需要“String”类型的值，但得到“Null”类型的值

Question

我必须在一个屏幕上添加新产品以及编辑现有产品。所以我通过分配初始值来使用 didchangedependency 方法来更新屏幕，代码中出现了问题。请帮助我。所以错误是

错误显示在这里：

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

      // ignore: unnecessary_null_comparison
      if (productId != null) {
        _editedproduct =
            Provider.of<Products>(context, listen: false).FindByID(productId);
        _imageUrlController.text = _editedproduct.imageUrl;
        _initValues = {
          'title': _editedproduct.title,
          'description': _editedproduct.description,
          'Price': _editedproduct.price.toString(),
          // 'imageUrl': _editedproduct.imageUrl,
          'imageUrl': '',
        };
      }
    }
    _isinit = false;
    super.didChangeDependencies();
  } 

在更改依赖项方法中，我尝试从一页获取参数，即产品 ID，但它显示为空。这是我已推送到编辑屏幕产品页面的屏幕截图，其中包含我试图获取此参数的参数。

这是为了推送添加产品页面

import 'dart:html';

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shoppingapp/Providers/Product.dart';
import 'package:shoppingapp/Providers/Products.dart';

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

class _EditProductScreenState extends State<EditProductScreen> {
  final _priceFocusNode = FocusNode();
  final _descriptionFocusNode = FocusNode();
  final _imageUrlController = TextEditingController();
  final _imageUrlFocusNode = FocusNode();
  final _form = GlobalKey<FormState>();
  var _editedproduct =
      Product(id: '', title: '', description: '', price: 0, imageUrl: '');
  var _isinit = true;

  var _initValues = {
    'title': '',
    'description': '',
    'price': '',
    'imageUrl': '',
  };
  @override
  void initState() {
    _imageUrlFocusNode.addListener(_updateImageUrl);
    super.initState();
  }

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

      // ignore: unnecessary_null_comparison
      if (productId != null) {
        _editedproduct =
            Provider.of<Products>(context, listen: false).FindByID(productId);
        _imageUrlController.text = _editedproduct.imageUrl;
        _initValues = {
          'title': _editedproduct.title,
          'description': _editedproduct.description,
          'Price': _editedproduct.price.toString(),
          // 'imageUrl': _editedproduct.imageUrl,
          'imageUrl': '',
        };
      }
    }
    _isinit = false;
    super.didChangeDependencies();
  }

  @override
  void dispose() {
    _imageUrlFocusNode.removeListener(_updateImageUrl);
    _priceFocusNode.dispose();
    _descriptionFocusNode.dispose();
    _imageUrlFocusNode.dispose();

    super.dispose();
  }

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

  void _saveForm() {
    final isValid = _form.currentState!.validate();
    if (isValid) {
      return;
    }
    _form.currentState!.save();
    if (_editedproduct.id != null) {
      Provider.of<Products>(context, listen: false)
          .updateProducts(_editedproduct.id, _editedproduct);
    } else {
      Provider.of<Products>(context, listen: false).addProducts(_editedproduct);
    }

