Flutter 錯誤：未處理的異常：類型“未來”<dynamic> ' 不是類型 'Future 的子類型<null> ?</null></dynamic>

Question

我正在關注 Max 的 Flutter 教程，並且在處理 http 請求時遇到錯誤部分：

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我正在從 URL 中刪除“.json”以創建錯誤，但它沒有正確處理錯誤對話框顯示&當按下平面按鈕時它的功能（pop），但它仍然保留在 CircularProgressIndicator 頁面上（加載/微調頁面）

編輯1：我正在談論的錯誤不是missing.json，我正在刪除.json 以查看我的代碼如何解決錯誤。

我的代碼：

在 8,577 毫秒內重新啟動應用程序。 在 1,869 毫秒內重新加載了 634 個庫中的 0 個。 E/flutter（3845）：[錯誤：flutter/lib/ui/ui_dart_state.cc（177）]未處理的異常：類型“未來”不是“未來”類型的子類型？ E/flutter (3845): #0 _FutureListener.handleError (dart:async/future_impl.dart:160:20) E/flutter (3845): #1
Future._propagateToListeners.handleError (dart:async/future_impl.dart:708:47) E/flutter (3845): #2
Future._propagateToListeners (dart:async/future_impl.dart:729:24) E/flutter (3845): #3 Future._completeWithValue (dart:async/future_impl.ZBB14127678960FAE97D8739:50EA20156Z:382:50EA20156Z:3824:9)#4/flutter
_AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:40:15) E/flutter (3845): #5
_completeOnAsyncReturn (dart:async-patch/async_patch.dart:311:13) E/flutter ( 3845): #6 _withClient (package:http/http.dart) package:http/http.dart:1 E/flutter ( 3845) : #7 _rootRunUnary (dart:async/zone.dart:1198:47)

./providers/products_provider.dart

import 'dart:convert';

import 'package:flutter/material.dart';
import './product.dart';
import 'package:http/http.dart' as http;

class Products with ChangeNotifier {
  List<Product> _items = [
    Product(
      id: 'p1',
      title: 'Red Shirt',
      description: 'A red shirt - it is pretty red!',
      price: 29.99,
      imageUrl:
          'https://cdn.pixabay.com/photo/2016/10/02/22/17/red-t-shirt-1710578_1280.jpg',
    ),
  ];

  List<Product> get items {
    return [..._items];
  }

  List<Product> get favsItems {
    return _items.where((element) => element.isFavourite).toList();
  }

  Future<void> addProduct(Product prod) {
    const url =
        'https://learning-5048e-default-rtdb.firebaseio.com/products';
    return http
        .post(url,
            body: json.encode({
              'title': prod.title,
              'description': prod.description,
              'price': prod.price,
              'imageUrl': prod.imageUrl,
              'isFavourite': prod.isFavourite
            }))
        .then((response) {
      final newProduct = Product(
          id: json.decode(response.body)['name'],
          title: prod.title,
          description: prod.description,
          price: prod.price,
          imageUrl: prod.imageUrl);

      _items.add(newProduct);
      notifyListeners();
    }).catchError((error){
      throw error;
    });
  }

  void updateProduct(String id, Product xxx) {
    final productIndex = _items.indexWhere((element) => element.id == id);
    _items[productIndex] = xxx;
    notifyListeners();
  }

  void deleteProduct(String id) {
    _items.removeWhere((element) => element.id == id);
    notifyListeners();
  }

  Product findById(productId) {
    return _items.firstWhere((element) => element.id == productId);
  }
}

./screens/edit_product_screen.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import '../providers/product.dart';
import '../providers/products_provider.dart';

class EditProductScreen extends StatefulWidget {
  static const routeName = '/edit-product';
  @override
  _EditProductScreenState createState() => _EditProductScreenState();
}

class _EditProductScreenState extends State<EditProductScreen> {
  final _priceFocusNode = FocusNode();
  final _describtionFocusNode = FocusNode();
  final _imageEditingController = TextEditingController();
  final _imageUrlFocusNode = FocusNode();
  final _form = GlobalKey<FormState>();
  var _editedProduct =
      Product(id: null, title: '', description: '', price: 0, imageUrl: '');
  var _initVals = {
    'title ': '',
    'description': '',
    'price': '',
    'imageUrl': ''
  };
  var _isinit = true;
  var _isLoading = false;

