Allow for custom subscription return types

docs/execution/subscriptions.md

@@ -28,10 +28,15 @@ toSchema(
 
 ### Subscription Hooks
 
-Through the hooks a new method was added `didGenerateSubscriptionType` which is called after a new subscription type is
-generated but before it is added to the schema. The other hook are still called so you can add logic for the types and
+#### `didGenerateSubscriptionType`
+This hook is called after a new subscription type is generated but before it is added to the schema. The other generator hooks are still called so you can add logic for the types and
 validation of subscriptions the same as queries and mutations.
 
+#### `isValidSubscriptionReturnType`
+This hook is called when generating the functions for each subscription. It allows for changing the rules of what classes can be used as the return type. By default, graphql-java supports `org.reactivestreams.Publisher`.
+
+To effectively use this hook, you should also override the `willResolveMonad` hook, and if you are using `graphql-kotlin-spring-server` you should override the `GraphQL` bean to specify a custom subscription execution strategy.
+
 ### Server Implementation
 
 The server that runs your GraphQL schema will have to support some method for subscriptions, like WebSockets.

graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/exceptions/InvalidSubscriptionTypeException.kt

@@ -22,7 +22,7 @@ import kotlin.reflect.KFunction
 
 class InvalidSubscriptionTypeException(kClass: KClass<*>, kFunction: KFunction<*>? = null) :
     GraphQLKotlinException(
-        "Schema requires all subscriptions to be public and return a type of Publisher. " +
+        "Schema requires all subscriptions to be public and return a valid type from the hooks. " +
         "${kClass.simpleName} has ${kClass.visibility} visibility modifier. " +
         if (kFunction != null) "The function return type is ${kFunction.returnType.getSimpleName()}" else ""
     )

graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/types/generateSubscription.kt

@@ -21,9 +21,7 @@ import com.expediagroup.graphql.exceptions.InvalidSubscriptionTypeException
 import com.expediagroup.graphql.generator.SchemaGenerator
 import com.expediagroup.graphql.generator.extensions.getValidFunctions
 import com.expediagroup.graphql.generator.extensions.isNotPublic
-import com.expediagroup.graphql.generator.extensions.isSubclassOf
 import graphql.schema.GraphQLObjectType
-import org.reactivestreams.Publisher
 
 internal fun generateSubscriptions(generator: SchemaGenerator, subscriptions: List<TopLevelObject>): GraphQLObjectType? {
     if (subscriptions.isEmpty()) {
@@ -34,18 +32,20 @@ internal fun generateSubscriptions(generator: SchemaGenerator, subscriptions: Li
     subscriptionBuilder.name(generator.config.topLevelNames.subscription)
 
     for (subscription in subscriptions) {
-        if (subscription.kClass.isNotPublic()) {
-            throw InvalidSubscriptionTypeException(subscription.kClass)
+        val kClass = subscription.kClass
+
+        if (kClass.isNotPublic()) {
+            throw InvalidSubscriptionTypeException(kClass)
         }
 
-        subscription.kClass.getValidFunctions(generator.config.hooks)
+        kClass.getValidFunctions(generator.config.hooks)
             .forEach {
-                if (it.returnType.isSubclassOf(Publisher::class).not()) {
-                    throw InvalidSubscriptionTypeException(subscription.kClass, it)
+                if (generator.config.hooks.isValidSubscriptionReturnType(kClass, it).not()) {
+                    throw InvalidSubscriptionTypeException(kClass, it)
                 }
 
                 val function = generateFunction(generator, it, generator.config.topLevelNames.subscription, subscription.obj)
-                val functionFromHook = generator.config.hooks.didGenerateSubscriptionField(subscription.kClass, it, function)
+                val functionFromHook = generator.config.hooks.didGenerateSubscriptionField(kClass, it, function)
                 subscriptionBuilder.field(functionFromHook)
             }
     }

graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/hooks/SchemaGeneratorHooks.kt

