接收方：null 尝试调用：长度答案

【问题标题】：Receiver: null Tried calling: length接收方：null 尝试调用：长度
【发布时间】：2021-01-13 08:02:39
【问题描述】：

import 'package:flutter/material.dart'; 导入'package:flutter_rest_api/rest_api.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.purple,
      ),
      home: EmployeePage(),
    );
  }
}

class EmployeePage extends StatefulWidget {
  @override
  _EmployeePageState createState() => _EmployeePageState();
}

class _EmployeePageState extends State<EmployeePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter REST API'),
      ),
      body: FutureBuilder(
        future: ApiService.getEmployees(),
        builder: (context, snapshot) {
          final employees = snapshot.data;
          if (snapshot.connectionState == ConnectionState.done) {
            return ListView.separated(
              separatorBuilder: (context, index) {
                return Divider(
                  height: 2,
                  color: Colors.black,
                );
              },
              itemBuilder: (context, index) {
                return ListTile(
                  title: Text(employees[index]['employee_name']),
                  subtitle: Text('Age: ${employees[index]['employee_age']}'),
                );
              },
              itemCount: employees.length,
            );
          }
          return Center(
            child: CircularProgressIndicator(),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.push(
            context,
            MaterialPageRoute(
              builder: (context) => AddNewEmployeePage(),
            ),
          );
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class AddNewEmployeePage extends StatefulWidget {
  AddNewEmployeePage({Key key}) : super(key: key);

  _AddNewEmployeePageState createState() => _AddNewEmployeePageState();
}

class _AddNewEmployeePageState extends State<AddNewEmployeePage> {
  final _employeeNameController = TextEditingController();
  final _employeeAge = TextEditingController();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('New Employee'),
      ),
      body: Center(
        child: Padding(
          padding: EdgeInsets.all(10),
          child: Column(
            children: <Widget>[
              TextField(
                controller: _employeeNameController,
                decoration: InputDecoration(hintText: 'Employee Name'),
              ),
              TextField(
                controller: _employeeAge,
                decoration: InputDecoration(hintText: 'Employee Age'),
                keyboardType: TextInputType.number,
              ),
              RaisedButton(
                child: Text(
                  'SAVE',
                  style: TextStyle(
                    color: Colors.white,
                  ),
                ),
                color: Colors.purple,
                onPressed: () {
                  final body = {
                    "name": _employeeNameController.text,
                    "age": _employeeAge.text,
                  };
                  ApiService.addEmployee(body).then((success) {
                    if (success) {
                      showDialog(
                        builder: (context) => AlertDialog(
                          title: Text('Employee has been added!!!'),
                          actions: <Widget>[
                            FlatButton(
                              onPressed: () {
                                Navigator.pop(context);
                                _employeeNameController.text = '';
                                _employeeAge.text = '';
                              },
                              child: Text('OK'),
                            )
                          ],
                        ),
                        context: context,
                      );
                      return;
                    }else{
                      showDialog(
                        builder: (context) => AlertDialog(
                          title: Text('Error Adding Employee!!!'),
                          actions: <Widget>[
                            FlatButton(
                              onPressed: () {
                                Navigator.pop(context);
                              },
                              child: Text('OK'),
                            )
                          ],
                        ),
                        context: context,
                      );
                      return;
                    }
                  });
                },
              )
            ],
          ),
        ),
      ),
    );
  }
}

我收到了错误 -

======== 小部件库捕获的异常 =============================== 在构建 FutureBuilder(dirty, state: _FutureBuilderState#e477a) 时引发了以下 NoSuchMethodError：在 null 上调用了 getter 'length'。接收方：空尝试调用：长度

【问题讨论】：

你试过添加snapshot.data.length而不是employees.length

标签： java android flutter dart amplify

【解决方案1】：

因为快照放在ConnectionState.done之前还没有返回数据。它将始终为空。

尝试将final employees = snapshot.data; 向下移动

应该是

     ...
     if (snapshot.connectionState == ConnectionState.done) {
        final employees = snapshot.data;
        return ListView.separated(
          separatorBuilder: (context, index) {
     ...

【讨论】：

【解决方案2】：

让我知道这些是否有效：

if (snapshot.connectionState == ConnectionState.done && snapshot.hasData) {
    List employees = []; //i suppose your data return a list
    employees = snapshot.data;
    return ListView.separated(
        itemCount: employees.length,
          separatorBuilder: (context, index) {
                          .....
          }

【讨论】：