Add ErrorMessages state to app state

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

@@ -46,12 +46,18 @@ import com.google.samples.apps.nowinandroid.navigation.TopLevelDestination.BOOKM
 import com.google.samples.apps.nowinandroid.navigation.TopLevelDestination.FOR_YOU
 import com.google.samples.apps.nowinandroid.navigation.TopLevelDestination.INTERESTS
 import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.mapNotNull
 import kotlinx.coroutines.flow.stateIn
+import kotlinx.coroutines.flow.update
 import kotlinx.datetime.TimeZone
+import java.util.UUID
 
 @Composable
 fun rememberNiaAppState(
@@ -117,6 +123,20 @@ class NiaAppState(
             initialValue = false,
         )
 
+    private val _errorMessages = MutableStateFlow<List<ErrorMessage>>(emptyList())
+    val errorMessage: StateFlow<String?> = _errorMessages.map{ it.firstOrNull()?.message }.stateIn(
+        scope = coroutineScope,
+        started = SharingStarted.WhileSubscribed(5_000),
+        initialValue = null,
+    )
+    fun addErrorMessage(error: String) {
+        _errorMessages.update { it + ErrorMessage(error) }
+    }
+
+    fun clearErrorMessage() {
+        _errorMessages.update { it.drop(1) }
+    }
+
     /**
      * Map of top level destinations to be used in the TopBar, BottomBar and NavRail. The key is the
      * route.
@@ -198,3 +218,8 @@ private fun NavigationTrackingSideEffect(navController: NavHostController) {
         }
     }
 }
+
+data class ErrorMessage(
+    val message: String,
+    val id: String = UUID.randomUUID().toString(),
+)