@@ -23,6 +23,7 @@ import com.expediagroup.graphql.exceptions.EmptyMutationTypeException
 import com.expediagroup.graphql.exceptions.EmptyObjectTypeException
 import com.expediagroup.graphql.exceptions.EmptyQueryTypeException
 import com.expediagroup.graphql.exceptions.EmptySubscriptionTypeException
+import com.expediagroup.graphql.generator.extensions.isSubclassOf
 import graphql.schema.FieldCoordinates
 import graphql.schema.GraphQLCodeRegistry
 import graphql.schema.GraphQLFieldDefinition
@@ -33,6 +34,7 @@ import graphql.schema.GraphQLSchema
 import graphql.schema.GraphQLSchemaElement
 import graphql.schema.GraphQLType
 import graphql.schema.GraphQLTypeUtil
+import org.reactivestreams.Publisher
 import kotlin.reflect.KClass
 import kotlin.reflect.KFunction
 import kotlin.reflect.KProperty
@@ -91,6 +93,15 @@ interface SchemaGeneratorHooks {
     @Suppress("Detekt.FunctionOnlyReturningConstant")
     fun isValidFunction(kClass: KClass<*>, function: KFunction<*>): Boolean = true
 
+    /**
+     * Called when looking at the subscription functions to determine if it is using a valid return type.
+     * By default, graphql-java supports org.reactivestreams.Publisher in the subscription execution strategy.
+     * If you want to provide a custom execution strategy, you may need to override this hook.
+     *
+     * NOTE: You will most likely need to also override the [willResolveMonad] hook to allow for your custom type to be generated.
+     */
+    fun isValidSubscriptionReturnType(kClass: KClass<*>, function: KFunction<*>): Boolean = function.returnType.isSubclassOf(Publisher::class)
+
     /**
      * Called after `willGenerateGraphQLType` and before `didGenerateGraphQLType`.
      * Enables you to change the wiring, e.g. apply directives to alter the target type.

graphql-kotlin-schema-generator/src/test/kotlin/com/expediagroup/graphql/generator/types/GenerateSubscriptionTest.kt

@@ -20,15 +20,20 @@ import com.expediagroup.graphql.TopLevelNames
 import com.expediagroup.graphql.TopLevelObject
 import com.expediagroup.graphql.exceptions.EmptySubscriptionTypeException
 import com.expediagroup.graphql.exceptions.InvalidSubscriptionTypeException
+import com.expediagroup.graphql.generator.extensions.getTypeOfFirstArgument
+import com.expediagroup.graphql.generator.extensions.isSubclassOf
 import com.expediagroup.graphql.hooks.SchemaGeneratorHooks
 import graphql.schema.GraphQLFieldDefinition
 import io.mockk.every
 import io.reactivex.rxjava3.core.Flowable
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flowOf
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
 import org.reactivestreams.Publisher
 import kotlin.reflect.KClass
 import kotlin.reflect.KFunction
+import kotlin.reflect.KType
 import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
 import kotlin.test.assertNotNull
@@ -123,6 +128,26 @@ internal class GenerateSubscriptionTest : TypeTestHelper() {
         assertEquals(3, result?.fieldDefinitions?.size)
         assertNotNull(result?.fieldDefinitions?.find { it.name == "changedField" })
     }
+
+    @Test
+    fun `given custom hooks that allow custom subscription return types, it should generate a valid schema`() {
+        val subscriptions = listOf(TopLevelObject(MyCustomSubscriptionClass()))
+
+        class CustomHooks : SchemaGeneratorHooks {
+            override fun isValidSubscriptionReturnType(kClass: KClass<*>, function: KFunction<*>) = function.returnType.isSubclassOf(Flow::class)
+            override fun willResolveMonad(type: KType): KType = when {
+                type.isSubclassOf(Flow::class) -> type.getTypeOfFirstArgument()
+                else -> this.willResolveMonad(type)
+            }
+        }
+
+        every { config.hooks } returns CustomHooks()
+
+        val result = generateSubscriptions(generator, subscriptions)
+
+        assertEquals(1, result?.fieldDefinitions?.size)
+        assertNotNull(result?.fieldDefinitions?.find { it.name == "number" })
+    }
 }
 
 class MyPublicTestSubscription {
@@ -138,6 +163,11 @@ class MyInvalidSubscriptionClass {
     fun number(): Int = 1
 }
 
+class MyCustomSubscriptionClass {
+    @Suppress("Detekt.FunctionOnlyReturningConstant")
+    fun number(): Flow<Int> = flowOf(1)
+}
+
 private class MyPrivateTestSubscription {
     fun counter(): Publisher<Int> = Flowable.just(3)
 }