Added new generate_to_json_function build option

Controls how `toJson` functionality is generated for all types processed
by this generator.

If `false` (the default), a private `_$ClassNameSerializerMixin` class is
created in the generated part file which contains a `toJson` method.

Mix in this class to the source class:

```dart
@JsonSerializable()
class Example extends Object with _$ExampleSerializerMixin {
  // ...
}
```

If `true`, then a top-level function is created that you can reference
from your class.

```dart
@JsonSerializable()
class Example {
  // ...
  Map<String, dynamic> toJson() => _$ExampleToJson(this);
}
```

Author: kevmoo

json_annotation/CHANGELOG.md

@@ -16,6 +16,9 @@
   `DisallowedNullValueException` that is thrown when corresponding keys exist in
   a source JSON map, but their values are `null`.
 
+* Updated documentation of `JsonSerializable.createToJson` to include details
+  of the new `generate_to_json_function` configuration option.
+
 ## 0.2.7+1
 
 * Small improvement to `UnrecognizedKeysException.message` output and

json_annotation/lib/src/json_serializable.dart

@@ -25,17 +25,31 @@ class JsonSerializable {
   /// ```
   final bool createFactory;
 
-  /// If `true` (the default), a private `_$ClassNameMixin` class is created
+  /// If `true` (the default), code for decoding JSON is generated fon this
+  /// class.
+  ///
+  /// By default, a private `_$ClassNameMixin` class is created
   /// in the generated part file which contains a `toJson` method.
   ///
   /// Mix in this class to the source class:
   ///
   /// ```dart
   /// @JsonSerializable()
-  /// class Example extends Object with _$ExampleMixin {
+  /// class Example extends Object with _$ExampleSerializerMixin {
   ///   // ...
   /// }
   /// ```
+  ///
+  /// If `json_serializable` is configured with
+  /// `generate_to_json_function: true`, then a top-level function is created
+  /// that you can reference from your class.
+  ///
+  /// ```dart
+  /// @JsonSerializable()
+  /// class Example {
+  ///   Map<String, dynamic> toJson() => _$ExampleToJson(this);
+  /// }
+  /// ```
   final bool createToJson;
 
   /// Whether the generator should include fields with `null` values in the

json_serializable/CHANGELOG.md

@@ -18,6 +18,10 @@
   `JsonSerializableGenerator.withDefaultHelpers` constructor.
 
 * Added `explicit_to_json` configuration option.
+  * See `JsonSerializableGenerator.explicitToJson` for details.
+
+* Added `generate_to_json_function` configuration option.
+  * See `JsonSerializableGenerator.generateToJsonFunction` for details.
 
