flutter 显示错误“此表达式的类型为 'void'，因此无法使用其值。”该怎么办？

Question

该表达式的类型为“void”，因此无法使用其值。 尝试检查您是否使用了正确的 API；可能有一个函数或调用返回了您意想不到的 void 。还要检查类型参数和变量，它们也可能为 void。

错误正好显示在“pop”上的第 65 行中。代码如下。

      class NewExpense extends StatefulWidget {
        const NewExpense({Key? key, required this.onAddExpense}): super(
                key: key,
              );

        final void Function(Expense expense) onAddExpense;

        @override
        State<StatefulWidget> createState() {
          return _NewExpenseState();
        }
      }

      class _NewExpenseState extends State<NewExpense> {
        final _titleController = TextEditingController();
        final _amountController = TextEditingController();
        DateTime? _selectedDate;
        Category _selectedCategory = Category.frameset;
        

        void _presentDatePicker() async {
          final now = DateTime.now();
          final firstDate = DateTime(now.year - 1, now.month, now.day);
          final pickedDate = await showDatePicker(
            context: context,
            initialDate: now,
            firstDate: firstDate,
            lastDate: now,
          );
          setState(() {
            _selectedDate = pickedDate;
          });
        }

        void _submitExpenseData() {
          double? enterAmount = double.tryParse(_amountController.text);
          final amountIsInvalid = enterAmount == null || enterAmount <= 0;
          if (_titleController.text.trim().isEmpty ||
              amountIsInvalid ||
              _selectedDate == null) {
            showDialog(
              context: context,
              builder: (ctx) => AlertDialog(
                title: const Text('Invalid Input'),
                content: const Text(
                    'Please make sure a valid title, amount, date and category was entered.'),
                actions: [
                  TextButton(
                    onPressed: () {
                      Navigator.pop(ctx);
                    },
                    child: Text('Okay'),
                  )
                ],
              ),
            );
            return;
          }

          widget.onAddExpense(
            Navigator.of(context).pop(Expense(
              title: _titleController.text,
              amount: enterAmount!,
              date: _selectedDate!,
              category: _selectedCategory)));
        
        }

        @override
        void dispose() {
          _titleController.dispose();
          _amountController.dispose();
          super.dispose();
        }

        @override
        
        
        
        Widget build(BuildContext context) {
          return Padding(
            padding: const EdgeInsets.fromLTRB(16, 16, 16, 16),
            child: Column(
              children: [
                TextField(
                  controller: _titleController,
                  maxLength: 50,
                  decoration: const InputDecoration(
                    label: Text('Title'),
                  ),
                ),
                Row(
                  children: [
                    Expanded(
                      child: TextField(
                        controller: _amountController,
                        keyboardType: TextInputType.number,
                        decoration: const InputDecoration(
                          prefixText: '₹ ',
                          label: Text('Amount'),
                        ),
                      ),
                    ),
                    const SizedBox(width: 16),
                    Expanded(
                      child: Row(
                        mainAxisAlignment: MainAxisAlignment.end,
                        crossAxisAlignment: CrossAxisAlignment.center,
                        children: [
                          Text(_selectedDate == null
                              ? 'No date selected'
                              : formatter.format(_selectedDate!)),
                          IconButton(
                            onPressed: _presentDatePicker,
                            icon: const Icon(
                              Icons.calendar_month,
                            ),
                          ),
                        ],
                      ),
                    ),
                  ],
                ),
                const SizedBox(
                  height: 16,
                ),
                Row(
                  children: [
                    DropdownButton(
                        value: _selectedCategory,
                        items: Category.values
                            .map(
                              (category) => DropdownMenuItem(
                                value: category,
                                child: Text(
                                  // ignore: sdk_version_since
                                  category.name.toUpperCase(),
                                ),
                              ),
                            )
                            .toList(),
                        onChanged: (value) {
                          if (value == null) {
                            return;
                          }
                          setState(() {
                            _selectedCategory = value as Category;
                          });
                        }),
                    const Spacer(),
                    TextButton(
                      onPressed: () {
                        Navigator.pop(context);
                      },
                      child: const Text('Cancel'),
                    ),
                    ElevatedButton(
                      onPressed: _submitExpenseData,
                      child: const Text('Save Expense'),
                    ),
                  ],
                ),
              ],
            ),
          );
        }
      }

Answer 1

我不知道你的项目流程是哪个。我注意到的一件事是，您在 onAddExpense 函数下使用 pop 方法，这就是问题所在。

因为你不能使用void方法作为返回。您需要在 main 方法之后调用 pop 方法，或者通过弹出页面作为您的目标返回 Expense()。

#像这样使用

widget.onAddExpense(
  Expense(
    title: _titleController.text,
    amount: enterAmount!,
    date: _selectedDate!,
    category: _selectedCategory,
  ),
);

Navigator.of(context).pop();

#或

Navigator.of(context).pop(
  Expense(
    title: _titleController.text,
    amount: enterAmount!,
    date: _selectedDate!,
    category: _selectedCategory,
  ),
);