颤振文本字段没有更新另一个变量

Question

我有以下小部件。

import 'package:flutter/material.dart';

class AddTaskScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    String? newTaskTitle;

    return Container(
      color: Color(0xFF757575),
      child: Container(
        padding: EdgeInsets.all(20),
        decoration: BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.only(
            topLeft: Radius.circular(20),
            topRight: Radius.circular(20),
          ),
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: [
            Text(
              'Add Task',
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 30,
                color: Colors.lightBlueAccent,
              ),
            ),
            TextField(
              autofocus: true,
              textAlign: TextAlign.center,
              onChanged: (String newText) {
                newTaskTitle = newText;
              },
            ),
            FlatButton(
              child: Text(
                'Add',
                style: TextStyle(
                  color: Colors.white,
                ),
              ),
              color: Colors.lightBlueAccent,
              onPressed: () {
                // this line prints null
                // even though I typed something in my textfield
                print(newTaskTitle);
              },
            )
          ],
        ),
      ),
    );
  }
}

在这里，我使用变量 newTaskTitle 更新我的文本字段值。当文本值更改时，它会成功更新。但是当我单击添加按钮时，它会将 newTaskTitle 打印为 null。我什至在文本字段的 onChanged 中打印它，它正在获取更新的值。但它没有在添加按钮的 onPressed 函数中获取更新的值。我如何获得更新的值，我在这里做错了什么？

Answer 1

用户TextEditingController 并传递_nameController.text。

import 'package:flutter/material.dart';
    
    class AddTaskScreen extends StatelessWidget {
TextEditingController _nameController = TextEditingController();
      @override
      Widget build(BuildContext context) {
        String? newTaskTitle;
    
        return Container(
          color: Color(0xFF757575),
          child: Container(
            padding: EdgeInsets.all(20),
            decoration: BoxDecoration(
              color: Colors.white,
              borderRadius: BorderRadius.only(
                topLeft: Radius.circular(20),
                topRight: Radius.circular(20),
              ),
            ),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.stretch,
              children: [
                Text(
                  'Add Task',
                  textAlign: TextAlign.center,
                  style: TextStyle(
                    fontSize: 30,
                    color: Colors.lightBlueAccent,
                  ),
                ),
                TextField(
                  controller: _nameController,
                  autofocus: true,
                  textAlign: TextAlign.center,
                  onChanged: (String newText) {
                    newTaskTitle = newText;
                  },
                ),
                FlatButton(
                  child: Text(
                    'Add',
                    style: TextStyle(
                      color: Colors.white,
                    ),
                  ),
                  color: Colors.lightBlueAccent,
                  onPressed: () {
                    // this line prints null
                    // even though I typed something in my textfield
                    print(_nameController.text);
                  },
                )
              ],
            ),
          ),
        );
      }
    }

Answer 2

那是因为您有一个带有String? newTaskTitle; 的无状态小部件

默认为 null 且不会更新。试试这个：

import 'package:flutter/material.dart';

class AddTaskScreen extends StatelfulWidget {

 //your constructor here

@override
_AddTaskScreenState createState() => _AddTaskScreenState();
}

class _AddTaskScreenState extends State<AddTaskScreen>{
  String? newTaskTitle;

  @override
  Widget build(BuildContext context) {
    

    return Container(
      color: Color(0xFF757575),
      child: Container(
        padding: EdgeInsets.all(20),
        decoration: BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.only(
            topLeft: Radius.circular(20),
            topRight: Radius.circular(20),
          ),
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: [
            Text(
              'Add Task',
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 30,
                color: Colors.lightBlueAccent,
              ),
            ),
            TextField(
              autofocus: true,
              textAlign: TextAlign.center,
              onChanged: (String newText) {
                setState((){
                 newTaskTitle = newText;
                });                   
              },
            ),
            FlatButton(
              child: Text(
                'Add',
                style: TextStyle(
                  color: Colors.white,
                ),
              ),
              color: Colors.lightBlueAccent,
              onPressed: () {
                // this line prints null
                // even though I typed something in my textfield
                print(newTaskTitle);
              },
            )
          ],
        ),
      ),
    );
  }
}