import 'package:flutter/services.dart';
import 'dart:math' as math;
class InputFormat extends TextInputFormatter {

  InputFormat({this.decimalRange = 2})
      : assert(decimalRange == null || decimalRange > 0);

  final int decimalRange;

  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue,
      TextEditingValue newValue) {
    
 
// 拿到錄入後的字元
    String nValue = newValue.text;
    //當前所選擇的文字區域
    TextSelection nSelection = newValue.selection;

    // 先來一波過濾，過濾出數字及小數點
    // 匹配包含數字和小數點的字元
    Pattern p = RegExp(r'(\d+\.?)|(\.?\d+)|(\.?)');
    nValue = p.allMatches(nValue)
        .map<String>((Match match) => match.group(0))
        .join();

    
 
// 用匹配完的字元判斷
    if (nValue.startsWith('.')) { //如果小數點開頭，我們給他補個0
      nValue = '0.';
    } else if (nValue.contains('.')) {
      //來驗證小數點位置
      if (nValue.substring(nValue.indexOf('.') + 1).length > decimalRange) {
        nValue = oldValue.text;
      } else {
        if (nValue.split('.').length > 2
 
) { //多個小數點，去掉後面的
          List<String> split = nValue.split('.');
          nValue = split[0] + '.' + split[1];
        }
      }
    }

    //使游標定位到最後一個字元後面
    nSelection = newValue.selection.copyWith(
      baseOffset: math.min(nValue.length, nValue.length + 1),
      extentOffset: math.min(nValue.length, nValue.length + 1),
    );

    return TextEditingValue(
        text: nValue,
        selection: nSelection,
        composing: TextRange.empty
    );
  }
}

使用：

keyboardType: TextInputType.numberWithOptions(decimal: true),
inputFormatters: [InputFormat()],