import 'package:flutter/services.dart';
  
  class MyNumberTextInputFormatter extends TextInputFormatter {
    static const defaultDouble = 0.001;
  
    ///允许的小数位数，-1代表不限制位数
    int digit;
    MyNumberTextInputFormatter({this.digit = -1});
    static double strToFloat(String str, [double defaultValue = defaultDouble]) {
    try {
       return double.parse(str);
     } catch (e) {
       return defaultValue;
     }
   }
 
   ///获取目前的小数位数
   static int getValueDigit(String value) {
     if (value.contains(".")) {
       return value.split(".")[1].length;
     } else {
       return -1;
     }
   }
 
   @override
   TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
     String value = newValue.text;
     int selectionIndex = newValue.selection.end;
     if (value == ".") {
      value = "0.";
       selectionIndex++;
     } else if (value == "-") {
       value = "-";
       selectionIndex++;
     } else if (value != "" && value != defaultDouble.toString() && strToFloat(value, defaultDouble) == defaultDouble || getValueDigit(value) > digit) {
       value = oldValue.text;
       selectionIndex = oldValue.selection.end;
     }
     return new TextEditingValue(
       text: value,
       selection: new TextSelection.collapsed(offset: selectionIndex),
     );
   }
 }