Support string and null as schema representation

pkgs/dart_model/lib/src/dart_model.g.dart

@@ -85,6 +85,16 @@ extension type Model.fromJson(Map<String, Object?> node) {
   Map<String, Library> get uris => (node['uris'] as Map).cast();
 }
 
+/// A Dart type of the form `T?` for an inner type `T`
+extension type NullableType.fromJson(Map<String, Object?> node) {
+  NullableType({
+    StaticType? inner,
+  }) : this.fromJson({
+          if (inner != null) 'inner': inner,
+        });
+  StaticType get inner => node['inner'] as StaticType;
+}
+
 /// Set of boolean properties.
 extension type Properties.fromJson(Map<String, Object?> node) {
   Properties({
@@ -131,3 +141,45 @@ extension type QualifiedName.fromJson(String string) {
 extension type Query.fromJson(Map<String, Object?> node) {
   Query() : this.fromJson({});
 }
+
+enum StaticTypeType {
+  unknown,
+  nullableType,
+  voidType;
+}
+
+extension type StaticType.fromJson(Map<String, Object?> node) {
+  static StaticType nullableType(NullableType nullableType) =>
+      StaticType.fromJson({'type': 'NullableType', 'value': nullableType.node});
+  static StaticType voidType(VoidType voidType) =>
+      StaticType.fromJson({'type': 'VoidType', 'value': voidType.string});
+  StaticTypeType get type {
+    switch (node['type'] as String) {
+      case 'NullableType':
+        return StaticTypeType.nullableType;
+      case 'VoidType':
+        return StaticTypeType.voidType;
+      default:
+        return StaticTypeType.unknown;
+    }
+  }
+
+  NullableType get asNullableType {
+    if (node['type'] != 'NullableType') {
+      throw StateError('Not a NullableType.');
+    }
+    return NullableType.fromJson(node['value'] as Map<String, Object?>);
+  }
+
+  VoidType get asVoidType {
+    if (node['type'] != 'VoidType') {
+      throw StateError('Not a VoidType.');
+    }
+    return VoidType.fromJson(node['value'] as String);
+  }
+}
+
+/// The type-hierarchy representation of the type `void`.
+extension type VoidType.fromJson(String string) {
+  VoidType(String string) : this.fromJson(string);
+}

tool/dart_model_generator/lib/generate_dart_model.dart

@@ -37,16 +37,45 @@ String generate(String schemaJson,
   final schema = JsonSchema.create(schemaJson,
       refProvider: LocalRefProvider(dartModelJson ??
           File('schemas/dart_model.schema.json').readAsStringSync()));
-  for (final def in schema.defs.entries) {
-    if (def.value.oneOf.isNotEmpty) {
-      result.add(_generateUnion(def.key, def.value.oneOf));
+  final allDefinitions = <String, JsonSchema>{
+    for (final def in schema.defs.entries) def.key: def.value,
+  };
+
+  for (final MapEntry(:key, :value) in allDefinitions.entries) {
+    if (value.oneOf.isNotEmpty) {
+      result.add(
+          _generateUnion(key, value.oneOf, allDefinitions: allDefinitions));
     } else {
-      result.add(_generateExtensionType(def.key, def.value));
+      result.add(_generateExtensionType(key, value));
     }
   }
   return DartFormatter().formatSource(SourceCode(result.join('\n'))).text;
 }
 
+/// The Dart type used to represent the JSON value for [definition].
+///
+/// This is most commonly a `Map<String, Object?>, but can also be a JSON
+/// primitive type for simpler definitions.
+String _dartJsonType(JsonSchema definition) {
+  return switch (definition.type) {
+    SchemaType.object => 'Map<String, Object?>',
+    SchemaType.string => 'String',
+    SchemaType.nullValue => 'Null',
+    _ => throw UnsupportedError('Unsupported type: ${definition.type}'),
+  };
+}
+
+/// Expands a [wrapper] expression evaluating to a generated extension type
+/// instance to obtain the underlying JSON representation.
+String _dartWrapperToJson(String wrapper, JsonSchema definition) {
+  return switch (definition.type) {
+    SchemaType.object => '$wrapper.node',
+    SchemaType.string => '$wrapper.string',
+    SchemaType.nullValue => 'null',
+    _ => throw UnsupportedError('Unsupported type: ${definition.type}'),
+  };
+}
+
 String _generateExtensionType(String name, JsonSchema definition) {
   final result = StringBuffer();
 
