你可以创建一个可重用的函数，接受你想格式化的numberOfDecimal，并利用toStringAsFixed()方法来格式化数字并将其转换回double。

供参考，toStringAsFixed方法不会四舍五入以5结尾的数字(例如:toStringAsFixed四舍五入2.275到2.27而不是2.28)。这是dart toStringAsFixed方法的默认行为(类似于Javascript的toFixed)

作为一种变通方法，我们可以在现有数字的最后一个十进制数后面加上1(例如:将0.0001加到2.275变成2.2751，而2.2751将正确舍入为2.28)

double roundOffToXDecimal(double number, {int numberOfDecimal = 2}) {
  // To prevent number that ends with 5 not round up correctly in Dart (eg: 2.275 round off to 2.27 instead of 2.28)
  String numbersAfterDecimal = number.toString().split('.')[1];
  if (numbersAfterDecimal != '0') {
    int existingNumberOfDecimal = numbersAfterDecimal.length;
    number += 1 / (10 * pow(10, existingNumberOfDecimal));
  }

  return double.parse(number.toStringAsFixed(numberOfDecimal));
}

// Example of usage:
var price = roundOffToXDecimal(2.275, numberOfDecimal: 2)
print(price); // 2.28

num.toStringAsFixed()轮。它将你的num (n)转换成一个带有你想要的小数点数的字符串(2)，然后在一行漂亮的代码中将它解析回你的num:

n = num.parse(n.toStringAsFixed(2));

上述解决方案没有适当地四舍五入数字。我使用:

double dp(double val, int places){ 
   num mod = pow(10.0, places); 
   return ((val * mod).round().toDouble() / mod); 
}

这个DART四舍五入的问题已经出现了很长一段时间(@LucasMeadows)，因为很明显直到现在这个问题还没有得到充分的解决(正如@DeepShah的观察所表明的那样)。

著名的舍入规则(未解决的问题):

“以数字5结尾的数字四舍五入:如果结果是偶数，则四舍五入;如果结果是奇数，则向下舍入。”

这是DART代码的解决方案:

double roundAccurately(double numToRound, int decimals) {

  // Step 1 - Prime IMPORTANT Function Parameters ...
  int iCutIndex = 0;
  String sDeciClipdNTR = "";
  num nMod = pow(10.0, decimals);
  String sNTR = numToRound.toString();
  int iLastDigitNTR = 0, i2ndLastDigitNTR = 0;
  debugPrint("Round => $numToRound to $decimals Decimal ${(decimals == 1) ? "Place" : "Places"} !!");   // Deactivate this 'print()' line in production code !!

  // Step 2 - Calculate Decimal Cut Index (i.e. string cut length) ...
  int iDeciPlaces = (decimals + 2);
  if (sNTR.contains('.')) {
    iCutIndex = sNTR.indexOf('.') + iDeciPlaces;
  } else {
    sNTR = sNTR + '.';
    iCutIndex = sNTR.indexOf('.') + iDeciPlaces;
  }

  // Step 3 - Cut input double to length of requested Decimal Places ...
  if (iCutIndex > sNTR.length) {                    // Check that decimal cutting is possible ...
    sNTR = sNTR + ("0" * iDeciPlaces);              // ... and fix (lengthen) the input double if it is too short.
    sDeciClipdNTR = sNTR.substring(0, iCutIndex);   // ... then cut string at indicated 'iCutIndex' !!
  } else {
    sDeciClipdNTR = sNTR.substring(0, iCutIndex);   // Cut string at indicated 'iCutIndex' !!
  }

  // Step 4 - Extract the Last and 2nd Last digits of the cut input double.
  int iLenSDCNTR = sDeciClipdNTR.length;
  iLastDigitNTR = int.parse(sDeciClipdNTR.substring(iLenSDCNTR - 1));   // Extract the last digit !!
  (decimals == 0)
    ? i2ndLastDigitNTR = int.parse(sDeciClipdNTR.substring(iLenSDCNTR - 3, iLenSDCNTR - 2))
    : i2ndLastDigitNTR = int.parse(sDeciClipdNTR.substring(iLenSDCNTR - 2, iLenSDCNTR - 1));

  // Step 5 - Execute the FINAL (Accurate) Rounding Process on the cut input double.
  double dAccuRound = 0;
  if (iLastDigitNTR == 5 && ((i2ndLastDigitNTR + 1) % 2 != 0)) {
    dAccuRound = double.parse(sDeciClipdNTR.substring(0, iLenSDCNTR - 1));
  } else {
    if (iLastDigitNTR < 5) {
      dAccuRound = double.parse(sDeciClipdNTR.substring(0, iLenSDCNTR - 1));
    } else {
      if (decimals == 0) {
        sDeciClipdNTR = sNTR.substring(0, iCutIndex - 2);
        dAccuRound = double.parse(sDeciClipdNTR) + 1;   // Finally - Round UP !!
      } else {
        double dModUnit = 1 / nMod;
        sDeciClipdNTR = sNTR.substring(0, iCutIndex - 1);
        dAccuRound = double.parse(sDeciClipdNTR) + dModUnit;   // Finally - Round UP !!
      }
    }
  }

  // Step 6 - Run final QUALITY CHECK !!
  double dResFin = double.parse(dAccuRound.toStringAsFixed(decimals));

  // Step 7 - Return result to function call ...
  debugPrint("Result (AccuRound) => $dResFin !!");   // Deactivate this 'print()' line in production code !!
  return dResFin;
}

这是一个完全手动的方法(可能有点过度)，但它是有效的。请测试一下(直到耗尽)，如果我没有做到，请告诉我。

double value = 2.8032739273;
String formattedValue = value.toStringAsFixed(3);

要将Dart中的double整入到小数点后的给定精度，可以使用Dart toStringAsFixed()方法中的内置解决方案，但必须将其转换回double

void main() {
  double step1 = 1/3;  
  print(step1); // 0.3333333333333333
  
  String step2 = step1.toStringAsFixed(2); 
  print(step2); // 0.33 
  
  double step3 = double.parse(step2);
  print(step3); // 0.33
}