如何修复在空颤动上调用的方法“包含密钥”[重复]

Posted 2023-02-23

【中文标题】如何修复在空颤动上调用的方法“包含密钥”[重复]

【英文标题】：How to fix Method "Contains key" called on null flutter [duplicate]

【发布时间】：2021-03-12 02:51:31

【问题描述】：

我是 Flutter 的初学者，我一直在开发电子商务应用程序。我使用 Provider 包将产品添加到购物车，但出现以下错误： 我的颤振医生也很好。仅当我尝试将商品添加到购物车时才遇到此问题。任何帮助将不胜感激。

══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown while handling a gesture:
The method 'containsKey' was called on null.
Receiver: null
Tried calling: containsKey("p1")
When the exception was thrown, this was the stack:
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:51:5)
#1      Cart.addItem 
package:shop_stop/…/components/cart.dart:33
#2      _ProductItemState.build.<anonymous closure> 
package:shop_stop/widgets/product_item.dart:80

我的购物车提供商：

class CartItem 
  final String id;
  final String title;
  final int quantity;
  final double price;

  CartItem(
    @required this.id,
    @required this.title,
    @required this.quantity,
    @required this.price,
  );


class Cart with ChangeNotifier 
  Map<String, CartItem> _items;

  Map<String, CartItem> get items 
    return ..._items;
  

  int get itemCount 
    return _items.length;
  

  void addItem(
    String productId,
    double price,
    String title,
  ) 
    if (_items.containsKey(productId)) 
      // change quantity...
      _items.update(
        productId,
        (existingCartItem) => CartItem(
          id: existingCartItem.id,
          title: existingCartItem.title,
          price: existingCartItem.price,
          quantity: existingCartItem.quantity + 1,
        ),
      );
     else 
      _items.putIfAbsent(
        productId,
        () => CartItem(
          id: DateTime.now().toString(),
          title: title,
          price: price,
          quantity: 1,
        ),
      );
    
    notifyListeners();
  

我的产品项目：

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shop_stop/screens/cart/components/cart.dart';

import '../views/product_details.dart';
import '../models/product.dart';

class ProductItem extends StatefulWidget 
  @override
  _ProductItemState createState() => _ProductItemState();


class _ProductItemState extends State<ProductItem> 
  @override
  Widget build(BuildContext context) 
    final products = Provider.of<Product>(context, listen: false);
    final cart = Provider.of<Cart>(context, listen: false);
    return Container(
      child: GestureDetector(
        onTap: () 
          Navigator.of(context)
              .pushNamed(ProductDetails.routeName, arguments: products.id);
        ,
        child: Card(
          color: Colors.white,
          elevation: 6,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(10)),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              //product image
              Image(image: AssetImage('assets/laptop.jpg')),
              Padding(
                padding: EdgeInsets.symmetric(vertical: 0, horizontal: 10),
                //item name and fav button
                child: Flex(
                  direction: Axis.horizontal,
                  children: <Widget>[
                    Expanded(
                        child: Text(
                      products.name,
                      style: Theme.of(context)
                          .textTheme
                          .headline1
                          .copyWith(fontSize: 16, color: Colors.black),
                    )),
                    //add-to-fav button
                    Consumer<Product>(
                      builder: (ctx, product, child) => IconButton(
                          icon: Icon(
                            products.isFav
                                ? Icons.favorite
                                : Icons.favorite_border,
                            color: Colors.red,
                          ),
                          onPressed: () 
                            products.toggleFav();
                          ),
                    )
                  ],
                ),
              ),
              // price and buy button
              Padding(
                padding: EdgeInsets.symmetric(horizontal: 10, vertical: 0),
                child: Flex(
                  direction: Axis.horizontal,
                  children: <Widget>[
                    Expanded(
                        child: Text(
                      "\$ " + products.price.toString(),
                      style: Theme.of(context).textTheme.headline1.copyWith(
                          color: Theme.of(context).primaryColor, fontSize: 15),
                    )),
                    SizedBox(
                      width: 60,
                      child: RaisedButton(
                        onPressed: () 
                          cart.addItem(
                              products.id, products.price, products.name);
                        ,
                        color: Colors.orange,
                        child: Text(
                          "Buy",
                          style: TextStyle(fontFamily: 'Prompt', fontSize: 14),
                        ),
                      ),
                    )
                  ],
                ),
              ),
            ],
          ),
        ),
      ),
    );
  

【参考方案1】：

将Map<String, CartItem> _items; 更改为Map<String, CartItem> _items = ;