    Navigator.of(context).pop();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Edit Product'),
        actions: [
          IconButton(
            icon: Icon(Icons.save),
            onPressed: _saveForm,
          ),
        ],
      ),
      body: Padding(
        padding: const EdgeInsets.all(15.0),
        child: Form(
          key: _form,
          child: ListView(
            children: [
              TextFormField(
                decoration: InputDecoration(
                  labelText: 'Title',
                ),
                initialValue: _initValues['title'],
                textInputAction: TextInputAction.next,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please provide a value.';
                  }
                  return null;
                },
                onFieldSubmitted: (_) {
                  FocusScope.of(context).requestFocus(_priceFocusNode);
                },
                onSaved: (value) {
                  _editedproduct = Product(
                      title: value as String,
                      price: _editedproduct.price,
                      description: _editedproduct.description,
                      imageUrl: _editedproduct.imageUrl,
                      id: _editedproduct.id,
                      isFavourite: _editedproduct.isFavourite);
                },
              ),
              TextFormField(
                initialValue: _initValues['price'],
                decoration: InputDecoration(
                  labelText: 'Price',
                ),
                textInputAction: TextInputAction.next,
                keyboardType: TextInputType.number,
                focusNode: _priceFocusNode,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please enter a  price ';
                  }
                  if (double.tryParse(value) == null) {
                    return 'Please Enter a Valid Number';
                  }
                  if (double.parse(value) <= 0) {
                    return 'Please Enter the number greather no than zero';
                  }
                },
                onSaved: (value) {
                  _editedproduct = Product(
                      title: _editedproduct.title,
                      price: double.parse(value!),
                      description: _editedproduct.description,
                      imageUrl: _editedproduct.imageUrl,
                      id: _editedproduct.id,
                      isFavourite: _editedproduct.isFavourite);
                },
              ),
              TextFormField(
                decoration: InputDecoration(
                  labelText: 'Description',
                ),
                initialValue: _initValues['description'],
                maxLines: 3,
                textInputAction: TextInputAction.next,
                keyboardType: TextInputType.multiline,
                focusNode: _descriptionFocusNode,
                validator: (value) {
                  if (value!.isEmpty) {
                    return 'Please enter a  description ';
                  }
                  if (value.length < 10) {
                    return 'Should be at least 10 characters long.';
                  }
                  return null;
                },
                onSaved: (value) {
                  _editedproduct = Product(
                      title: _editedproduct.title,
                      price: _editedproduct.price,
                      description: value as String,
                      imageUrl: _editedproduct.imageUrl,
                      id: _editedproduct.id,
                      isFavourite: _editedproduct.isFavourite);
                },
              ),
              Row(
                crossAxisAlignment: CrossAxisAlignment.end,
                children: [
                  Container(
                    width: 100,
                    height: 100,
                    margin: EdgeInsets.only(top: 8, right: 10),
                    decoration: BoxDecoration(
                        border: Border.all(width: 1, color: Colors.grey)),
                    child: _imageUrlController.text.isEmpty
                        ? Text('Enter a URL')
                        : FittedBox(
                            child: Image.network(
                              _imageUrlController.text,
                              fit: BoxFit.cover,
                            ),
                          ),
                  ),
                  Expanded(
                    child: TextFormField(
                      decoration: InputDecoration(labelText: 'Image URl'),
                      keyboardType: TextInputType.url,
                      textInputAction: TextInputAction.done,
                      controller: _imageUrlController,
                      focusNode: _imageUrlFocusNode,
                      validator: (value) {
                        if (value!.isEmpty) {
                          return 'Please enter a  URL ';
                        }
                        if (!value.startsWith('http') &&
                            !value.startsWith('https')) {
                          return 'Please Enter a valid URL';
                        }
                        if (!value.endsWith('.png') &&
                            !value.endsWith('.jpg') &&
                            !value.endsWith('.jpeg')) {
                          return 'Please enter a valid image URL';
                        }
                        return null;
                      },
                      onFieldSubmitted: (_) {
                        _saveForm();
                      },
                      onSaved: (value) {
                        _editedproduct = Product(
                            title: _editedproduct.title,
                            price: _editedproduct.price,
                            description: _editedproduct.description,
                            imageUrl: value as String,
                            id: _editedproduct.id,
                            isFavourite: _editedproduct.isFavourite);
                      },
                    ),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Answer 1

虽然实现了空安全，但大多数教程都是在它之前完成的，因此在您尝试检查返回ModalRoute值null是否返回值并根据该值执行操作的情况下，好的旧方法if(productId == null)现在不起作用。相反，你可以这样做：

final productId = ModalRoute.of(context)!.settings.arguments == null ? "NULL":ModalRoute.of(context)!.settings.arguments as String;

if(productId != "NULL"){
 
   //Do stuff here

}

如果ModalRoute给你null，你将字符串“NULL”分配给你的变量productId，否则你只需分配它返回的任何值。

然后您可以简单地检查 的值是否为productId并NULL进行相应操作。