Create a general error handler which handles multiple errors - Nia 1435

app/src/main/kotlin/com/google/samples/apps/nowinandroid/MainActivity.kt

@@ -37,6 +37,7 @@ import com.google.samples.apps.nowinandroid.MainActivityUiState.Loading
 import com.google.samples.apps.nowinandroid.core.analytics.AnalyticsHelper
 import com.google.samples.apps.nowinandroid.core.analytics.LocalAnalyticsHelper
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.NiaTheme
@@ -64,6 +65,9 @@ class MainActivity : ComponentActivity() {
     @Inject
     lateinit var networkMonitor: NetworkMonitor
 
+    @Inject
+    lateinit var errorMonitor: ErrorMonitor
+
     @Inject
     lateinit var timeZoneMonitor: TimeZoneMonitor
 
@@ -135,6 +139,7 @@ class MainActivity : ComponentActivity() {
         setContent {
             val appState = rememberNiaAppState(
                 networkMonitor = networkMonitor,
+                errorMonitor = errorMonitor,
                 userNewsResourceRepository = userNewsResourceRepository,
                 timeZoneMonitor = timeZoneMonitor,
             )

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/NiaApp.kt

@@ -16,6 +16,7 @@
 
 package com.google.samples.apps.nowinandroid.ui
 
+import android.content.Context
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.WindowInsets
@@ -30,6 +31,7 @@ import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Scaffold
+import androidx.compose.material3.SnackbarDuration
 import androidx.compose.material3.SnackbarDuration.Indefinite
 import androidx.compose.material3.SnackbarDuration.Short
 import androidx.compose.material3.SnackbarHost
@@ -52,6 +54,7 @@ import androidx.compose.ui.composed
 import androidx.compose.ui.draw.drawWithContent
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.platform.testTag
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.semantics.semantics
@@ -69,6 +72,10 @@ import com.google.samples.apps.nowinandroid.core.designsystem.component.NiaTopAp
 import com.google.samples.apps.nowinandroid.core.designsystem.icon.NiaIcons
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.GradientColors
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.LocalGradientColors
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.MESSAGE
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.OFFLINE
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.UNKNOWN
 import com.google.samples.apps.nowinandroid.feature.settings.SettingsDialog
 import com.google.samples.apps.nowinandroid.navigation.NiaNavHost
 import com.google.samples.apps.nowinandroid.navigation.TopLevelDestination
@@ -81,6 +88,8 @@ fun NiaApp(
     modifier: Modifier = Modifier,
     windowAdaptiveInfo: WindowAdaptiveInfo = currentWindowAdaptiveInfo(),
 ) {
+    val context = LocalContext.current
+
     val shouldShowGradientBackground =
         appState.currentTopLevelDestination == TopLevelDestination.FOR_YOU
     var showSettingsDialog by rememberSaveable { mutableStateOf(false) }
@@ -95,16 +104,30 @@ fun NiaApp(
         ) {
             val snackbarHostState = remember { SnackbarHostState() }
 
-            val isOffline by appState.isOffline.collectAsStateWithLifecycle()
+            val stateMessage by appState.stateMessage.collectAsStateWithLifecycle()
 
-            // If user is not connected to the internet show a snack bar to inform them.
-            val notConnectedMessage = stringResource(R.string.not_connected)
-            LaunchedEffect(isOffline) {
-                if (isOffline) {
-                    snackbarHostState.showSnackbar(
-                        message = notConnectedMessage,
-                        duration = Indefinite,
-                    )
+            LaunchedEffect(stateMessage) {
+                stateMessage?.let { message ->
+
+                    // Text and Duration values dictated by the UI
+                    val (text, duration) = getSnackbarValues(context, message)
+
+                    // Determine whether user clicked action button
+                    val snackBarResult = snackbarHostState.showSnackbar(
+                        message = text,
+                        actionLabel = message.label,
+                        duration = duration,
+                    ) == ActionPerformed
+
+                    // Handle result action
+                    if (snackBarResult) {
+                        message.onConfirm?.invoke()
+                    } else {
+                        message.onDelay?.invoke()
+                    }
+
+                    // Remove Message from List
+                    appState.errorMonitor.clearMessage(message)
                 }
             }
 
