flutter - 如何在没有错误的情况下将Future <void> main转换为dart flutter 中的Future <List>

Question

我想将下面的代码变成:

Future<List> fetchPrices() async {details in code below} 

没有错误。

要更改的代码在这里:

Future<void> main(List<String> arguments) async {
  Map<String, num> quotes = await Forex.fx(
    quoteProvider: QuoteProvider.yahoo,
    base: 'USD',
    quotes: <String>['EUR', 'JPY']
  );
  print('Number of quotes retrieved: ${quotes.keys.length}.');
  print('Exchange rate USDEUR: ${quotes['USDEUR']}.');
  print('Exchange rate USDJPY: ${quotes['USDJPY']}.');
  quotes = await Forex.fx(
    quoteProvider: QuoteProvider.ecb,
    base: 'JPY',
    quotes: <String>['EUR', 'USD']
  );
  print('Number of quotes retrieved: ${quotes.keys.length}.');
  print('Exchange rate JPYEUR: ${quotes['JPYEUR']}.');
  print('Exchange rate JPYUSD: ${quotes['JPYUSD']}.');
}

Answer 1

根据您的代码示例Forex.fx()返回Map<String, num>。因此，您在这里有两个选择。

Future<List<String>> fooFunction(List<String> arguments) async {
  Map<String, num> quotes = await Forex.fx(
      quoteProvider: QuoteProvider.yahoo,
      base: 'USD',
      quotes: <String>['EUR', 'JPY']
  );
  return quotes.keys;
}

Future<List<num>> fooFunction(List<String> arguments) async {
  Map<String, num> quotes = await Forex.fx(
      quoteProvider: QuoteProvider.yahoo,
      base: 'USD',
      quotes: <String>['EUR', 'JPY']
  );
  return quotes.values;
}