1-) FLUTTER - Tarih girişi button ile showDatePicker CupertinoDatePicker kullanımı

EKRAN GÖRÜNTÜSÜ

1- DEĞİŞKENLER VE SHOWDATEPİCKER METOTU

@override
void initState() {
  // TODO: implement initState
  super.initState();

  _dogumTar = new TextEditingController();

}

DateTime? _chosenDateTime;
String secilenTarih = "";

late TextEditingController _dogumTar;

// Show the modal that contains the CupertinoDatePicker
void _showDatePicker(ctx) {
  // showCupertinoModalPopup is a built-in function of the cupertino library

  showCupertinoModalPopup(
      context: ctx,
      builder: (_) => Container(
            height: 300,
            color: const Color.fromARGB(255, 255, 255, 255),
            child: Column(
              children: [
                SizedBox(
                  height: 200,
                  child: CupertinoDatePicker(
                    mode: CupertinoDatePickerMode.date,
                    initialDateTime: DateTime(1980, 01, 01),
                    onDateTimeChanged: (value) {
                      setState(
                        () {
                          _chosenDateTime = value;
                          print(value.toString());
                          print(formatDate(value, [yyyy, '-', mm, '-', dd]));
                          secilenTarih =
                              formatDate(value, [yyyy, '-', mm, '-', dd]);

                          _dogumTar.text = secilenTarih;
                        },
                      );
                    },
                  ),
                ),

                // Close the modal
                CupertinoButton(
                  child: const Text('OK'),
                  onPressed: () {
                    if(secilenTarih==""){
                      secilenTarih= "1980-01-01";
                    }

                    setState(() {
                      _dogumTar.text = secilenTarih;
                    });

                    Navigator.of(ctx).pop();
                  },
                )
              ],
            ),
          ));
}

2- TEXTFORMFİELD

Padding(
  padding: EdgeInsets.only(top: 10),
  child: Container(
    child: TextFormField(
      controller: _dogumTar, // DEĞERİ BUNUN SAYESİNDE ATAMIŞ OLURUZ
      onTap: () {
        // Below line stops keyboard from appearing
        FocusScope.of(context)
            .requestFocus(new FocusNode());

        _showDatePicker(context);

      },
      style: TextStyle(
          fontSize: 17, fontWeight: FontWeight.bold),
      keyboardType: TextInputType.emailAddress,
      decoration: InputDecoration(
        labelText: "Doğum Tarihini Seçiniz",
        border: OutlineInputBorder(),
        labelStyle: TextStyle(fontSize: 15),
      ),
    ),
  ),
),

NOT : formatDate için aşağıki paketi eklemen lazım

flutter pub add date_format

pubspec.yaml

date_format: ^2.0.5

main.dart -> import --> import 'package:date_format/date_format.dart';