Improve error message when a field cannot be (de)serialized

Author: kevmoo

json_serializable/CHANGELOG.md

@@ -1,4 +1,7 @@
-## 0.2.6
+## 0.3.0
+
+* **BREAKING** `UnsupportedTypeError` added a new required constructor argument:
+  `reason`.
 
 * Make `null` field handling smarter. If a field is classified as not
   `nullable`, then use this knowledge when generating serialization code –  even

json_serializable/lib/src/json_serializable_generator.dart

@@ -344,13 +344,7 @@ void $toJsonMapHelperName(String key, dynamic value) {
       return _serialize(
           field.type, accessOverride, _nullable(field, classIncludeNullable));
     } on UnsupportedTypeError catch (e) {
-      var extra = (field.type != e.type) ? ' because of type `${e.type}`' : '';
-
-      var message = 'Could not generate `toJson` code for '
-          '`${friendlyNameForElement(field)}`$extra.';
-
-      throw new InvalidGenerationSourceError(message,
-          todo: 'Make sure all of the types are serializable.');
+      throw _createInvalidGenerationError('toJson', field, e);
     }
   }
 
@@ -360,8 +354,8 @@ void $toJsonMapHelperName(String key, dynamic value) {
       _allHelpers
           .map((h) => h.serialize(targetType, expression, nullable, _serialize))
           .firstWhere((r) => r != null,
-              orElse: () =>
-                  throw new UnsupportedTypeError(targetType, expression));
+              orElse: () => throw new UnsupportedTypeError(
+                  targetType, expression, _notSupportedWithTypeHelpersMsg));
 
   String _deserializeForField(FieldElement field, bool classSupportNullable,
       {ParameterElement ctorParam}) {
@@ -373,13 +367,7 @@ void $toJsonMapHelperName(String key, dynamic value) {
       return _deserialize(
           targetType, 'json[$jsonKey]', _nullable(field, classSupportNullable));
     } on UnsupportedTypeError catch (e) {
-      var extra = (field.type != e.type) ? ' because of type `${e.type}`' : '';
-
-      var message = 'Could not generate `fromJson` code for '
-          '`${friendlyNameForElement(field)}`$extra.';
-
-      throw new InvalidGenerationSourceError(message,
-          todo: 'Make sure all of the types are serializable.');
+      throw _createInvalidGenerationError('fromJson', field, e);
     }
   }
 
@@ -388,8 +376,8 @@ void $toJsonMapHelperName(String key, dynamic value) {
           .map((th) =>
               th.deserialize(targetType, expression, nullable, _deserialize))
           .firstWhere((r) => r != null,
-              orElse: () =>
-                  throw new UnsupportedTypeError(targetType, expression));
+              orElse: () => throw new UnsupportedTypeError(
+                  targetType, expression, _notSupportedWithTypeHelpersMsg));
 }
 
 String _safeNameAccess(FieldElement field) {
@@ -457,3 +445,17 @@ int _offsetFor(FieldElement e) {
   }
   return e.nameOffset;
 }
+
+final _notSupportedWithTypeHelpersMsg =
+    'None of the provided `TypeHelper` instances support the defined type.';
+
+InvalidGenerationSourceError _createInvalidGenerationError(
+    String targetMember, FieldElement field, UnsupportedTypeError e) {
+  var extra = (field.type != e.type) ? ' because of type `${e.type}`' : '';
+
+  var message = 'Could not generate `$targetMember` code for '
+      '`${friendlyNameForElement(field)}`$extra.\n${e.reason}';
+
+  return new InvalidGenerationSourceError(message,
+      todo: 'Make sure all of the types are serializable.');
+}

json_serializable/lib/src/type_helper.dart

@@ -78,8 +78,9 @@ const simpleJsonTypeChecker = const TypeChecker.any(const [
 class UnsupportedTypeError extends Error {
   final String expression;
   final DartType type;
+  final String reason;
 
-  UnsupportedTypeError(this.type, this.expression);
+  UnsupportedTypeError(this.type, this.expression, this.reason);
 }
 
 DartType _getImplementationType(DartType type, TypeChecker checker) {

json_serializable/lib/src/type_helpers/map_helper.dart

@@ -21,18 +21,7 @@ class MapHelper extends TypeHelper {
     var keyArg = args[0];
     var valueType = args[1];
 
