[Dart] Add support for advanced union features

dart/lib/flat_buffers.dart

@@ -1022,6 +1022,42 @@ class ListReader<E> extends Reader<List<E>> {
   }
 }
 
+/// The reader of lists of objects. Lazy by default - see [lazy].
+class UnionListReader<E> extends Reader<List<E?>> {
+  final Reader<E>? Function(int index) _getElementReader;
+
+  /// Enables lazy reading of the list
+  ///
+  /// If true, the returned unmodifiable list lazily reads objects on access.
+  /// Therefore, the underlying buffer must not change while accessing the list.
+  ///
+  /// If false, reads the whole list immediately on access.
+  final bool lazy;
+
+  const UnionListReader(this._getElementReader, {this.lazy = true});
+
+  @override
+  @pragma('vm:prefer-inline')
+  int get size => _sizeofUint32;
+
+  @override
+  List<E?> read(BufferContext bc, int offset) {
+    final listOffset = bc.derefObject(offset);
+    return lazy
+        ? _FbUnionList<E>(_getElementReader, bc, listOffset)
+        : List<E?>.generate(
+            bc.buffer.getUint32(listOffset, Endian.little),
+            (int index) {
+              final reader = _getElementReader(index);
+              if (reader == null) return null;
+              int offset = listOffset + size + _sizeofUint32 * index;
+              return reader.read(bc, offset);
+            },
+            growable: true,
+          );
+  }
+}
+
 /// Object that can read a value at a [BufferContext].
 abstract class Reader<T> {
   const Reader();
@@ -1121,6 +1157,25 @@ abstract class TableReader<T> extends Reader<T> {
   }
 }
 
+/// A reader that wraps another reader if the type is a union.
+///
+/// This is useful for reading unions that can be stucts and need an extra
+/// derefObject call.
+class UnionReader<T> extends Reader<T?> {
+  final Reader<T>? reader;
+
+  const UnionReader(this.reader);
+
+  @override
+  int get size => 4;
+
+  @override
+  T? read(BufferContext bc, int offset) {
+    if (reader is StructReader) offset = bc.derefObject(offset);
+    return reader?.read(bc, offset);
+  }
+}
+
 /// Reader of lists of unsigned 32-bit integer values.
 ///
 /// The returned unmodifiable lists lazily read values on access.
@@ -1311,6 +1366,29 @@ class _FbGenericList<E> extends _FbList<E> {
   }
 }
 
+/// Lazy list of union objects
+class _FbUnionList<E> extends _FbList<E?> {
+  final Reader<E>? Function(int index) getElementReader;
+
+  List<E?>? _items;
+
+  _FbUnionList(this.getElementReader, BufferContext bp, int offset)
+      : super(bp, offset);
+
+  @override
+  @pragma('vm:prefer-inline')
+  E? operator [](int i) {
+    _items ??= List<E?>.filled(length, null);
+    var item = _items![i];
+    final reader = getElementReader(i);
+    if (item == null && reader != null) {
+      item = reader.read(bc, offset + 4 + 4 * i);
+      _items![i] = item;
+    }
+    return item!;
+  }
+}
+
 /// The base class for immutable lists read from flat buffers.
 abstract class _FbList<E> extends Object with ListMixin<E> implements List<E> {
   final BufferContext bc;

dart/test/flat_buffers_test.dart

@@ -10,6 +10,7 @@ import './bool_structs_generated.dart' as example4;
 import './monster_test_my_game.example2_generated.dart' as example2;
 import './monster_test_my_game.example_generated.dart' as example;
 import 'enums_generated.dart' as example3;
+import 'union_vector_generated.dart' as example5;
 
 main() {
   defineReflectiveSuite(() {
@@ -18,6 +19,7 @@ main() {
     defineReflectiveTests(CheckOtherLangaugesData);
     defineReflectiveTests(GeneratorTest);
     defineReflectiveTests(ListOfEnumsTest);
+    defineReflectiveTests(UnionVectorTest);
   });
 }
 
