错误：flutter/lib/ui/ui_dart_state.cc(157) 未处理的异常：类型 'Future<dynamic>' 不是类型 'FutureOr<Null> 的子类型答案

【问题标题】：ERROR:flutter/lib/ui/ui_dart_state.cc(157) Unhandled Exception: type 'Future<dynamic>' is not a subtype of type 'FutureOr<Null>错误：flutter/lib/ui/ui_dart_state.cc(157) 未处理的异常：类型 'Future<dynamic>' 不是类型 'FutureOr<Null> 的子类型
【发布时间】：2020-05-09 10:32:19
【问题描述】：

我正在尝试在 showDialog 中处理来自 http 请求的错误，然后抛出一个错误，但我正面临这个错误

错误

E/flutter (18769): #13 TextInput._handleTextInputInvocation 包：flutter/.../services/text_input.dart:968 E/flutter (18769): #14 MethodChannel._handleAsMethodCall 包：flutter/…/services/platform_channel.dart:402 E/flutter (18769)：#15 MethodChannel.setMethodCallHandler。包：flutter/…/services/platform_channel.dart:370 E/颤振 (18769)：#16
_DefaultBinaryMessenger.handlePlatformMessage 包：flutter/…/services/binding.dart:200 E/flutter (18769): #17
_调用3。 (dart:ui/hooks.dart:303:15) E/flutter (18769): #18 _rootRun (dart:async/zone.dart:1126:13) E/flutter (18769): #19 _CustomZone.run (dart:async/zone.dart:1023:19) E/颤振（18769）：#20 _CustomZone.runGuarded (dart:async/zone.dart:925:7) E/flutter (18769): #21 _invoke3 (dart:ui/hooks.dart:302:10) E/flutter (18769): #22
_dispatchPlatformMessage (dart:ui/hooks.dart:162:5)


  Future<void> addProduct(Product product) {
    const url = 'https://flutter-shop-768a7.firebaseio.com/products.jon';
    return http
        .post(url,
            body: json.encode({
              'title': product.title,
              'description': product.description,
              'imageUrl': product.imageUrl,
              'price': product.price,
              'isFavorite': product.isFavorite
            }))
        .then((response) {
      final newProduct = Product(
          title: product.title,
          description: product.description,
          imageUrl: product.imageUrl,
          price: product.price,
          id: json.decode(response.body)['name']);
      // _items.insert(index, element)
      _items.add(newProduct);
      notifyListeners();
    }).catchError((error) {
      throw error;
    });
  }

     Provider.of<Products>(context, listen: false)
          .addProduct(_edditedProduct)
          .catchError((error) {
        return showDialog(
          context: context,
          builder: (ctx) => AlertDialog(
            title: Text('An Error occurred!'),
            content: Text('Someghing went wrong'),
            actions: <Widget>[
              FlatButton(
                  child: Text('ok'),
                  onPressed: () async => Navigator.of(context).pop())
            ],
          ),
        );
      }).then((_) {
        print('this is then function');
        setState(() {
          _isLoading = false;
        });
        Navigator.pop(context);
      });

【问题讨论】：

标签： http flutter dart

【解决方案1】：

因为你的函数类型是 Future 而你的返回类型必须是 Future 但是当您遇到错误时，您的响应会抛出错误并返回 Null，因此最好像这样编写异步函数

addProduct(Product product) async {
    const url = 'https://flutter-shop-768a7.firebaseio.com/products.json';
    await http
        .post(url,
            body: json.encode({
              'title': product.title,
              'description': product.description,
              'imageUrl': product.imageUrl,
              'price': product.price,
              'isFavorite': product.isFavorite
            }))
        .then((response) {
      final newProduct = Product(
          title: product.title,
          description: product.description,
          imageUrl: product.imageUrl,
          price: product.price,
          id: json.decode(response.body)['name']);
      // _items.insert(index, element)
      _items.add(newProduct);
      notifyListeners();
    }).catchError((error) {
      throw error;
    });
  }

你的网址不正确，把'https://flutter-shop-768a7.firebaseio.com/products.jon'改成'https://flutter-shop-768a7.firebaseio.com/products.json'

【讨论】：

【解决方案2】：

请像这样在下面添加“Null”。添加问题解决后，我也面临同样的问题。

    return showDialog<Null>(
      context: context,
      builder: (ctx) => AlertDialog(
        title: Text('Error occurred!'),
        content: Text('Something went wrong...'),
        actions: [
          FlatButton(
              onPressed: () {
                Navigator.of(ctx).pop();
              },
              child: Text('Okay')),
        ],
      ),
    );

【讨论】：

【解决方案3】：

这是因为您没有指定.then((response) {})方法的返回类型，解决这个问题只需更改

 .then((response) {
  final newProduct = Product(
      title: product.title,
      description: product.description,
      imageUrl: product.imageUrl,
      price: product.price,
      id: json.decode(response.body)['name']);
  // _items.insert(index, element)
  _items.add(newProduct);
  notifyListeners();
})

到

.then<void>((response) {
  final newProduct = Product(
      title: product.title,
      description: product.description,
      imageUrl: product.imageUrl,
      price: product.price,
      id: json.decode(response.body)['name']);
  // _items.insert(index, element)
  _items.add(newProduct);
  notifyListeners();
})de here

【讨论】：