@@ -271,3 +294,11 @@ private fun NavDestination?.isRouteInHierarchy(route: KClass<*>) =
     this?.hierarchy?.any {
         it.hasRoute(route)
     } ?: false
+
+private fun getSnackbarValues(context: Context, message: MessageData): Pair<String, SnackbarDuration> {
+    return when (message.type) {
+        OFFLINE -> context.getString(R.string.not_connected) to SnackbarDuration.Indefinite
+        is MESSAGE -> (message.type as MESSAGE).value to SnackbarDuration.Long
+        UNKNOWN -> context.getString(R.string.unknown_error) to SnackbarDuration.Short
+    }
+}

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/NiaAppState.kt

@@ -31,8 +31,11 @@ import androidx.navigation.compose.rememberNavController
 import androidx.navigation.navOptions
 import androidx.tracing.trace
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType
 import com.google.samples.apps.nowinandroid.core.ui.TrackDisposableJank
 import com.google.samples.apps.nowinandroid.feature.bookmarks.navigation.navigateToBookmarks
 import com.google.samples.apps.nowinandroid.feature.foryou.navigation.navigateToForYou
@@ -53,6 +56,7 @@ import kotlinx.datetime.TimeZone
 @Composable
 fun rememberNiaAppState(
     networkMonitor: NetworkMonitor,
+    errorMonitor: ErrorMonitor,
     userNewsResourceRepository: UserNewsResourceRepository,
     timeZoneMonitor: TimeZoneMonitor,
     coroutineScope: CoroutineScope = rememberCoroutineScope(),
@@ -63,13 +67,15 @@ fun rememberNiaAppState(
         navController,
         coroutineScope,
         networkMonitor,
+        errorMonitor,
         userNewsResourceRepository,
         timeZoneMonitor,
     ) {
         NiaAppState(
             navController = navController,
             coroutineScope = coroutineScope,
             networkMonitor = networkMonitor,
+            errorMonitor = errorMonitor,
             userNewsResourceRepository = userNewsResourceRepository,
             timeZoneMonitor = timeZoneMonitor,
         )
@@ -81,6 +87,7 @@ class NiaAppState(
     val navController: NavHostController,
     coroutineScope: CoroutineScope,
     networkMonitor: NetworkMonitor,
+    val errorMonitor: ErrorMonitor,
     userNewsResourceRepository: UserNewsResourceRepository,
     timeZoneMonitor: TimeZoneMonitor,
 ) {
@@ -115,6 +122,27 @@ class NiaAppState(
             initialValue = false,
         )
 
+    private val errorMessages: StateFlow<List<MessageData?>> = errorMonitor.messages.stateIn(
+        scope = coroutineScope,
+        started = SharingStarted.WhileSubscribed(5_000),
+        initialValue = emptyList(),
+    )
+
+    val stateMessage: StateFlow<MessageData?> = combine(isOffline, errorMessages) { offline, errors ->
+        if (offline) {
+            // Priority is given to Offline Error Message over other types
+            MessageData(type = MessageType.OFFLINE)
+        }
+        // Otherwise, Display first from error monitor list if exists
+        else {
+            errors.firstOrNull()
+        }
+    }.stateIn(
+        scope = coroutineScope,
+        started = SharingStarted.WhileSubscribed(5_000),
+        initialValue = null,
+    )
+
     /**
      * Map of top level destinations to be used in the TopBar, BottomBar and NavRail. The key is the
      * route.

