return 类型 'Null' 不是闭包上下文所要求的 'Widget'

Question

迁移到 flutter null-safety 后我遇到了一些问题：

return 类型 'Null' 不是闭包上下文所要求的 'Widget'

参数类型'Null'无法赋值给参数类型'AuthenticationRepository'

参数类型'Null'无法赋值给参数类型'DatabaseRepository'

参数类型'Null'无法赋值给参数类型'RemoteConfig'

class _PlacesState extends State<_Places> {
  bool _isLoading = false;
  final TextEditingController _typeAheadController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return TypeAheadFormField<Map<String, dynamic>>(
      key: widget.formFieldKey,
      textFieldConfiguration: TextFieldConfiguration(
        autofocus: true,
        decoration: InputDecoration(
          hintText: 'Place',
          contentPadding: EdgeInsets.only(left: 8, right: 8),
          border: InputBorder.none,
        ),
        controller: _typeAheadController,
      ),
      suggestionsCallback: (String? pattern) async {
        final s = pattern?.trim();
        if (s == null || s == '' || s.length < 4) return null;
        _isLoading = true;
        try {
          await Future.delayed(Duration(seconds: 1));
          final data = await (_request(s) as FutureOr<Map<String, dynamic>>);
          final result = data['hits'] as List;
          return result.cast<Map<String, dynamic>>();
        } finally {
          _isLoading = false;
        }
      } as FutureOr<Iterable<Map<String, dynamic>>> Function(String),
      loadingBuilder: (BuildContext context) {
        if (!_isLoading) return null; **<-- Error is here**
        return Align(
          child: Padding(
            padding: EdgeInsets.symmetric(vertical: 8),
            child: ExtendedProgressIndicator(),
          ),
        );
      },

--

    void main() {
  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
    // Build our app and trigger a frame.
    await tester.pumpWidget(App(
      authenticationRepository: null, <-- The argument type 'Null' can't be assigned to the parameter type 'AuthenticationRepository'
      databaseRepository: null, <-- The argument type 'Null' can't be assigned to the parameter type 'DatabaseRepository'
      remoteConfig: null, <-- The argument type 'Null' can't be assigned to the parameter type 'RemoteConfig'
    ));

    // Verify that our counter starts at 0.
    expect(find.text('0'), findsOneWidget);
    expect(find.text('1'), findsNothing);

    // Tap the '+' icon and trigger a frame.
    await tester.tap(find.byIcon(Icons.add));
    await tester.pump();

    // Verify that our counter has incremented.
    expect(find.text('0'), findsNothing);
    expect(find.text('1'), findsOneWidget);
  });
}

Answer 1

在您的 App 小部件中，放置可为 null 的存储库。

final AuthenticationRepository? authenticationRepository

或者模拟您的存储库并将其提供给 App 而不是提供 null。

return 类型 'Null' 不是闭包上下文所要求的 'Widget'

The return type 'Null' isn't a 'Widget', as required by the closure's context

null

dart

flutter

dart-null-safety