奇怪的异步行为

Question

我试图了解 Dart 如何执行异步调用。

给定以下代码：

main() {
  test("test", () async{
    await myCall( );
    print( "done");
  });
}

Future myCall( ) async{

  ["0","1","2"].forEach( (item) async{
    print( "about to callback");
    await futurePrint( item);
    print( "done  callback");
  });
  print( "done mycall");
  return new Future.value();
}

Future futurePrint( String str){
  print( "string = ${str}");
  return new Future.value();
}

我希望输出是

about to future print 
string = 0 
done future print 
about to future print 
string = 1
done future print 
about to future print 
string = 2 
done future print 
done mycall
done 

但代码实际输出的是

print( "done mycall");
about to future print
string = 0
about to future print
string = 1
about to future print
string = 2
done
done future print
done future print
done future print

为什么“done myCall”出现在打印的第一行，而不是最后一行？ 为什么在“完成”打印之后会出现“完成的未来打印”？

如何让myCall 仅在数组迭代完成时返回？

Answer 1

["0","1","2"].forEach(<callback>) 不等待<callback> 返回的期货。

如果你这样做

Future myCall( ) async {

  for(item in ["0","1","2"]){
    print( "about to callback");
    await futurePrint( item);
    print( "done  callback");
  });
  print( "done mycall");
  return new Future.value();
}

它应该符合你的预期。