app/src/main/res/values/strings.xml

@@ -17,4 +17,5 @@
 <resources>
     <string name="app_name">Now in Android</string>
     <string name="not_connected">⚠️ You aren’t connected to the internet</string>
+    <string name="unknown_error">Unknown Error</string>
 </resources>

app/src/testDemo/kotlin/com/google/samples/apps/nowinandroid/ui/NiaAppScreenSizesScreenshotTests.kt

@@ -32,6 +32,7 @@ import com.github.takahirom.roborazzi.captureRoboImage
 import com.google.samples.apps.nowinandroid.core.data.repository.TopicsRepository
 import com.google.samples.apps.nowinandroid.core.data.repository.UserDataRepository
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.NiaTheme
@@ -80,6 +81,9 @@ class NiaAppScreenSizesScreenshotTests {
     @Inject
     lateinit var networkMonitor: NetworkMonitor
 
+    @Inject
+    lateinit var errorMonitor: ErrorMonitor
+
     @Inject
     lateinit var timeZoneMonitor: TimeZoneMonitor
 
@@ -123,6 +127,7 @@ class NiaAppScreenSizesScreenshotTests {
                     NiaTheme {
                         val fakeAppState = rememberNiaAppState(
                             networkMonitor = networkMonitor,
+                            errorMonitor = errorMonitor,
                             userNewsResourceRepository = userNewsResourceRepository,
                             timeZoneMonitor = timeZoneMonitor,
                         )

app/src/testDemo/kotlin/com/google/samples/apps/nowinandroid/ui/NiaAppStateTest.kt

@@ -29,6 +29,7 @@ import androidx.navigation.testing.TestNavHostController
 import com.google.samples.apps.nowinandroid.core.data.repository.CompositeUserNewsResourceRepository
 import com.google.samples.apps.nowinandroid.core.testing.repository.TestNewsRepository
 import com.google.samples.apps.nowinandroid.core.testing.repository.TestUserDataRepository
+import com.google.samples.apps.nowinandroid.core.testing.util.TestErrorMonitor
 import com.google.samples.apps.nowinandroid.core.testing.util.TestNetworkMonitor
 import com.google.samples.apps.nowinandroid.core.testing.util.TestTimeZoneMonitor
 import dagger.hilt.android.testing.HiltAndroidTest
@@ -60,6 +61,8 @@ class NiaAppStateTest {
     // Create the test dependencies.
     private val networkMonitor = TestNetworkMonitor()
 
+    private var errorMonitor = TestErrorMonitor()
+
     private val timeZoneMonitor = TestTimeZoneMonitor()
 
     private val userNewsResourceRepository =
@@ -79,6 +82,7 @@ class NiaAppStateTest {
                     navController = navController,
                     coroutineScope = backgroundScope,
                     networkMonitor = networkMonitor,
+                    errorMonitor = errorMonitor,
                     userNewsResourceRepository = userNewsResourceRepository,
                     timeZoneMonitor = timeZoneMonitor,
                 )
@@ -101,6 +105,7 @@ class NiaAppStateTest {
         composeTestRule.setContent {
             state = rememberNiaAppState(
                 networkMonitor = networkMonitor,
+                errorMonitor = errorMonitor,
                 userNewsResourceRepository = userNewsResourceRepository,
                 timeZoneMonitor = timeZoneMonitor,
             )
@@ -119,6 +124,7 @@ class NiaAppStateTest {
                 navController = NavHostController(LocalContext.current),
                 coroutineScope = backgroundScope,
                 networkMonitor = networkMonitor,
+                errorMonitor = errorMonitor,
                 userNewsResourceRepository = userNewsResourceRepository,
                 timeZoneMonitor = timeZoneMonitor,
             )
@@ -139,6 +145,7 @@ class NiaAppStateTest {
                 navController = NavHostController(LocalContext.current),
                 coroutineScope = backgroundScope,
                 networkMonitor = networkMonitor,
+                errorMonitor = errorMonitor,
                 userNewsResourceRepository = userNewsResourceRepository,
                 timeZoneMonitor = timeZoneMonitor,
             )

app/src/testDemo/kotlin/com/google/samples/apps/nowinandroid/ui/SnackbarInsetsScreenshotTests.kt

