Support custom error types

store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/FetcherResult.kt

@@ -5,5 +5,6 @@ sealed class FetcherResult<out Network : Any> {
     sealed class Error : FetcherResult<Nothing>() {
         data class Exception(val error: Throwable) : Error()
         data class Message(val message: String) : Error()
+        data class Custom<E : Any>(val error: E) : Error()
     }
 }

store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/StoreReadResponse.kt

@@ -44,8 +44,8 @@ sealed class StoreReadResponse<out Output> {
         StoreReadResponse<Output>()
 
     /**
-     * No new data event dispatched by Store to signal the [Fetcher] returned no data (i.e the
-     * returned [kotlinx.coroutines.Flow], when collected, was empty).
+     * No new data event dispatched by Store to signal the [Fetcher] returned no data (i.e., the
+     * returned [kotlinx.coroutines.flow.Flow], when collected, was empty).
      */
     data class NoNewData(override val origin: StoreReadResponseOrigin) : StoreReadResponse<Nothing>()
 
@@ -62,6 +62,11 @@ sealed class StoreReadResponse<out Output> {
             val message: String,
             override val origin: StoreReadResponseOrigin
         ) : Error()
+
+        data class Custom<E : Any>(
+            val error: E,
+            override val origin: StoreReadResponseOrigin
+        ) : Error()
     }
 
     /**
@@ -105,6 +110,26 @@ sealed class StoreReadResponse<out Output> {
         else -> null
     }
 
+    private fun errorOrNull(): Throwable? {
+        if (this is Error.Exception) {
+            return error
+        }
+
+        return null
+    }
+
+    /**
+     * @returns Error if there is one, else null.
+     */
+    @Suppress("UNCHECKED_CAST")
+    fun <E : Any> errorOrNull(): E? {
+        if (this is Error.Custom<*>) {
+            return (this as? Error.Custom<E>)?.error
+        }
+
+        return errorOrNull() as? E
+    }
+
     @Suppress("UNCHECKED_CAST")
     internal fun <T> swapType(): StoreReadResponse<T> = when (this) {
         is Error -> this
@@ -141,4 +166,11 @@ sealed class StoreReadResponseOrigin {
 fun StoreReadResponse.Error.doThrow(): Nothing = when (this) {
     is StoreReadResponse.Error.Exception -> throw error
     is StoreReadResponse.Error.Message -> throw RuntimeException(message)
+    is StoreReadResponse.Error.Custom<*> -> {
+        if (error is Throwable) {
+            throw error
+        } else {
+            throw RuntimeException("Non-throwable custom error: $error")
+        }
+    }
 }

store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/FetcherController.kt

@@ -92,6 +92,10 @@ internal class FetcherController<Key : Any, Network : Any, Output : Any, Local :
                             it.error,
                             origin = StoreReadResponseOrigin.Fetcher()
                         )
+                        is FetcherResult.Error.Custom<*> -> StoreReadResponse.Error.Custom(
+                            it.error,
+                            StoreReadResponseOrigin.Fetcher()
+                        )
                     }
                 }.onEmpty {
                     val origin =

store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/FetcherResponseTests.kt

@@ -211,6 +211,32 @@ class FetcherResponseTests {
         )
     }
 
+    @Test
+    fun givenAFetcherThatEmitsCustomErrorWhenStreamingThenCustomErrorShouldBeEmitted() = testScope.runTest {
+        data class TestCustomError(val errorMessage: String)
+        val customError = TestCustomError("Test custom error")
+
+        val store = StoreBuilder.from(
+            fetcher = Fetcher.ofResultFlow { _: Int ->
+                flowOf(
+                    FetcherResult.Error.Custom(customError)
+                )
+            }
+        ).buildWithTestScope()
+
+        assertEmitsExactly(
+            store.stream(StoreReadRequest.fresh(1)),
+            listOf(
+                StoreReadResponse.Loading(origin = StoreReadResponseOrigin.Fetcher()),
+                StoreReadResponse.Error.Custom(
+                    error = customError,
+                    origin = StoreReadResponseOrigin.Fetcher()
+                )
+            )
+        )
+    }
+
+
     private fun <Key : Any, Output : Any> StoreBuilder<Key, Output>.buildWithTestScope() =
         scope(testScope).build()
 }