我在这里见过很多方法。然而，我认为这比其他答案更有效或至少更简洁。

TextField(
   controller: TextEditingController(text: "Initial Text here"),
)

(来自邮件列表。这个答案不是我想出来的。)

class _FooState extends State<Foo> {
  TextEditingController _controller;

  @override
  void initState() {
    super.initState();
    _controller = new TextEditingController(text: 'Initial value');
  }

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        new TextField(
          // The TextField is first built, the controller has some initial text,
          // which the TextField shows. As the user edits, the text property of
          // the controller is updated.
          controller: _controller,
        ),
        new RaisedButton(
          onPressed: () {
            // You can also use the controller to manipuate what is shown in the
            // text field. For example, the clear() method removes all the text
            // from the text field.
            _controller.clear();
          },
          child: new Text('CLEAR'),
        ),
      ],
    );
  }
}

当你使用TextEditingController

如果使用TextEditingController，请将其文本字段设置为所需的值

TextEditingController txtController = TextEditingController()..text = 'Your initial text value'; 
TextField( controller: txtController ..... ) 

当你不使用TextEditingController

如果你没有使用texteditingcontroller，直接从TextField小部件使用initialValue字段:

TextFormField( initialValue: "Your initial text value" )

你可以使用TextFormField代替TextField，并使用initialValue属性。例如

TextFormField(initialValue: "I am smart")

如果您使用文本表单字段并将数据从上一页传递到下一页文本表单字段，请使用此选项

class ProfilePage extends StatefulWidget {

  late final String fname;

    ProfilePage({required this.fname});

  @override
  State<ProfilePage> createState() => _ProfilePageState();
}

class _ProfilePageState extends State<ProfilePage> {
  final _form = GlobalKey<FormState>();
  
  late var FullNameController = TextEditingController(text: widget.fname);
  
}

简单高效的方法

分配控制器到你的TextFormField或TextField和在initState你可以初始化它像这样的初始值

_controller = TextEditingController(text: 'Your initial value');