@@ -63,6 +63,7 @@ import com.github.takahirom.roborazzi.captureRoboImage
 import com.google.samples.apps.nowinandroid.core.data.repository.TopicsRepository
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
 import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeUserDataRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.NiaTheme
@@ -113,6 +114,9 @@ class SnackbarInsetsScreenshotTests {
     @Inject
     lateinit var networkMonitor: NetworkMonitor
 
+    @Inject
+    lateinit var errorMonitor: ErrorMonitor
+
     @Inject
     lateinit var timeZoneMonitor: TimeZoneMonitor
 
@@ -254,6 +258,7 @@ class SnackbarInsetsScreenshotTests {
                             NiaTheme {
                                 val appState = rememberNiaAppState(
                                     networkMonitor = networkMonitor,
+                                    errorMonitor = errorMonitor,
                                     userNewsResourceRepository = userNewsResourceRepository,
                                     timeZoneMonitor = timeZoneMonitor,
                                 )

app/src/testDemo/kotlin/com/google/samples/apps/nowinandroid/ui/SnackbarScreenshotTests.kt

@@ -36,6 +36,7 @@ import com.github.takahirom.roborazzi.captureRoboImage
 import com.google.samples.apps.nowinandroid.core.data.repository.TopicsRepository
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
 import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeUserDataRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.NiaTheme
@@ -86,6 +87,9 @@ class SnackbarScreenshotTests {
     @Inject
     lateinit var networkMonitor: NetworkMonitor
 
+    @Inject
+    lateinit var errorMonitor: ErrorMonitor
+
     @Inject
     lateinit var timeZoneMonitor: TimeZoneMonitor
 
@@ -203,6 +207,7 @@ class SnackbarScreenshotTests {
                         NiaTheme {
                             val appState = rememberNiaAppState(
                                 networkMonitor = networkMonitor,
+                                errorMonitor = errorMonitor,
                                 userNewsResourceRepository = userNewsResourceRepository,
                                 timeZoneMonitor = timeZoneMonitor,
                             )

core/data-test/src/main/kotlin/com/google/samples/apps/nowinandroid/core/data/test/EmptyErrorMonitor.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+package com.google.samples.apps.nowinandroid.core.data.test
+
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flowOf
+import javax.inject.Inject
+
+class EmptyErrorMonitor @Inject constructor() : ErrorMonitor {
+
+    override fun addMessageByString(message: String): MessageData {
+        TODO("Not yet implemented")
+    }
+
+    override fun addMessageByData(message: MessageData) {
+        TODO("Not yet implemented")
+    }
+
+    override fun clearMessage(message: MessageData) {
+        TODO("Not yet implemented")
+    }
+
+    override fun clearAllMessages() {
+        TODO("Not yet implemented")
+    }
+
+    override val messages: Flow<List<MessageData?>>
+        get() = flowOf(emptyList())
+}

core/data-test/src/main/kotlin/com/google/samples/apps/nowinandroid/core/data/test/TestDataModule.kt

@@ -27,6 +27,7 @@ import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeRecent
 import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeSearchContentsRepository
 import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeTopicsRepository
 import com.google.samples.apps.nowinandroid.core.data.test.repository.FakeUserDataRepository
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import dagger.Binds
@@ -70,6 +71,11 @@ internal interface TestDataModule {
         networkMonitor: AlwaysOnlineNetworkMonitor,
     ): NetworkMonitor
 
+    @Binds
+    fun bindsErrorMonitor(
+        errorMonitor: EmptyErrorMonitor,
+    ): ErrorMonitor
+
     @Binds
     fun binds(impl: DefaultZoneIdTimeZoneMonitor): TimeZoneMonitor
 }

core/data/src/main/kotlin/com/google/samples/apps/nowinandroid/core/data/di/DataModule.kt

