feat(directive): support GQL directive

refactor scalars setter & getter
refactor enum values

example/queries_examples.dart

@@ -21,7 +21,7 @@ class LoginQuery extends Object with Fields implements GQLOperation {
   ViewerResolver viewer = new ViewerResolver();
 
   @override
-  OperationType get type => OperationType.query;
+  String get type => queryType;
 
   @override
   String get name => 'LoginQuery';
@@ -39,7 +39,7 @@ class AddTestCommentMutation extends Object
   AddCommentMutation addComment = new AddCommentMutation();
 
   @override
-  OperationType get type => OperationType.mutation;
+  String get type => mutationType;
 
   @override
   String get name => 'AddTestCommentMutation';
@@ -352,11 +352,17 @@ class RepositoriesResolver extends Object
  */
 
 class LoginResolver extends Object
-    with Scalar<String>, Alias
+    with Scalar<String>, Alias, Directives
     implements GQLField {
   @override
   String get name => 'login';
 
+  @override
+  String get directive => includeDirective;
+
+  @override
+  String get directiveValue => 'false';
+
   @override
   LoginResolver clone() => new LoginResolver()..aliasId = aliasId;
 }

lib/src/encoder.dart

@@ -50,20 +50,14 @@ class GQLEncoder extends Converter<GQLOperation, String> {
 
   String _encodeOperation(GQLOperation operation) {
     final GQLField field = operation;
-    final rootResolver = _encodeOperationResolvers(operation);
-    final operationType =
-        operation.type == OperationType.mutation ? 'mutation' : 'query';
-    var args = '';
+    final rootField = _encodeOperationFields(operation);
+    final args = (field is Arguments) ? '(${field.args})' : '';
 
-    if (field is Arguments) {
-      args = '(${field.args})';
-    }
-
-    return '$operationType ${operation.name} $args { $rootResolver }';
+    return '${operation.type} ${operation.name} $args { $rootField }';
   }
 
-  String _encodeOperationResolvers(GQLField operation) =>
-      _extractFields(operation).map(_encodeResolver).join(' ');
+  String _encodeOperationFields(GQLField operation) =>
+      _extractFields(operation).map(_encodeField).join(' ');
 
   String _encodeOperationInlineFragments(GQLField operation) =>
       _extractFragments(operation).map(_encodeInlineFragment).join(' ');
@@ -72,24 +66,27 @@ class GQLEncoder extends Converter<GQLOperation, String> {
       _extractNestedFragments(operation).map(_encodeFragment).join('\n');
 
   String _encodeFragment(GQLFragment fragment) {
-    final rootResolver = _encodeOperationResolvers(fragment);
+    final rootField = _encodeOperationFields(fragment);
     final fragmentsGQL = _encodeNestedOperationFragments(fragment);
 
-    return 'fragment ${fragment.name} on ${fragment.onType} { $rootResolver }${fragmentsGQL.isNotEmpty ? fragmentsGQL : ''}';
+    return 'fragment ${fragment.name} on ${fragment.onType} { $rootField }${fragmentsGQL.isNotEmpty ? fragmentsGQL : ''}';
   }
 
-  String _encodeResolver(GQLField operation) {
-    final childrenGQL = _encodeOperationResolvers(operation);
+  String _encodeField(GQLField operation) {
+    final childrenGQL = _encodeOperationFields(operation);
     final childrenFragment = _encodeOperationInlineFragments(operation);
 
     final alias = (operation is Alias) ? '${operation.alias}: ' : '';
     final name = operation.name != null ? '${operation.name} ' : '';
     final args = (operation is Arguments) ? '(${operation.args}) ' : '';
+    final directive = (operation is Directives)
+        ? '@${operation.directive}(if: ${operation.directiveValue}) '
+        : '';
     final fields = (childrenGQL.isNotEmpty || childrenFragment.isNotEmpty)
         ? '{ $childrenGQL $childrenFragment }'
         : '';
 
-    return '$alias$name$args$fields';
+    return '$alias$name$args$directive$fields';
   }
 
