创建一个helper函数来测量API调用所花费的时间，然后在返回结果之前添加适当的延迟，这是相当简单的：

Future<R> callWithMinimumDuration<R>(
  Future<R> Function() func,
  Duration minimumDuration,
) async {
  var stopwatch = Stopwatch()..start();
  var result = await func();
  var delta = minimumDuration - stopwatch.elapsed;
  if (delta > Duration.zero) {
    await Future.delayed(delta);
  }
  return result;
}

然后你会调用它：

var result = await callWithMinimumDuration(apiCall, Duration(milliseconds: 1500));

或者，您可以使用Future.wait将原始Future与Future.delayed组合。但是，Future.wait希望Future具有异构类型，因此您需要转换（无论是显式还是隐式，如果允许隐式dynamic转换）：

Future<R> callWithMinimumDuration<R>(
  Future<R> Function() func,
  Duration minimumDuration,
) async =>
    (await Future.wait<Object?>([func(), Future.delayed(minimumDuration)]))[0]
        as R;

或者使用an assignment technique to deal with heterogeneous Future s：

Future<R> callWithMinimumDuration<R>(
  Future<R> Function() func,
  Duration minimumDuration,
) async {
  late R result;
  await Future.wait([
    () async {
      result = await func();
    }(),
    Future.delayed(minimumDuration),
  ]);
  return result;
}

您也可以使用one of the .wait extensions，但正确的错误处理将需要更多的工作：

Future<R> callWithMinimumDuration<R>(
  Future<R> Function() func,
  Duration minimumDuration,
) async {
  try {
    return (await (func(), Future.delayed(minimumDuration)).wait).$1;
  } on ParallelWaitError catch (e, st) {
    Error.throwWithStackTrace(e.errors.$1, st);
  }
}