Add Text in TextField in button click in flutter

i want to add text in click of my button into text field but problem is it's add text in end of the textfield value i want to add text where user want to add text in middle of text field or any where .. i try :

  TextEditingController symbolTextField = TextEditingController();

  symbolTextField.text =
                                symbolTextField.text + items[index];

but my items value is add in end of the textfield value

example video:

Solution 1: Spike L

Make use of controller.selection, I created a example for you:

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController controller;

  @override
  void initState() {
    super.initState();
    controller = TextEditingController();
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Expanded(
              child: TextField(
                controller: controller,
              ),
            ),
            ElevatedButton(
              onPressed: () => insert("SSS"),
              child: Text('Add "SSS"'),
            ),
          ],
        ),
      ),
    );
  }

  void insert(String content) {
    var text = controller.text;
    var pos = controller.selection.start;
    controller.value = TextEditingValue(
      text: text.substring(0, pos) + content + text.substring(pos),
      selection: TextSelection.collapsed(offset: pos + content.length),
    );
  }
}