@@ -55,6 +84,7 @@ String _generateExtensionType(String name, JsonSchema definition) {
   final jsonType = switch (definition.type) {
     SchemaType.object => 'Map<String, Object?> node',
     SchemaType.string => 'String string',
+    SchemaType.nullValue => 'Null _',
     _ => throw UnsupportedError('Schema type ${definition.type}.'),
   };
   if (definition.description != null) {
@@ -96,6 +126,8 @@ String _generateExtensionType(String name, JsonSchema definition) {
       }
     case SchemaType.string:
       result.writeln('$name(String string) : this.fromJson(string);');
+    case SchemaType.nullValue:
+      result.writeln('$name(): this.fromJson(null);');
     default:
       throw UnsupportedError('Unsupported type: ${definition.type}');
   }
@@ -142,32 +174,39 @@ String _generateExtensionType(String name, JsonSchema definition) {
 /// and `asFoo` getters that "cast" to each type.
 ///
 /// On the wire the union type is: `{"type": <name>, "value": <value>}`
-String _generateUnion(String name, List<JsonSchema> oneOf) {
+String _generateUnion(
+  String name,
+  List<JsonSchema> oneOf, {
+  required Map<String, JsonSchema> allDefinitions,
+}) {
   final result = StringBuffer();
-  final types =
-      oneOf.map((s) => _refName(s.schemaMap![r'$ref'] as String)).toList();
+  final unionEntries = oneOf
+      .map((s) => _refName(s.schemaMap![r'$ref'] as String))
+      .map((name) => (name, allDefinitions[name]!));
 
   // TODO(davidmorgan): add description(s).
   result
     ..writeln('enum ${name}Type {')
-    ..writeln(['unknown'].followedBy(types.map(_firstToLowerCase)).join(', '))
+    ..writeln(['unknown']
+        .followedBy(unionEntries.map((e) => _firstToLowerCase(e.$1)))
+        .join(', '))
     ..writeln(';}');
 
   // TODO(davidmorgan): add description.
   result.writeln('extension type $name.fromJson(Map<String, Object?> node) {');
-  for (final type in types) {
+  for (final (type, def) in unionEntries) {
     final lowerType = _firstToLowerCase(type);
     result
       ..writeln('static $name $lowerType($type $lowerType) =>')
       ..writeln('$name.fromJson({')
       ..writeln("'type': '$type',")
-      ..writeln("'value': $lowerType.node});");
+      ..writeln("'value': ${_dartWrapperToJson(lowerType, def)}});");
   }
 
   result
     ..writeln('${name}Type get type {')
     ..writeln("switch(node['type'] as String) {");
-  for (final type in types) {
+  for (final (type, _) in unionEntries) {
     final lowerType = _firstToLowerCase(type);
     result.writeln("case '$type': return ${name}Type.$lowerType;");
   }
@@ -176,12 +215,13 @@ String _generateUnion(String name, List<JsonSchema> oneOf) {
     ..writeln('}')
     ..writeln('}');
 
-  for (final type in types) {
+  for (final (type, schema) in unionEntries) {
     result
       ..writeln('$type get as$type {')
       ..writeln("if (node['type'] != '$type') "
           "{ throw StateError('Not a $type.'); }")
-      ..writeln("return $type.fromJson(node['value'] as Map<String, Object?>);")
+      ..writeln(
+          "return $type.fromJson(node['value'] as ${_dartJsonType(schema)});")
       ..writeln('}');
   }
   result.writeln('}');