feat(mvi-presenter) Added onError method in presenter to be called when exception occurs

Author: PiotrBandurskiVA

mvi-presenter/src/main/java/pl/valueadd/mvi/presenter/BaseMviPresenter.kt

@@ -25,7 +25,7 @@ abstract class BaseMviPresenter<VS : IBaseViewState, PS : IBasePartialState, VI
      * Hot observable of [view state][IBaseViewState].
      */
     val stateObservable: Observable<VS>
-        get() = viewStateBehaviorSubject
+        get() = viewStateBehaviorSubject.share()
 
     /**
      * Returns [view][IBaseView] but may throw [ViewNotAttachedException] if called in wrong place.
@@ -37,9 +37,8 @@ abstract class BaseMviPresenter<VS : IBaseViewState, PS : IBasePartialState, VI
      * @see subscribeViewStateConsumer
      * @see viewStateConsumerDisposable
      */
-    protected open val viewStateSubscriptionScheduler: Scheduler by lazy {
+    protected open val viewStateSubscriptionScheduler: Scheduler =
         Schedulers.io()
-    }
 
     /**
      * @see subscribeViewStateConsumer
@@ -76,7 +75,7 @@ abstract class BaseMviPresenter<VS : IBaseViewState, PS : IBasePartialState, VI
     /**
      * A subject to pass emission of wrapped intents to currently binded view's consumer.
      */
-    private val viewStateBehaviorSubject by lazy {
+    private val viewStateBehaviorSubject: BehaviorSubject<VS> by lazy {
         BehaviorSubject.createDefault(currentState)
     }
 
@@ -179,6 +178,14 @@ abstract class BaseMviPresenter<VS : IBaseViewState, PS : IBasePartialState, VI
      */
     protected open fun providePresenterIntents(): List<Observable<out PS>> = listOf()
 
+    /**
+     * Method which is called when an Exception occurred during observing external
+     * observables or reducing states.
+     */
+    protected open fun onError(throwable: Throwable) {
+        // no-op
+    }
+
     /**
      * Binds provided view's intents to wrapper subject.
      *
@@ -217,6 +224,7 @@ abstract class BaseMviPresenter<VS : IBaseViewState, PS : IBasePartialState, VI
         viewStateReducerDisposable =
             currentViewIntentsSubject
                 .flatMap(::mapViewIntentToPartialState)
+                .doOnError(::onError)
                 .mergeWith(Observable.merge(providePresenterIntents()))
                 .scan(currentState, this::reduce)
                 .distinctUntilChanged()

mvi-presenter/src/test/java/pl/valueadd/mvi/presenter/BaseMviPresenterTest.kt

@@ -23,6 +23,9 @@ class BaseMviPresenterTest {
     @MockK
     private lateinit var mockReducer: TestReducer
 
+    @MockK
+    private lateinit var mockTestLogger: TestErrorLogger
+
     private lateinit var presenter: TestPresenter
 
     private lateinit var presenterPublishSubject: PublishSubject<TestPartialState>
@@ -33,7 +36,8 @@ class BaseMviPresenterTest {
         presenter = TestPresenter(
             mockMapper,
             mockReducer,
-            presenterPublishSubject
+            presenterPublishSubject,
+            mockTestLogger
         )
     }
 
@@ -215,6 +219,42 @@ class BaseMviPresenterTest {
         assert(presenterPublishSubject.hasObservers() == false)
     }
 
+    @Test
+    fun `Should call onError when error occurs during observing presenter intents`() {
+        // Given
+        val viewIntentsSubject = PublishSubject.create<TestViewIntent>()
+        val mockView = createMockView(viewIntentsSubject)
+        val mockThrowable: Throwable = mockk(relaxed = true)
+        every { mockThrowable.stackTrace } returns emptyArray()
+        every { mockThrowable.cause } returns null
+        every { mockTestLogger.logError(any()) } returns Unit
+        presenter.attachView(mockView)
+
+        // When
+        presenterPublishSubject.onError(mockThrowable)
+
+        // Then
+        verify(exactly = 1) { mockTestLogger.logError(mockThrowable) }
+    }
+
+    @Test
+    fun `Should call onError when error occurs during observing view intents`() {
+        // Given
+        val viewIntentsSubject = PublishSubject.create<TestViewIntent>()
+        val mockView = createMockView(viewIntentsSubject)
+        val mockThrowable: Throwable = mockk(relaxed = true)
+        every { mockThrowable.stackTrace } returns emptyArray()
+        every { mockThrowable.cause } returns null
+        every { mockTestLogger.logError(any()) } returns Unit
+        presenter.attachView(mockView)
+
+        // When
+        viewIntentsSubject.onError(mockThrowable)
+
+        // Then
+        verify(exactly = 1) { mockTestLogger.logError(mockThrowable) }
+    }
+
     private fun createMockView(viewIntentsObservable: Observable<TestViewIntent>): IBaseView<TestViewState, TestViewIntent> {
         return mockk {
             every { provideViewIntents() } returns listOf(viewIntentsObservable)
@@ -227,7 +267,8 @@ class BaseMviPresenterTest {
 private class TestPresenter(
     private val mapper: TestViewIntentToPartialStateMapper,
     private val reducer: TestReducer,
-    private val presenterObservable: Observable<TestPartialState>
+    private val presenterObservable: Observable<TestPartialState>,
+    private val testErrorLogger: TestErrorLogger
 ) : BaseMviPresenter<TestViewState, TestPartialState, TestViewIntent, IBaseView<TestViewState, TestViewIntent>>(Schedulers.trampoline()) {
     override val viewStateSubscriptionScheduler = Schedulers.trampoline()
     override val viewStateObservationScheduler = Schedulers.trampoline()
@@ -245,6 +286,10 @@ private class TestPresenter(
     override fun providePresenterIntents(): List<Observable<out TestPartialState>> {
         return listOf(presenterObservable)
     }
+
+    override fun onError(throwable: Throwable) {
+        testErrorLogger.logError(throwable)
+    }
 }
 
 private class TestViewState(var someProperty: Int = 0) :
@@ -265,4 +310,10 @@ private class TestReducer {
     fun reduce(previousState: TestViewState, action: TestPartialState): TestViewState {
         throw RuntimeException("Should be mocked")
     }
+}
+
+private class TestErrorLogger {
+    fun logError(throwable: Throwable) {
+        throw RuntimeException("Should be mocked")
+    }
 }