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

效果很好

var price=99.012334554
price = price.roundTodouble();
print(price); // 99.01

void main() {
  int decimals = 2;
  int fac = pow(10, decimals);
  double d = 1.234567889;
  d = (d * fac).round() / fac;
  print("d: $d");
}

打印: 1.23

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。

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

如果您想要一个双精度结果，将一个IEEE-754二进制浮点数舍入到特定的十进制位数本身就是有问题的。

In the same way that fractions such as 1/3 can't be exactly represented with a finite number of decimal digits, many (really infinitely many) decimal numbers can't be represented with a finite number of binary digits. For example, the decimal number 0.1 cannot be exactly represented in binary. While you could try to round 0.09999 to 0.1, as a double it would actually be "rounded" to 0.1000000000000000055511151231257827021181583404541015625. Most of the other answers that claim to round doubles with decimal precision actually return the nearest representable double.

你能做的就是使字符串表示形式看起来像一个漂亮的四舍五入的数字，这就是double.toStringAsFixed()所做的。这也是为什么当你打印0.100000000…，如果实现试图打印用户友好的值，则可能会看到0.1。然而，不要被愚弄了:double值实际上永远不会精确到0.1，如果你用这样不精确的值重复计算，就会累积错误。

请注意，以上所有内容都是浮点数工作方式所固有的，并不是Dart所特有的。还看到:

浮点数学坏了吗? 如果你不理解上面的解释，Tom Scott也做了一个很好的视频来解释浮点数是如何工作的。

底线:如果关心十进制精度，就不要使用二进制浮点类型。如果你和钱打交道，这一点尤其重要。

相反，你应该使用:

Integers. For example, if you are dealing with currency, instead of using double dollars = 1.23;, use int cents = 123;. Your calculations then always will be exact, and you can convert to the desired units only when displaying them to the user (and likewise can convert in the opposite direction when reading input from the user). A type designed to represent decimal numbers with arbitrary precision. For example, package:decimal provides a Decimal type. With such a type, some of the other answers (such as multiplying by 100, rounding, and then dividing by 100) then would be appropriate. (But really you should use Decimal.round directly.)