 ## 0.5.6
 

json_serializable/README.md

@@ -93,6 +93,7 @@ targets:
           any_map: true
           checked: true
           explicit_to_json: true
+          generate_to_json_function: true
 ```
 
 [example]: https://github.com/dart-lang/json_serializable/blob/master/example

json_serializable/lib/builder.dart

@@ -31,6 +31,8 @@ Builder jsonSerializable(BuilderOptions options) {
     checked: optionsMap.remove('checked') as bool,
     anyMap: optionsMap.remove('any_map') as bool,
     explicitToJson: optionsMap.remove('explicit_to_json') as bool,
+    generateToJsonFunction:
+        optionsMap.remove('generate_to_json_function') as bool,
   );
 
   if (optionsMap.isNotEmpty) {

json_serializable/lib/src/generator_helper.dart

@@ -33,6 +33,10 @@ Future<String> generate(JsonSerializableGenerator generator, Element element,
 }
 
 class _GeneratorHelper {
+  /// Name of the parameter used when generating top-level `toJson` functions
+  /// if [JsonSerializableGenerator.generateToJsonFunction] is `true`.
+  static const _toJsonParamName = 'instance';
+
   final ClassElement _element;
   final JsonSerializable _annotation;
   final JsonSerializableGenerator _generator;
@@ -76,42 +80,7 @@ class _GeneratorHelper {
       return set;
     });
 
-    if (_annotation.createToJson) {
-      //
-      // Generate the mixin class
-      //
-      _buffer.writeln('abstract class ${_mixinClassName(true)} {');
-
-      // write copies of the fields - this allows the toJson method to access
-      // the fields of the target class
-      for (var field in accessibleFields) {
-        //TODO - handle aliased imports
-        _buffer.writeln('  ${field.type} get ${field.name};');
-      }
-
-      _buffer.write('  Map<String, dynamic> toJson() ');
-
-      var writeNaive = accessibleFields.every(_writeJsonValueNaive);
-
-      if (_generator.useWrappers) {
-        _buffer.writeln('=> new ${_wrapperClassName(false)}(this);');
-      } else {
-        if (writeNaive) {
-          // write simple `toJson` method that includes all keys...
-          _writeToJsonSimple(accessibleFields);
-        } else {
-          // At least one field should be excluded if null
-          _writeToJsonWithNullChecks(accessibleFields);
-        }
-      }
-
-      // end of the mixin class
-      _buffer.writeln('}');
-
-      if (_generator.useWrappers) {
-        _writeWrapper(accessibleFields);
-      }
-    }
+    _writeToJson(accessibleFields);
 
     return _buffer.toString();
   }
@@ -274,12 +243,67 @@ class _GeneratorHelper {
     }
   }
 
+  void _writeToJson(Set<FieldElement> accessibleFields) {
+    if (!_annotation.createToJson) {
+      return;
+    }
+
+    if (_generator.generateToJsonFunction) {
+      var functionName = '${_prefix}ToJson${_genericClassArguments(true)}';
+      _buffer.write('Map<String, dynamic> $functionName'
+          '($_targetClassReference $_toJsonParamName) ');
+    } else {
+      //
+      // Generate the mixin class
+      //
+      _buffer.writeln('abstract class ${_mixinClassName(true)} {');
+
+      // write copies of the fields - this allows the toJson method to access
+      // the fields of the target class
+      for (var field in accessibleFields) {
+        //TODO - handle aliased imports
+        _buffer.writeln('  ${field.type} get ${field.name};');
+      }
+
+      _buffer.write('  Map<String, dynamic> toJson() ');
+    }
+
+    var writeNaive = accessibleFields.every(_writeJsonValueNaive);
+
+    if (_generator.useWrappers) {
+      var param = _generator.generateToJsonFunction ? _toJsonParamName : 'this';
+      _buffer.writeln('=> new ${_wrapperClassName(false)}($param);');
+    } else {
+      if (writeNaive) {
+        // write simple `toJson` method that includes all keys...
+        _writeToJsonSimple(accessibleFields);
+      } else {
+        // At least one field should be excluded if null
+        _writeToJsonWithNullChecks(accessibleFields);
+      }
+    }
+
+    if (!_generator.generateToJsonFunction) {
+      // end of the mixin class
+      _buffer.writeln('}');
+    }
+
+    if (_generator.useWrappers) {
+      _writeWrapper(accessibleFields);
+    }
+  }
+
   void _writeWrapper(Iterable<FieldElement> fields) {
     _buffer.writeln();
     // TODO(kevmoo): write JsonMapWrapper if annotation lib is prefix-imported
+
+    var fieldType = _generator.generateToJsonFunction
+        ? _targetClassReference
+        : _mixinClassName(false);
+
     _buffer.writeln('''
 class ${_wrapperClassName(true)} extends \$JsonMapWrapper {
-  final ${_mixinClassName(false)} _v;
+  final $fieldType _v;
   ${_wrapperClassName()}(this._v);
 ''');
 
@@ -334,6 +358,14 @@ class ${_wrapperClassName(true)} extends \$JsonMapWrapper {
     _buffer.writeln('}');
   }
 
+  String _fieldAccess(FieldElement field) {
+    var fieldAccess = field.name;
+    if (_generator.generateToJsonFunction) {
+      fieldAccess = '$_toJsonParamName.$fieldAccess';
+    }
+    return fieldAccess;
+  }
+