@@ -27,7 +27,9 @@ import com.google.samples.apps.nowinandroid.core.data.repository.SearchContentsR
 import com.google.samples.apps.nowinandroid.core.data.repository.TopicsRepository
 import com.google.samples.apps.nowinandroid.core.data.repository.UserDataRepository
 import com.google.samples.apps.nowinandroid.core.data.util.ConnectivityManagerNetworkMonitor
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
+import com.google.samples.apps.nowinandroid.core.data.util.StateErrorMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneBroadcastMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import dagger.Binds
@@ -69,6 +71,11 @@ abstract class DataModule {
         networkMonitor: ConnectivityManagerNetworkMonitor,
     ): NetworkMonitor
 
+    @Binds
+    internal abstract fun bindsErrorMonitor(
+        errorMonitor: StateErrorMonitor,
+    ): ErrorMonitor
+
     @Binds
     internal abstract fun binds(impl: TimeZoneBroadcastMonitor): TimeZoneMonitor
 }

core/data/src/main/kotlin/com/google/samples/apps/nowinandroid/core/data/util/ErrorMonitor.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+package com.google.samples.apps.nowinandroid.core.data.util
+
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import kotlinx.coroutines.flow.Flow
+
+/**
+ * Interface for handling messages.
+ */
+interface ErrorMonitor {
+    fun addMessageByString(message: String): MessageData
+
+    fun addMessageByData(message: MessageData)
+
+    fun clearMessage(message: MessageData)
+
+    fun clearAllMessages()
+
+    val messages: Flow<List<MessageData?>>
+}

core/data/src/main/kotlin/com/google/samples/apps/nowinandroid/core/data/util/StateErrorMonitor.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+package com.google.samples.apps.nowinandroid.core.data.util
+
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.update
+import javax.inject.Inject
+
+/**
+ * Interface implementation for handling general errors.
+ */
+
+class StateErrorMonitor @Inject constructor() : ErrorMonitor {
+    /**
+     * List of [MessageData] to be shown
+     */
+    override val messages = MutableStateFlow<List<MessageData>>(emptyList())
+
+    /**
+     * Creates a [MessageData] and adds it to the list.
+     * @param message: String value for message to add.
+     */
+    override fun addMessageByString(message: String): MessageData {
+        val data = MessageData(type = MessageType.MESSAGE(message))
+        messages.update { it + data }
+
+        return data
+    }
+
+    /**
+     * Add a [MessageData] to the list.
+     * @param message: [MessageData] to add.
+     */
+    override fun addMessageByData(message: MessageData) {
+        messages.update { it + message }
+    }
+
+    /**
+     * Removes the [MessageData] from the list.
+     */
+    override fun clearMessage(message: MessageData) {
+        messages.update { list -> list.filterNot { it == message } }
+    }
+
+    /**
+     * Remove all from list, reset to empty list
+     */
+    override fun clearAllMessages() {
+        messages.update { emptyList() }
+    }
+}

core/data/src/test/kotlin/com/google/samples/apps/nowinandroid/core/data/util/StateErrorMonitorTest.kt

