Introspection supports union types

graphql_schema/lib/src/schema.dart

@@ -19,6 +19,8 @@ part 'scalar.dart';
 
 part 'type.dart';
 
+part 'union.dart';
+
 part 'validation_result.dart';
 
 class GraphQLSchema {

graphql_schema/lib/src/union.dart

@@ -0,0 +1,18 @@
+part of graphql_schema.src.schema;
+
+class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
+    with
+        _NonNullableMixin<Value, Serialized> {
+  final List<GraphQLType<Value, Serialized>> possibleTypes;
+  final String description;
+
+  GraphQLUnionType(this.possibleTypes, {this.description}) {
+    assert(possibleTypes.every((
+        t) => t is GraphQLUnionType), 'The member types of a Union type must all be Object base types; Scalar, Interface and Union types may not be member types of a Union. Similarly, wrapping types may not be member types of a Union');
+    assert(possibleTypes
+        .isNotEmpty, 'A Union type must define one or more member types');
+  }
+
+  @override
+  String get name => possibleTypes.map((t) => t.name).join(' | ')
+}

graphql_server/lib/introspection.dart

@@ -122,6 +122,8 @@ GraphQLObjectType _reflectSchemaTypes() {
         resolve: (type, _) {
           if (type is GraphQLObjectType && type.isInterface) {
             return type.possibleTypes;
+          } else if (type is GraphQLUnionType) {
+            return type.possibleTypes;
           } else {
             return null;
           }
@@ -205,9 +207,11 @@ GraphQLObjectType _createTypeType() {
           return 'NON_NULL';
         else if (t is GraphQLEnumType)
           return 'ENUM';
+        else if (t is GraphQLUnionType)
+          return 'UNION';
         else
           throw new UnsupportedError(
-              'Cannot get the kind of $t.'); // TODO: union
+              'Cannot get the kind of $t.');
       },
     ),
     field(
@@ -445,8 +449,10 @@ Iterable<GraphQLType> _fetchAllTypesFromType(GraphQLType type) {
     types.addAll(_fetchAllTypesFromObject(type));
   } else if (type is GraphQLEnumType) {
     types.add(type);
+  } else if ( type is GraphQLUnionType) {
+    for (var t in type.possibleTypes) {
+      types.addAll(_fetchAllTypesFromType(t));
+    }
   }
-
-  // TODO:  union
   return types;
 }