  @override
  void initState() {
    _imageUrlFocusNode.addListener(_updateImage);
    super.initState();
  }

  @override
  void didChangeDependencies() {
    if (_isinit) {
      final productId = ModalRoute.of(context).settings.arguments as String;

      if (productId != null) {
        _editedProduct =
            Provider.of<Products>(context, listen: false).findById(productId);
        print(_editedProduct.id);
        _initVals = {
          'title': _editedProduct.title,
          'description': _editedProduct.description,
          'price': _editedProduct.price.toString(),
          // 'imageUrl': _editedProduct.imageUrl
          // 'imageUrl': ''
        };

        _imageEditingController.text = _editedProduct.imageUrl;
      }
    }
    _isinit = false;

    super.didChangeDependencies();
  }

  @override
  void dispose() {
    _imageUrlFocusNode.removeListener(_updateImage);
    _priceFocusNode.dispose();
    _describtionFocusNode.dispose();
    _imageEditingController.dispose();
    _imageUrlFocusNode.dispose();

    super.dispose();
  }

  void _updateImage() {
    if (!_imageEditingController.text.endsWith('jpg') &&
        !_imageEditingController.text.endsWith('png') &
            !_imageEditingController.text.startsWith('http') &&
        !_imageEditingController.text.startsWith('https')) {
      return;
    }

    if (!_imageUrlFocusNode.hasFocus) {
      setState(() {});
    }
  }

  void saveForm() {
    // final isValid = _form.currentState.validate();
    if (!_form.currentState.validate()) {
      return;
    }
    _form.currentState.save();
    setState(() {
      _isLoading = true;
    });
    if (_editedProduct.id != null) {
      Provider.of<Products>(context, listen: false)
          .updateProduct(_editedProduct.id, _editedProduct);

