A template of a tab used in one of my first flutter projects.

gistfile1.txt

import 'package:flutter/material.dart';
class WidGetName extends StatefulWidget {
  @override
  _WidGetNameState createState() => _WidGetNameState();
}

class _WidGetNameState extends State<WidGetName> with AutomaticKeepAliveClientMixin<WidGetName> {
  //Define FormKey here
  GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  //Define list to go to dropdown list here.
  var dropdownList = ["item 1", "item 2"];
  //Define default Dropdown Item here. e.g. var selectedMachinery  = "Lorry";
  var selectedDropdownItem = "item 1";
  //Define the Model Instance Here e.g. MachineryModel _machinery = MachineryModel();
  MachineryModel _machinery = MachineryModel();
  //Define list of added items here. E.g List<MachineryModel> _items = [];. Should be empty at start so it is populated when new items are added.
  List<MachineryModel> _items = [];

  // Define variables that will be populated with data from the dialog form here. e.g. var timeDeployedString

  //Define the showInformationDialog here to show the dialog form here when the floatingActionButton is pressed.
  Future<void> showInformationDialog(BuildContext context) async {
    //Give default values to the empty variables defined above.
    selectedDropdownItem  = "item 1";
    //return the showDialog method here to create the dialog.
    return await showDialog(
        context: context,
        builder: (context) {
          //Define textEditingControllers here. e.g. final TextEditingController _textEditingControllerPlateNo = TextEditingController();

          final TextEditingController _textEditingControllerPlateNo = TextEditingController();
          //Create the stateful builder here to build a stateful alert dialog.
          return StatefulBuilder(builder: (context, setState) {
            //Define stateful variables here inside of the Stateful Builder. e.g. TimeOfDay _timeMachineryDeployed;
            TimeOfDay _timeMachineryDeployed;
            TimeOfDay _timeMachineryPicked;
            // Define future methods here that will require data input from the user e.g.
            /*
            Future<Null> selectMachineryDeployed(BuildContext context) async {
              _timeMachineryDeployed = await showTimePicker(
                context: context,
                initialTime: TimeOfDay(hour: 8, minute: 0),
              );
              setState(() {
                if(_timeMachineryDeployed != null) {
                  timeDeployedString = '${_timeMachineryDeployed.hour.toString().padLeft(2, '0')}:${_timeMachineryDeployed.minute.toString().padLeft(2, '0')}';
                }
                else {
                  timeDeployedString = "Select Time";
                }
              });
            }
             */

            //Return the alert dialog here with a form.
            return AlertDialog(
              //Use a singlechild scroll view to make the form scrollable.
              content: SingleChildScrollView(
                child: Form(
                  key: _formKey,
                  child: Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    mainAxisSize: MainAxisSize.min,
                    children: <Widget>[
                      Text("Pick a Machinery"),
                      Row(
                        children: [
                          Icon(
                            Icons.car_rental,
                            color: Colors.amber,
                          ),
                          SizedBox(width: 20,),
                          /*
                          This is a sample of a stateful DropDown Button.
                          DropdownButton(
                            hint: Text("Pick a Machinery"),
                            items: machinery.map((String dropDownStringItem){
                              return DropdownMenuItem<String>(
                                  value: dropDownStringItem,

                                  child: Text(dropDownStringItem)
                              );
                            }).toList(),
                            onChanged: (String newSelectedMachinery) {
                              setState(() {
                                this.selectedMachinery = newSelectedMachinery;
                              });
                            },
                            value: selectedMachinery,
                          ),
                           */
                        ],
                      ),
                      SizedBox(height: 10,),
                      Container(
                        width: 400,
                        /*
                        This is a sample of a TextFormField with validator and controller.
                          TextFormField(
                          controller: _textEditingControllerPlateNo,
                          validator: (value) {
                            return value.isNotEmpty ? null : "Cannot be Empty";
                          },
                          onSaved: (val) => setState(() => _machinery.plateNo = val),
                          decoration: InputDecoration(
                            border: OutlineInputBorder(),
                            icon: Icon(Icons.construction, color: yellowColor,),
                            labelText: 'Enter the Plate Number',
                          ),
                        ),
                         */
                      ),
                      SizedBox(height: 10,),
                      Row(
                        children: [
                          Icon(
                            Icons.watch_later_outlined,
                            color: Colors.amber,
                          ),
                          //Here
                          SizedBox(width: 10,),
                          Column(
                            crossAxisAlignment: CrossAxisAlignment.start,
                            children: <Widget>[