-    // We're not going to handle converting key types at the moment
-    // So the only safe types for key are dynamic/Object/String
-    var safeKey = keyArg.isDynamic ||
-        keyArg.isObject ||
-        _stringTypeChecker.isExactlyType(keyArg);
-
-    if (!safeKey) {
-      // TODO: should add some logic to `UnsupportedTypeError` to allow more
-      //       details to be provided – such as this case where the `key` in
-      //       `targetType` is not supported.
-      throw new UnsupportedTypeError(targetType, expression);
-    }
+    _checkSafeKeyType(expression, keyArg);
 
     var subFieldValue = serializeNested(valueType, _closureArg, nullable);
 
@@ -62,15 +51,7 @@ class MapHelper extends TypeHelper {
     var keyArg = typeArgs.first;
     var valueArg = typeArgs.last;
 
-    // We're not going to handle converting key types at the moment
-    // So the only safe types for key are dynamic/Object/String
-    var safeKey = keyArg.isDynamic ||
-        keyArg.isObject ||
-        _stringTypeChecker.isExactlyType(keyArg);
-
-    if (!safeKey) {
-      throw new UnsupportedTypeError(keyArg, expression);
-    }
+    _checkSafeKeyType(expression, keyArg);
 
     // this is the trivial case. Do a runtime cast to the known type of JSON
     // map values - `Map<String, dynamic>`
@@ -96,6 +77,19 @@ class MapHelper extends TypeHelper {
 
     return commonNullPrefix(nullable, expression, result);
   }
+
+  void _checkSafeKeyType(String expression, DartType keyArg) {
+    // We're not going to handle converting key types at the moment
+    // So the only safe types for key are dynamic/Object/String
+    var safeKey = keyArg.isDynamic ||
+        keyArg.isObject ||
+        _stringTypeChecker.isExactlyType(keyArg);
+
+    if (!safeKey) {
+      throw new UnsupportedTypeError(keyArg, expression,
+          'The type of the Map key must be `String`, `Object` or `dynamic`.');
+    }
+  }
 }
 
 final _coreMapChecker = const TypeChecker.fromUrl('dart:core#Map');

json_serializable/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_serializable
-version: 0.2.6-dev
+version: 0.3.0-dev
 author: Dart Team <misc@dartlang.org>
 description: Generates utilities to aid in serializing to/from JSON.
 homepage: https://github.com/dart-lang/json_serializable

json_serializable/test/json_serializable_test.dart

@@ -63,37 +63,41 @@ void main() {
   });
 
   group('unserializable types', () {
+    final noSupportHelperFyi =
+        'Could not generate `toJson` code for `Stopwatch watch`.\n'
+        'None of the provided `TypeHelper` instances support the defined type.';
+
     test('for toJson', () async {
       expect(
           _runForElementNamed('NoSerializeFieldType'),
-          throwsInvalidGenerationSourceError(
-              'Could not generate `toJson` code for `Stopwatch watch`.',
+          throwsInvalidGenerationSourceError(noSupportHelperFyi,
               'Make sure all of the types are serializable.'));
     });
 
     test('for fromJson', () async {
       expect(
           _runForElementNamed('NoDeserializeFieldType'),
           throwsInvalidGenerationSourceError(
-              'Could not generate `fromJson` code for `Stopwatch watch`.',
+              noSupportHelperFyi.replaceFirst('toJson', 'fromJson'),
               'Make sure all of the types are serializable.'));
     });
 
+    final mapKeyFyi = 'Could not generate `toJson` code for '
+        '`Map<int, DateTime> intDateTimeMap` because of type `int`.\n'
+        'The type of the Map key must be `String`, `Object` or `dynamic`.';
+
     test('for toJson in Map key', () async {
       expect(
           _runForElementNamed('NoSerializeBadKey'),
           throwsInvalidGenerationSourceError(
-              'Could not generate `toJson` code for '
-              '`Map<int, DateTime> intDateTimeMap`.',
-              'Make sure all of the types are serializable.'));
+              mapKeyFyi, 'Make sure all of the types are serializable.'));
     });
 
     test('for fromJson', () async {
       expect(
           _runForElementNamed('NoDeserializeBadKey'),
           throwsInvalidGenerationSourceError(
-              'Could not generate `fromJson` code for '
-              '`Map<int, DateTime> intDateTimeMap` because of type `int`.',
+              mapKeyFyi.replaceFirst('toJson', 'fromJson'),
               'Make sure all of the types are serializable.'));
     });
   });