   void _writeToJsonWithNullChecks(Iterable<FieldElement> fields) {
     _buffer.writeln('{');
 
@@ -346,13 +378,15 @@ class ${_wrapperClassName(true)} extends \$JsonMapWrapper {
     var directWrite = true;
 
     for (var field in fields) {
-      var safeFieldAccess = field.name;
+      var safeFieldAccess = _fieldAccess(field);
       var safeJsonKeyString = _safeNameAccess(field);
 
       // If `fieldName` collides with one of the local helpers, prefix
       // access with `this.`.
       if (safeFieldAccess == generatedLocalVarName ||
           safeFieldAccess == toJsonMapHelperName) {
+        assert(!_generator.generateToJsonFunction,
+            'This code path should only be hit during the mixin codepath.');
         safeFieldAccess = 'this.$safeFieldAccess';
       }
 
@@ -394,8 +428,9 @@ class ${_wrapperClassName(true)} extends \$JsonMapWrapper {
     _buffer.writeln('=> <String, dynamic>{');
 
     _buffer.writeAll(fields.map((field) {
+      var access = _fieldAccess(field);
       var value =
-          '${_safeNameAccess(field)}: ${_serializeField(field, field.name)}';
+          '${_safeNameAccess(field)}: ${_serializeField(field, access)}';
       return '        $value';
     }), ',\n');
 

json_serializable/lib/src/json_part_builder.dart

@@ -27,13 +27,15 @@ Builder jsonPartBuilder({
   bool anyMap: false,
   bool checked: false,
   bool explicitToJson: false,
+  bool generateToJsonFunction: false,
 }) {
   return new PartBuilder([
     new JsonSerializableGenerator(
       useWrappers: useWrappers,
       anyMap: anyMap,
       checked: checked,
       explicitToJson: explicitToJson,
+      generateToJsonFunction: generateToJsonFunction,
     ),
     const JsonLiteralGenerator()
   ], header: header, formatOutput: formatOutput);

json_serializable/lib/src/json_serializable_generator.dart

@@ -88,6 +88,33 @@ class JsonSerializableGenerator
   /// ```
   final bool explicitToJson;
 
+  /// Controls how `toJson` functionality is generated for all types processed
+  /// by this generator.
+  ///
+  /// If `false` (the default), a private `_$ClassNameSerializerMixin` class is
+  /// created in the generated part file which contains a `toJson` method.
+  ///
+  /// Mix in this class to the source class:
+  ///
+  /// ```dart
+  /// @JsonSerializable()
+  /// class Example extends Object with _$ExampleSerializerMixin {
+  ///   // ...
+  /// }
+  /// ```
+  ///
+  /// If `true`, then a top-level function is created that you can reference
+  /// from your class.
+  ///
+  /// ```dart
+  /// @JsonSerializable()
+  /// class Example {
+  ///   // ...
+  ///   Map<String, dynamic> toJson() => _$ExampleToJson(this);
+  /// }
+  /// ```
+  final bool generateToJsonFunction;
+
   /// Creates an instance of [JsonSerializableGenerator].
   ///
   /// If [typeHelpers] is not provided, three built-in helpers are used:
@@ -98,10 +125,12 @@ class JsonSerializableGenerator
     bool anyMap: false,
     bool checked: false,
     bool explicitToJson: false,
+    bool generateToJsonFunction: false,
   })  : this.useWrappers = useWrappers ?? false,
         this.anyMap = anyMap ?? false,
         this.checked = checked ?? false,
         this.explicitToJson = explicitToJson ?? false,
+        this.generateToJsonFunction = generateToJsonFunction ?? false,
         this._typeHelpers = typeHelpers ?? _defaultHelpers;
 
   /// Creates an instance of [JsonSerializableGenerator].
@@ -114,11 +143,13 @@ class JsonSerializableGenerator
     bool useWrappers: false,
     bool anyMap: false,
     bool checked: false,
+    bool generateToJsonFunction: false,
   }) =>
       new JsonSerializableGenerator(
           useWrappers: useWrappers,
           anyMap: anyMap,
           checked: checked,
+          generateToJsonFunction: generateToJsonFunction,
           typeHelpers:
               new List.unmodifiable(typeHelpers.followedBy(_defaultHelpers)));
 

json_serializable/test/config_test.dart

@@ -101,13 +101,15 @@ const _validConfig = const {
   'use_wrappers': true,
   'any_map': true,
   'checked': true,
-  'explicit_to_json': true
+  'explicit_to_json': true,
+  'generate_to_json_function': true,
 };
 
 const _invalidConfig = const {
   'header': true,
   'use_wrappers': 42,
   'any_map': 42,
   'checked': 42,
-  'explicit_to_json': 42
+  'explicit_to_json': 42,
+  'generate_to_json_function': 42,
 };

json_serializable/test/default_value/default_value.checked.dart

@@ -23,9 +23,7 @@ dvi.DefaultValue fromJson(Map<String, dynamic> json) =>
     _$DefaultValueFromJson(json);
 
 @JsonSerializable()
-class DefaultValue extends Object
-    with _$DefaultValueSerializerMixin
-    implements dvi.DefaultValue {
+class DefaultValue implements dvi.DefaultValue {
   @JsonKey(defaultValue: true)
   bool fieldBool;
 
@@ -62,4 +60,6 @@ class DefaultValue extends Object
 
   factory DefaultValue.fromJson(Map<String, dynamic> json) =>
       _$DefaultValueFromJson(json);
+
+  Map<String, dynamic> toJson() => _$DefaultValueToJson(this);
 }