Question

我想了解Dart如何执行异步调用。

给出以下代码：

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();
}

我希望输出为

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＆＃39;完成myCall＆＃39;发生在第一行打印，而不是最后一行？为什么未来的印刷品会在完成＆＃39;之后发生。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();
}

它应该做你期望的。

奇怪的异步行为

1 个答案: