How to change from Stateless Widget to Stateful Widget?

Hover on to the StatelessWidget class and use

Android Studio:

• Mac: option + enter

• Windows: alt + enter

Visual Studio Code:

• Mac: cmd + .

• Windows: ctrl + .

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Output (Your solution):

Here is the working code.

class DetailPage extends StatefulWidget {
  final Activity activity;

  DetailPage({Key key, this.activity}) : super(key: key);

  @override
  _DetailPageState createState() => _DetailPageState();
}

class _DetailPageState extends State<DetailPage> {
  bool _tracking = false, _mapped = false; // you need this
  String _schedule;

  @override
  Widget build(BuildContext context) {
    final levelIndicator = Container(
      child: Container(
        child: LinearProgressIndicator(backgroundColor: Color.fromRGBO(209, 224, 224, 0.2), value: 2.0, valueColor: AlwaysStoppedAnimation(Colors.green)),
      ),
    );

    final topContentText = Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: <Widget>[
        SizedBox(height: 120.0),
        Container(
          width: 90.0,
          child: Divider(color: Colors.green),
        ),
        SizedBox(height: 10.0),
        Text(
          widget.activity.activityName,
          style: TextStyle(color: Colors.white, fontSize: 45.0),
        ),
        SizedBox(height: 30.0),
        Row(
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            Expanded(
                flex: 6,
                child: Padding(
                    padding: EdgeInsets.only(left: 10.0),
                    child: Text(
                      "Last Run: 3-2-19\n" + "Last Avg Strain: 34%\n" + "Last Run Time: 00:45:23",
                      style: TextStyle(color: Colors.white),
                    ))),
            // Expanded(flex: 1, child: newRow)
          ],
        ),
      ],
    );

    final topContent = Stack(
      children: <Widget>[
        Container(
          height: MediaQuery.of(context).size.height * 0.45,
          padding: EdgeInsets.all(40.0),
          width: MediaQuery.of(context).size.width,
          decoration: BoxDecoration(
            gradient: LinearGradient(
              begin: Alignment.centerLeft,
              end: Alignment.centerRight,
              colors: [Color.fromRGBO(33, 147, 176, 100), Color.fromRGBO(109, 213, 237, 100)],
            ),
          ),
          child: Center(
            child: topContentText,
          ),
        ),
        Positioned(
          left: 235.0,
          top: 180.0,
          child: InkWell(
            onTap: () {
              Navigator.pop(context);
            },
            child: CircleAvatar(
              radius: 80.0,
              backgroundColor: Colors.white,
            ),
          ),
        ),
        Positioned(
          left: 8.0,
          top: 60.0,
          child: InkWell(
            onTap: () {
              Navigator.pop(context);
            },
            child: Icon(Icons.arrow_back, color: Colors.white),
          ),
        )
      ],
    );

    final bottomContentText = Text(
      "Config:",
      style: TextStyle(fontSize: 18.0),
    );
    final mappedCheckbox = CheckboxListTile(
      title: Text("Mapped"),
      value: _mapped,
      onChanged: (newValue) => setState(() => _mapped = newValue),
      controlAffinity: ListTileControlAffinity.leading, //  <-- leading Checkbox
    );
    final rtCheckBox = CheckboxListTile(
      title: Text("Real-time Tracking"),
      value: _tracking,
      onChanged: (newValue) => setState(() => _tracking = newValue),
      controlAffinity: ListTileControlAffinity.leading, //  <-- leading Checkbox
    );
    final descriptionText = Text(
      "Description:",
      style: TextStyle(fontSize: 12.0),
    );
    final description = TextFormField(
      decoration: InputDecoration(
        hintText: 'Enter an activity description',
      ),
    );
    final scheduledFor = Text(
      "Scheduled for:",
      style: TextStyle(fontSize: 12.0),
    );
    final dropdown = DropdownButton<String>(
      value: _schedule,
      items: <String>['Now (Default)', 'B', 'C', 'D'].map((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
      hint: Text("Now (Default)"),
      onChanged: (newValue) {
        setState(() {
          _schedule = newValue;
        });
      },
    );
    final readButton = Container(
        padding: EdgeInsets.symmetric(vertical: 16.0),
        width: 170, //MediaQuery.of(context).size.width,
        child: RaisedButton(
          onPressed: () => {},
          color: Colors.lightBlue,
          child: Text("Start", style: TextStyle(color: Colors.white, fontSize: 20)),
        ));
    final bottomContent = Container(
      width: MediaQuery.of(context).size.width,
      padding: EdgeInsets.all(40.0),
      child: Center(
        child: Column(
          children: <Widget>[bottomContentText, mappedCheckbox, rtCheckBox, descriptionText, description, Text("\n"), scheduledFor, dropdown, readButton],
        ),
      ),
    );

    return Scaffold(
      body: Column(
        children: <Widget>[topContent, bottomContent],
      ),
    );
  }
}

For VSCode (Visual Studio Code) use ctrl + '.' keys while the cursor on the stateless widget to convert it to stateful widget.