Custom Input with date picker

hi,
i am new to flutter and tried creating a text field with date picker and on selection of date the date get displayed in the text field and pass it to https call.
For this i have created a custom input widget (by extending stateless widget) returning GestureDetector with TextFormField. onTap() the date picker picker get opened. But on selection of the date, the selected value is not getting displayed on custom field.

CustomInput :

Widget build(BuildContext context) {
    return GestureDetector(
      onTap: this.onTap,

      behavior: HitTestBehavior.opaque,
      child: TextFormField(
        enabled: this.onTap != null ? false : !this.disabled,
        decoration: InputDecoration(
          hintText: this.hintText,
          prefixIcon: Icon(this.icon, color: Colors.black87, size: 20),
          fillColor: Colors.white.withOpacity(0.05),
          contentPadding: EdgeInsets.all(10),
          focusedBorder: Styles.inputBorder,
          disabledBorder: Styles.inputBorder,
          enabledBorder: Styles.inputBorder,
          border: Styles.inputBorder,
          filled: true,
        ),
        validator: this.validator ?? (_) => null,
        onSaved: this.onSave ?? (_) {},
      ),
    );
  }

Date selection:

   Flexible(
                        child: StreamBuilder(
                          initialData: 'Pick a Date',
                          stream: this._startDateStream.stream,
                          builder:
                              (_, AsyncSnapshot<String> startDateSnapshot) =>
                                  CustomInput(
                            hintText: startDateSnapshot.data,
                            icon: Icons.calendar_today,

                            onTap: () async {
                              final DateTime startDate = await showDatePicker(
                                context: context,
                                firstDate: DateTime.now(),
                                initialDate: DateTime.now(),
                                lastDate: DateTime(2100),
                                initialDatePickerMode: DatePickerMode.day,
                              ).then(
                                      (value) => this._data.pickDate = value);
                            },