calculus-logic-operand-input.dart

class OperandInput extends StatefulWidget {
  final String label;
  final num initValue;
  final num memory;
  final ValueChanged<String> onChanged;

  const OperandInput(
      {Key key, this.label, this.initValue, this.memory, this.onChanged})
      : super(key: key);

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

class _OperandInputState extends State<OperandInput> {
  TextEditingController controller;

  @override
  void initState() {
    super.initState();
    controller = TextEditingController(text: '${widget.initValue}');
  }
  
  void onFromMemory() {
    if (widget.memory == null) return;

    setState(() {
      controller.text = '${widget.memory}';
    });
    widget.onChanged(controller.text);
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Row(
        children: <Widget>[
          ...
          Flexible(
            child: TextField(
              ...
              controller: controller,
              onChanged: widget.onChanged,
            ),
          ),
          Padding(
            child: RaisedButton(
                ...
                onPressed: onFromMemory                
            ),
          ),
        ],
      ),
    );
  }
}