@@ -0,0 +1,147 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+import com.google.samples.apps.nowinandroid.core.data.util.StateErrorMonitor
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.runTest
+import org.junit.Before
+import org.junit.Test
+import kotlin.test.assertEquals
+
+/**
+ * Unit tests for [StateErrorMonitor].
+ */
+
+class StateErrorMonitorTest {
+
+    // Subject under test.
+    private lateinit var state: StateErrorMonitor
+
+    private lateinit var messages: List<MessageData?>
+
+    private val testString = "Test Error Message"
+    private val testData = MessageData(MessageType.MESSAGE("Test Error Message 1"))
+    private val testData2 = MessageData(MessageType.MESSAGE("Test Error Message 2"))
+
+    @Before
+    fun setup() {
+        state = StateErrorMonitor()
+        messages = emptyList()
+    }
+
+    @Test
+    fun whenErrorIsNotAdded_NullIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+        assertEquals(
+            emptyList(),
+            messages,
+        )
+    }
+
+    @Test
+    fun whenErrorIsAddedByString_ErrorMessageIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+
+        val expect = state.addMessageByString(testString)
+
+        assertEquals(
+            expect,
+            messages.firstOrNull(),
+        )
+    }
+
+    @Test
+    fun whenErrorIsAddedByData_ErrorMessageIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+
+        state.addMessageByData(testData)
+
+        assertEquals(
+            testData,
+            messages.firstOrNull(),
+        )
+    }
+
+    @Test
+    fun whenErrorsAreAdded_FirstErrorMessageIsPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            state.addMessageByData(testData)
+            state.addMessageByString(testString)
+
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+
+            assertEquals(
+                testData,
+                messages.firstOrNull(),
+            )
+        }
+
+    @Test
+    fun whenErrorIsCleared_ErrorMessageIsNotPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+            state.addMessageByData(testData)
+            state.clearMessage(testData)
+
+            assertEquals(
+                emptyList(),
+                messages,
+            )
+        }
+
+    @Test
+    fun whenErrorsAreCleared_NextErrorMessageIsPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+            state.addMessageByData(testData)
+            state.addMessageByData(testData2)
+
+            state.clearMessage(testData)
+
+            assertEquals(
+                testData2,
+                messages.firstOrNull(),
+            )
+        }
+}

core/model/src/main/kotlin/com/google/samples/apps/nowinandroid/core/model/data/MessageData.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+package com.google.samples.apps.nowinandroid.core.model.data
+
+/**
+ * [MessageData]
+ * Class to hold messages type objects with actions
+ */
+data class MessageData(
+    val type: MessageType,
+    val label: String? = null,
+    val onConfirm: (() -> Unit)? = null,
+    val onDelay: (() -> Unit)? = null,
+)
+
+/**
+ * Specified Errors
+ */
+sealed class MessageType {
+    data object OFFLINE : MessageType()
+    data class MESSAGE(val value: String) : MessageType()
+    data object UNKNOWN : MessageType()
+}

core/testing/src/main/kotlin/com/google/samples/apps/nowinandroid/core/testing/util/TestErrorMonitor.kt

@@ -0,0 +1,71 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * 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
+ *
+ *     https://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.
+ */
+
+package com.google.samples.apps.nowinandroid.core.testing.util
+
+import com.google.samples.apps.nowinandroid.core.data.util.ErrorMonitor
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.MESSAGE
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.OFFLINE
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType.UNKNOWN
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
+
+class TestErrorMonitor : ErrorMonitor {
+
+    private val _messages = MutableStateFlow<List<MessageData>>(emptyList())
+
+    override fun addMessageByString(message: String): MessageData {
+        TODO("Not yet implemented")
+    }
+
+    override fun addMessageByData(message: MessageData) {
+        TODO("Not yet implemented")
+    }
+
+    override fun clearMessage(message: MessageData) {
+        TODO("Not yet implemented")
+    }
+
+    override fun clearAllMessages() {
+        _messages.value = emptyList()
+    }
+
+    override val messages: Flow<List<MessageData?>>
+        get() = _messages
+
+    /**
+     * Test-only API to add message types
+     */
+
+    fun setOfflineMessage() {
+        _messages.value = listOf(OFFLINE_MESSAGE)
+    }
+
+    fun addMessage() {
+        _messages.value.plus(MESSAGE_MESSAGE)
+    }
+
+    fun addUnknownMessage() {
+        _messages.value.plus(UNKNOWN_MESSAGE)
+    }
+
+    companion object {
+        val OFFLINE_MESSAGE = MessageData(OFFLINE)
+        val MESSAGE_MESSAGE = MessageData(MESSAGE("Message"), "Title", {}, {})
+        val UNKNOWN_MESSAGE = MessageData(UNKNOWN)
+    }
+}