   String _encodeInlineFragment(GQLFragment fragment) => '...${fragment.name}';

lib/src/graphql.dart

@@ -12,3 +12,4 @@ part 'graphql_arguments.dart';
 part 'graphql_scalars.dart';
 part 'graphql_fields.dart';
 part 'graphql_fragments.dart';
+part 'graphql_directives.dart';

lib/src/graphql_directives.dart

@@ -0,0 +1,31 @@
+// Copyright Thomas Hourlier. All rights reserved.
+// Use of this source code is governed by a MIT-style license
+// that can be found in the LICENSE file.
+
+part of graphql_client.definitions;
+
+/// The GQL directive: `include`
+const String includeDirective = 'include';
+
+/// The GQL directive: `skip`
+const String skipDirective = 'skip';
+
+/// A GraphQL directive.
+///
+/// It could be applied as a mixin on a [GQLField].
+/// Eg.
+/// A GQL query with a field using an directive.
+/// ```
+/// query MyQuery {
+///   name @include(if: true)
+/// }
+/// ```
+abstract class Directives implements GQLField {
+  /// The directive type.
+  ///
+  /// Should be [includeDirective] or [skipDirective]
+  String get directive;
+
+  /// The GQL directive.
+  String get directiveValue;
+}

lib/src/graphql_operations.dart

@@ -4,19 +4,16 @@
 
 part of graphql_client.definitions;
 
-/// A GQL operation type.
-enum OperationType {
-  /// Represents the GQL type: `query`,
-  query,
+/// The GQL type: `query`,
+const String queryType = 'query';
 
-  /// Represents the GQL type: `mutation`,
-  mutation,
+/// The GQL type: `mutation`,
+const String mutationType = 'mutation';
 
-  /// Represents the GQL type: `subscription`,
-  ///
-  /// WARNING: This is not supported.
-  subscription,
-}
+/// The GQL type: `subscription`,
+///
+/// WARNING: This is not supported.
+const String subscriptionType = 'subscription';
 
 /// A GQL field.
 ///
@@ -39,10 +36,10 @@ abstract class GQLField {
 
 /// A GQL operation.
 ///
-/// This could be whether a: [OperationType.query] or [OperationType.mutation].
+/// This could be whether a: [queryType], [mutationType] or [subscriptionType].
 abstract class GQLOperation extends GQLField {
   /// The query type.
-  OperationType get type;
+  String get type;
 }
 
 /// A GQL Fragment.

lib/src/graphql_scalars.dart

@@ -9,21 +9,8 @@ part of graphql_client.definitions;
 /// It could be applied as a mixin on a [GQLField].
 /// This mixin should be used on a field that has no nested field and is scalar.
 abstract class Scalar<T> implements GQLField {
-  T _value;
-
-  /// Returns the scalar [GQLField] value.
-  ///
-  /// Throws a [StateError] if the value hasn't been set yet.
-  T get value {
-    if (_value == null) {
-      throw new StateError("This Scalar resolver hasn't been resolved.");
-    }
-
-    return _value;
-  }
-
-  /// Sets the [GQLField] value.
-  set value(T v) => _value = v;
+  /// The scalar [GQLField] value.
+  T value;
 }
 
 /// A GQL collection field.
@@ -32,58 +19,18 @@ abstract class Scalar<T> implements GQLField {
 /// This mixin should be used on a field that is a collection
 /// (ie. has the GQL List type).
 abstract class ScalarCollection<T extends GQLField> implements GQLField {
-  List<T> _nodes;
-  List<T> _edges;
-  int _totalCount;
+  /// The nodes collection of [GQLField].
+  List<T> nodes;
+
+  /// The edges collection of [GQLField].
+  List<T> edges;
+
+  /// The collection length of [GQLField].
+  int totalCount;
 
   /// Returns the node [GQLField] that will be used to resolve each list item.
   GQLField get nodesResolver => null;
 
   /// Returns the edge [GQLField] that will be used to resolve each list item.
   GQLField get edgesResolver => null;
-
-  /// Returns the nodes collection of [GQLField].
-  ///
-  /// Throws a [StateError] if the value hasn't been set yet.
-  List<T> get nodes {
-    if (_nodes == null) {
-      throw new StateError(
-          "This ScalarCollection resolver hasn't been resolved.");
-    }
-
-    return _nodes;
-  }
-
-  /// Sets the nodes collection of [GQLField].
-  set nodes(List<T> v) => _nodes = v;
-
-  /// Returns the edges collection of [GQLField].
-  ///
-  /// Throws a [StateError] if the value hasn't been set yet.
-  List<T> get edges {
-    if (_edges == null) {
-      throw new StateError(
-          "This ScalarCollection resolver hasn't been resolved.");
-    }
-
-    return _edges;
-  }
-
-  /// Sets the edges collection of [GQLField].
-  set edges(List<T> v) => _edges = v;
-
-  /// Returns the number of resolved items.
-  ///
-  /// Throws a [StateError] if the value hasn't been set yet.
-  int get totalCount {
-    if (_totalCount == null) {
-      throw new StateError(
-          "This ScalarCollection resolver hasn't been resolved.");
-    }
-
-    return _totalCount;
-  }
-
-  /// Sets the number of resolved items.
-  set totalCount(int v) => _totalCount = v;
 }