TextField formatter

textfield.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

main() {
    runApp(new Home());
}

class MyFormatter extends TextInputFormatter {
    MyFormatter();

    @override
    TextEditingValue formatEditUpdate(TextEditingValue oldValue,  TextEditingValue newValue) {
        var val = newValue.text + "-";
        return new TextEditingValue(text: val, composing: newValue.composing, selection: newValue.selection);
    }
}
class Home extends StatelessWidget {
    @override
      Widget build(BuildContext context) {
        var widget = new MaterialApp(
            home: new Scaffold(
                body: new Center(
                    child: new Container(
                        width: 300.0,
                        child: new TextField(
                        keyboardType:TextInputType.number,
                        inputFormatters: [
                            new MyFormatter()
                        ],
                    )
                    )
                    
                )
            ),
        );

        return widget;
    }
}