                              Divider(),
                              /*
                              This is a sample of a time picker button.
                              TextButton.icon(
                                //icon: Icon(Icons.alarm),
                                icon: Icon(
                                  Icons.watch_later_outlined,
                                  color: blackColor,
                                ),
                                label: Text(
                                  timeDeployedString,
                                  style: TextStyle(
                                    letterSpacing: 2,
                                    fontSize: 18,
                                    color: Colors.black,
                                  ),
                                ),
                                onPressed: (){
                                  selectMachineryDeployed(context);
                                },
                              ),
                               */

                            ],
                          ),
                        ],
                      ),
                      SizedBox(height: 10,),
                    ],
                  ),
                ),
              ),
              /*
              This is a sample of the actions part of the alert dialog.
              actions: [
                Center(
                  child: Row(
                    children: [
                      TextButton(
                          onPressed: () {
                              Navigator.of(context).pop();
                          },
                          child: Text("CANCEL")
                      ),
                      TextButton(
                          onPressed: () => _onSubmit(),
                          child: Text("ADD")
                      )
                    ],
                  ),
                )
              ],
               */
              actions: [],
            );
          });
        }
    );
  }
  /*
  This is a sample of the onSubmit Method.
  _onSubmit() {
    print(timeDeployedString);
    print(timePickedString);
    if(_formKey.currentState.validate()) {
      var form = _formKey.currentState;
      form.save();
      _machinery.description = selectedMachinery;
      _machinery.deployedFrom = timeDeployedString;
      _machinery.deployedTo = timePickedString;
      setState (() {
        _items.add(MachineryModel(id: null, description: _machinery.description, plateNo: _machinery.plateNo, deployedFrom: _machinery.deployedFrom, deployedTo: _machinery.deployedTo));
      });
      form.reset();
      Navigator.of(context).pop();
    }
  }
   */
  @override
  bool get wantKeepAlive => true;

  @override
  Widget build(BuildContext context) {
    super.build(context);
    return Scaffold(
      appBar: AppBar(
        leading: Container(),
        title: Text(
          'Machinery',
          style: TextStyle(
              color: darkGreyColor
          ),
        ),
        centerTitle: true,
        backgroundColor: Colors.white,
        actions: <Widget>[
          IconButton(
              icon: Icon(Icons.handyman),
              onPressed: () {})
        ],
      ),
      body: Column(
        children: [
          _displayList()
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () async {
          await showInformationDialog(context);
        },
        tooltip: 'Increment',
        child: Icon(
          Icons.add,
          color: yellowColor,
        ),
        backgroundColor: darkGreyColor,
      ),
    );
  }
  //The List to display items on the screen once added should come here. e.g.
  /*
    _machineryList() {
    return Expanded(
        child: Card(
          //margin: EdgeInsets.fromLTRB(20, 30, 20, 0),
          child: ListView.builder(
            padding: EdgeInsets.all(8),
            itemBuilder: (context, index) {
              return Column(
                children: <Widget>[
                  ListTile(
                    leading: Icon(
                      Icons.car_rental,
                      color: yellowColor,
                      size: 40,
                    ),
                    title: Text(
                      _items[index].description.toUpperCase(),
                      style: TextStyle(
                        color: darkGreyColor,
                        fontWeight: FontWeight.bold,
                      ),
                    ),
                    subtitle: Text(_items[index].plateNo),
                    trailing: Icon(
                      Icons.edit,
                      color: darkGreyColor,
                    ),
                  ),
                  Divider(
                    height: 5,
                  )
                ],
              );
            },
            itemCount: _items.length,
          ),
        )
    );
  }
   */
  _displayList () {

  }
  onSubmit() {

  }
}