arctano_eval/lib/num_format.dart

import 'package:flutter/services.dart';

class NumFormat extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(TextEditingValue o, TextEditingValue n) {
    TextSelection ns = n.selection;
    String truncated = n.text;
    final double v = double.tryParse(n.text)!;

    if (v < -5.0 || v > 5.0) {
      truncated = "0.0";
    }

    return TextEditingValue(
      text: truncated,
      selection: ns,
    );
  }
}