add Type System Extension syntax example

example/type-system-extension/README.md

@@ -0,0 +1,11 @@
+# Type System Extension example
+
+https://facebook.github.io/graphql/draft/#sec-Type-Extensions
+
+```
+$ go run ./server/server.go
+2018/10/25 12:46:45 connect to http://localhost:8080/ for GraphQL playground
+
+$ curl -X POST 'http://localhost:8080/query' --data-binary '{"query":"{ todos { id text state verified } }"}'
+{"data":{"todos":[{"id":"Todo:1","text":"Buy a cat food","state":"NOT_YET","verified":false},{"id":"Todo:2","text":"Check cat water","state":"DONE","verified":true},{"id":"Todo:3","text":"Check cat meal","state":"DONE","verified":true}]}}
+```

example/type-system-extension/directive.go

@@ -0,0 +1,44 @@
+package type_system_extension
+
+import (
+	"context"
+	"log"
+
+	"github.com/99designs/gqlgen/graphql"
+)
+
+func EnumLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("enum logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}
+
+func FieldLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("field logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}
+
+func InputLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("input object logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}
+
+func ObjectLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("object logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}
+
+func ScalarLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("scalar logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}
+
+func UnionLogging(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+	rc := graphql.GetResolverContext(ctx)
+	log.Printf("union logging: %v, %s, %T, %+v", rc.Path(), rc.Field.Name, obj, obj)
+	return next(ctx)
+}