dart - 那么Dart等待移除吗？

Question

是否等待删除“然后”？

为什么在以下情况下使用then会导致此错误:

// Error: [dart] The method 'then' isn't defined for the class 'String'. [undefined_method]

here的修改后的代码

import 'dart:async';

Future<void> printDailyNewsDigest() async {
  var newsDigest = await gatherNewsReports();
  // Error: [dart] The method 'then' isn't defined for the class 'String'. [undefined_method]
  newsDigest.then(print);
  // print(newsDigest);
}

main() {
  printDailyNewsDigest();
  printWinningLotteryNumbers();
  printWeatherForecast();
  printBaseballScore();
}

printWinningLotteryNumbers() {
  print('Winning lotto numbers: [23, 63, 87, 26, 2]');
}

printWeatherForecast() {
  print("Tomorrow's forecast: 70F, sunny.");
}

printBaseballScore() {
  print('Baseball score: Red Sox 10, Yankees 0');
}

const news = '<gathered news goes here>';
const oneSecond = Duration(seconds: 1);

// Imagine that this function is more complex and slow. :)
Future<String> gatherNewsReports() =>
    Future.delayed(oneSecond, () => news);    

Answer 1

await不会“删除” then，但它允许像then一样以更方便的语法编写异步代码，并且可以替换then。
然后，编译器将await重新写回then。
await延迟以下代码的执行，直到完成等待的异步操作的结果并返回结果值。
结果值不再是Future，因此then()不可用。