      setState(() {
        _isLoading = false;
      });
      Navigator.of(context).pop();
    } else {
      Provider.of<Products>(context, listen: false)
          .addProduct(_editedProduct)
          .catchError((onError) {
        return showDialog(
            context: context,
            builder: (ctx) => AlertDialog(
                  title: Text('Error!'),
                  content: Text('Error Occured!'),
                  actions: <Widget>[
                    FlatButton(
                        onPressed: () {
                          Navigator.of(ctx).pop();
                        },
                        child: Text('OK'))
                  ],
                ));
      }).then((_) {
        setState(() {
          _isLoading = false;
        });
        Navigator.of(context).pop();
      });
    }
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Edit Product'),
        actions: [IconButton(icon: Icon(Icons.save), onPressed: saveForm)],
      ),
      body: _isLoading
          ? Center(child: CircularProgressIndicator())
          : Padding(
              padding: EdgeInsets.all(15),
              child: Form(
                  key: _form,
                  child: ListView(children: [
                    TextFormField(
                      initialValue: _initVals['title'],
                      decoration: InputDecoration(labelText: 'Title'),
                      textInputAction: TextInputAction.next,
                      onFieldSubmitted: (_) {
                        FocusScope.of(context).requestFocus(_priceFocusNode);
                      },
                      onSaved: (val) {
                        _editedProduct = Product(
                          id: _editedProduct.id,
                          title: val,
                          description: _editedProduct.description,
                          price: _editedProduct.price,
                          imageUrl: _editedProduct.imageUrl,
                          isFavourite: _editedProduct.isFavourite,
                        );
                      },
                      validator: (value) {
                        if (value.isEmpty) {
                          return 'Please Provide A Valid Title Name';
                        }
                        return null;
                      },
                    ),
                    TextFormField(
                      initialValue: _initVals['price'],
                      decoration: InputDecoration(labelText: 'Price'),
                      textInputAction: TextInputAction.next,
                      keyboardType: TextInputType.number,
                      focusNode: _priceFocusNode,
                      onFieldSubmitted: (_) {
                        Focus.of(context).requestFocus(_describtionFocusNode);
                      },
                      onSaved: (val) {
                        _editedProduct = Product(
                          id: _editedProduct.id,
                          title: _editedProduct.title,
                          description: _editedProduct.description,
                          price: double.parse(val),
                          imageUrl: _editedProduct.imageUrl,
                          isFavourite: _editedProduct.isFavourite,
                        );
                      },
                      validator: (value) {
                        if (value.isEmpty) {
                          return 'Please Enter A Price';
                        }
                        if (double.tryParse(value) == null ||
                            double.tryParse(value) <= 0) {
                          return 'Please Enter A Valid Price';
                        }
                        return null;
                      },
                    ),
                    TextFormField(
                      initialValue: _initVals['description'],
                      decoration: InputDecoration(labelText: 'Description'),
                      keyboardType: TextInputType.text,
                      maxLines: 3,
                      focusNode: _describtionFocusNode,
                      onSaved: (val) {
                        _editedProduct = Product(
                          id: _editedProduct.id,
                          title: _editedProduct.title,
                          description: val,
                          price: _editedProduct.price,
                          imageUrl: _editedProduct.imageUrl,
                          isFavourite: _editedProduct.isFavourite,
                        );
                      },
                      validator: (value) {
                        if (value.isEmpty) {
                          return 'Please Enter A Valid Description';
                        }
                        return null;
                      },
                    ),
                    Row(crossAxisAlignment: CrossAxisAlignment.end, children: [
                      Container(
                        child: _imageEditingController.text.isEmpty
                            ? Text('Insert Image')
                            : FittedBox(
                                fit: BoxFit.cover,
                                child: Image.network(
                                    _imageEditingController.text)),
                        margin: EdgeInsets.only(top: 10, right: 10),
                        width: 100,
                        height: 100,
                        decoration: BoxDecoration(border: Border.all(width: 1)),
                      ),
                      Expanded(
                        child: TextFormField(
                          //   initialValue: _initVals['imageUrl'],
                          decoration: InputDecoration(labelText: 'Image URL'),
                          keyboardType: TextInputType.url,
                          textInputAction: TextInputAction.done,
                          controller: _imageEditingController,
                          focusNode: _imageUrlFocusNode,
                          onFieldSubmitted: (_) {
                            saveForm();
                          },
                          onSaved: (value) {
                            _editedProduct = Product(
                              id: _editedProduct.id,
                              title: _editedProduct.title,
                              description: _editedProduct.description,
                              price: _editedProduct.price,
                              imageUrl: value,
                              isFavourite: _editedProduct.isFavourite,
                            );
                          },
                          validator: (value) {
                            if (value.isEmpty) {
                              return 'Please Enter A Valid Image URL';
                            }
                            if (!value.endsWith('jpg') &&
                                !value.endsWith('png') &&
                                !value.endsWith('bmp')) {
                              return 'Please Enter A valid Url For The Image';
                            }
                            return null;
                          },
                        ),
                      )
                    ])
                  ])),
            ),
    );
  }
}

Answer 1

向此 URL 發出 POST 請求

https://learning-5048e-default-rtdb.firebaseio.com/products

回應是：

append .json to your request URI to use the REST API.

接下來，jsonDecode 嘗試解碼此響應，並在失敗時拋出FormatException ，因為很明顯，它不是 JSON 格式。

似乎 URL 本身有故障。 正如響應所暗示的，在 URL 的末尾添加.json：

https://learning-5048e-default-rtdb.firebaseio.com/products.json

我試過了，當發出帶有正文的 POST 請求時，它會返回一些 json 響應，這可能是您正在尋找的。

Answer 2

好吧，在課程的下一個視頻中，他說明了（try catch）方法，我修改了代碼，它按預期工作：D！

不知道之前代碼的錯誤修復，可能是dart較新版本的代碼支持，可能是一些舊方法，我不知道