineExceptionHandler { _, throwable ->
      launch(errorDispatcher) {
         onError.invoke(throwable)
      }
   }

   return this.launch(exceptionHandler + dispatcher) {
      safeAction.invoke()
   }
}

Detailed materials of this method are collected here:

License

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Disclaimer

(En) All information and source code are provided AS-IS, without express or implied warranties. Use of the source code or parts of it is at your sole discretion and risk. Citymobil LLC takes reasonable measures to ensure the relevance of the information posted in this repository, but it does not assume responsibility for maintaining or updating this repository or its parts outside the framework established by the company independently and without notifying third parties.

(Ru) Вся информация и исходный код предоставляются в исходном виде, без явно выраженных или подразумеваемых гарантий. Использование исходного кода или его части осуществляются исключительно по вашему усмотрению и на ваш риск. Компания ООО “Ситимобил” принимает разумные меры для обеспечения актуальности информации, размещенной в данном репозитории, но она не принимает на себя ответственности за поддержку или актуализацию данного репозитория или его частей вне рамок, устанавливаемых компанией самостоятельно и без уведомления третьих лиц.

Evleaps / SafeCoroutines