asynchronous - 在Dart异步方法中，可以共享运行某些不共享任何依赖项的行吗？

Question

假设您有一个异步方法主体，如下所示，该方法主体具有一个返回Future的方法调用，并在其后打印一个不需要该方法结果的输出。是否可以添加一些结构来使打印语句独立执行？还是这个语法糖迫使异步方法主体按顺序完全运行？

Future<String> test2() {
  Completer c = new Completer();

  new Timer(new Duration(seconds: 2), () {
    c.complete('test2: done');
  });

  return c.future;
}

Future<String> test1() async {
  String result = await test2();
  print(result);

  // Why is this not executed immediately before test2 call completes?
  print('hello');

  return 'test1: done';
}

void main() {
  test1().then((result) => print(result));
}

后续:我在下面添加了对test1()的重写，该重写已链接了异步方法调用。我真的很想知道如何使用异步语法糖来简化这种用例。如何使用新语法重写该块？

Future<String> test1() async {
  test2().then((result) {
    print(result);
    test2().then((result) {
      print(result);
    });
  });

  // This should be executed immediately before
  // the test2() chain completes.
  print('hello');

  return 'test1: done';
}

Answer 1

编辑以跟进:

我不确定您到底要做什么，但是如果要在打印之前等待链完成，则必须执行以下操作:

Future<String> test1() async {
  String result;
  var result1 = await test2();
  print("Printing result1 $result1 ; this will wait for the first call to test2() to finish to print");
  var result2 = await test2();
  print("Printing result2 $result2 ; this will wait for the second call to test2() to print");  

  // This should be executed immediately before
  // the test2() chain completes.
  print('hello');

  return 'test1: done';
}

现在，如果您调用 test1()并等待其返回“test1:done”，则必须等待它。

main() async {
  var result = await test1();
  print(result); // should print "hello" and "test1: done"
}

如果要独立于先前的将来结果执行代码，请不要输入 await关键字。

Future<String> test1() async {
  test2().then((String result) => print(result)); // the call to test2 will not stop the flow of test1(). Once test2's execution will be finished, it'll print the result asynchronously

  // This will be printed without having to wait for test2 to finish.
  print('hello');

  return 'test1: done';
}

关键字 await使流程停止，直到 test2()完成。