Added serialize + deserialize + validate to union type

graphql_schema/lib/src/union.dart

@@ -1,18 +1,56 @@
 part of graphql_schema.src.schema;
 
 class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
-    with
-        _NonNullableMixin<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');
+    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(' | ')
-}
+  String get name => possibleTypes.map((t) => t.name).join(' | ');
+
+  @override
+  Serialized serialize(Value value) {
+    for (var type in possibleTypes) {
+      try {
+        return type.serialize(value);
+      } catch (_) {}
+    }
+
+    throw new ArgumentError();
+  }
+
+  @override
+  Value deserialize(Serialized serialized) {
+    for (var type in possibleTypes) {
+      try {
+        return type.deserialize(serialized);
+      } catch (_) {}
+    }
+
+    throw new ArgumentError();
+  }
+
+  @override
+  ValidationResult<Serialized> validate(String key, Serialized input) {
+    List<String> errors = [];
+
+    for (var type in possibleTypes) {
+      var result = type.validate(key, input);
+
+      if (result.successful) {
+        return result;
+      } else {
+        errors.addAll(result.errors);
+      }
+    }
+
+    return new ValidationResult._failure(errors);
+  }
+}