@@ -934,6 +936,81 @@ class ObjectAPITest {
   }
 }
 
+@reflectiveTest
+class UnionVectorTest {
+  void test_unionVector() {
+    final movie = example5.MovieT(
+      mainCharacterType: example5.CharacterTypeId.Rapunzel,
+      mainCharacter: example5.RapunzelT(hairLength: 42),
+      charactersType: [
+        example5.CharacterTypeId.MuLan,
+        example5.CharacterTypeId.Rapunzel,
+        example5.CharacterTypeId.Belle,
+        example5.CharacterTypeId.BookFan,
+        example5.CharacterTypeId.Other,
+        example5.CharacterTypeId.Unused,
+      ],
+      characters: [
+        example5.AttackerT(swordAttackDamage: 10),
+        example5.RapunzelT(hairLength: 203),
+        example5.BookReaderT(booksRead: 21),
+        example5.BookReaderT(booksRead: 500),
+        "Hello",
+        "World",
+      ],
+    );
+
+    final fbb = Builder();
+    fbb.finish(movie.pack(fbb));
+
+    final movie2 = example5.Movie(fbb.buffer);
+    expect(
+      movie2.toString().replaceAllMapped(
+          RegExp('([a-zA-Z0-9]+){'), (match) => match.group(1)! + 'T{'),
+      movie.toString(),
+    );
+
+    final movie3 = movie2.unpack();
+    expect(movie3.toString(), movie.toString());
+
+    final movie4 = example5.MovieT(
+      mainCharacter: "String",
+      mainCharacterType: example5.CharacterTypeId.Other,
+    );
+
+    fbb.reset();
+    fbb.finish(movie4.pack(fbb));
+
+    final movie5 = example5.Movie(fbb.buffer);
+    expect(
+      movie5.toString().replaceAllMapped(
+          RegExp('([a-zA-Z0-9]+){'), (match) => match.group(1)! + 'T{'),
+      movie4.toString(),
+    );
+
+    final movie6 = movie5.unpack();
+    expect(movie6.toString(), movie4.toString());
+
+    final movie7 = example5.MovieT(
+      mainCharacter: example5.AttackerT(swordAttackDamage: 43),
+      mainCharacterType: example5.CharacterTypeId.MuLan,
+    );
+
+    fbb.reset();
+    fbb.finish(movie7.pack(fbb));
+
+    final movie8 = example5.Movie(fbb.buffer);
+    expect(
+      movie8.toString().replaceAllMapped(
+          RegExp('([a-zA-Z0-9]+){'), (match) => match.group(1)! + 'T{'),
+      movie7.toString(),
+    );
+
+    final movie9 = movie8.unpack();
+    expect(movie9.toString(), movie7.toString());
+  }
+}
+
 class StringListWrapperImpl {
   final BufferContext bp;
   final int offset;

dart/test/keyword_test_keyword_test_generated.dart

@@ -112,6 +112,18 @@ class _KeywordsInUnionTypeIdReader extends fb.Reader<KeywordsInUnionTypeId> {
       KeywordsInUnionTypeId.fromValue(const fb.Uint8Reader().read(bc, offset));
 }
 
+class _KeywordsInUnionReader extends fb.UnionReader {
+  _KeywordsInUnionReader(KeywordsInUnionTypeId? type) : super(_get(type));
+
+  static fb.Reader? _get(KeywordsInUnionTypeId? type) {
+    switch (type?.value) {
+      case 1: return KeywordsInTable.reader;
+      case 2: return KeywordsInTable.reader;
+      default: return null;
+    }
+  }
+}
+
 class KeywordsInTable {
   KeywordsInTable._(this._bc, this._bcOffset);
   factory KeywordsInTable(List<int> bytes) {
@@ -262,13 +274,7 @@ class Table2 {
   final int _bcOffset;
 
   KeywordsInUnionTypeId? get typeType => KeywordsInUnionTypeId._createOrNull(const fb.Uint8Reader().vTableGetNullable(_bc, _bcOffset, 4));
-  dynamic get type {
-    switch (typeType?.value) {
-      case 1: return KeywordsInTable.reader.vTableGetNullable(_bc, _bcOffset, 6);
-      case 2: return KeywordsInTable.reader.vTableGetNullable(_bc, _bcOffset, 6);
-      default: return null;
-    }
-  }
+  dynamic get type => _KeywordsInUnionReader(typeType).vTableGetNullable(_bc, _bcOffset, 6);
 
   @override
   String toString() {
@@ -277,7 +283,7 @@ class Table2 {
 
   Table2T unpack() => Table2T(
       typeType: typeType,
-      type: type?.unpack());
+      type: type is String ? type : type?.unpack());
 
   static int pack(fb.Builder fbBuilder, Table2T? object) {
     if (object == null) return 0;
@@ -295,7 +301,7 @@ class Table2T implements fb.Packable {
 
   @override
   int pack(fb.Builder fbBuilder) {
-    final int? typeOffset = type?.pack(fbBuilder);
+    final int? typeOffset = type is String ? fbBuilder.writeString(type) : type?.pack(fbBuilder);
     fbBuilder.startTable(2);
     fbBuilder.addUint8(0, typeType?.value);
     fbBuilder.addOffset(1, typeOffset);
@@ -353,7 +359,7 @@ class Table2ObjectBuilder extends fb.ObjectBuilder {
   /// Finish building, and store into the [fbBuilder].
   @override
   int finish(fb.Builder fbBuilder) {
-    final int? typeOffset = _type?.getOrCreateOffset(fbBuilder);
+    final int? typeOffset = _type is String ? fbBuilder.writeString(_type) : _type?.getOrCreateOffset(fbBuilder);
     fbBuilder.startTable(2);
     fbBuilder.addUint8(0, _typeType?.value);
     fbBuilder.addOffset(1, typeOffset);