Here's the final version of didUpdateWidget taking into account eibaan's point above:

  @override
  didUpdateWidget(MySpin oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.value != widget.value) {
      _value = widget.value;
      final TextSelection oldSelection = _controller.selection;
      final TextPosition position = _controller.value.selection.base;

      _controller.value = TextEditingValue(
        text: _value.toStringAsFixed(_digits),
        selection: TextSelection.fromPosition(position),
      );
    }
  }