颤振等待不等待

Question

为什么程序在执行打印语句之前不等待函数返回列表？

我认为这是因为我将 forEach 循环设为异步，但我需要将其设为异步以获取作为 Future 的 newSummary。

Future syncToCloud() async{
  final List<Map<String,dynamic>> _events = await events();
  print(_events.length);
}
  
  
Future<List<Map<String, dynamic>>> events() async {
  List<Map<String, dynamic>> maps = await db.query('data');
  List<Map<String, dynamic>> newMaps=[];

  maps.forEach((element)async{
    Map<String, dynamic> newElement = {};

    if(element['summary']!=''){
      newElement['summary'] = await newSummary(element['summary']);
      print(newElement['summary']);
    }
    else{
      newElement['summary'] = element['summary'];
    }
    newMaps.add(newElement);
  });
  
    return newMaps;
}
  

void main()async{
  await syncToCloud();
}

Answer 1

请替换以下内容

  maps.forEach((element)async{
    Map<String, dynamic> newElement = {};

    if(element['summary']!=''){
      newElement['summary'] = await newSummary(element['summary']);
      print(newElement['summary']);
    }
    else{
      newElement['summary'] = element['summary'];
    }
    newMaps.add(newElement);
  });

有了这个

  await Future.wait(  maps.forEach((element)async{
    Map<String, dynamic> newElement = {};

    if(element['summary']!=''){
      newElement['summary'] = await newSummary(element['summary']);
      print(newElement['summary']);
    }
    else{
      newElement['summary'] = element['summary'];
    }
    newMaps.add(newElement);
  }));

Answer 2

你在一个不起作用的回调中执行你的return [{'hi':5}]。您应该等待 Future.delayed 然后像这样返回列表await Future.delayed(Duration(seconds: 2));。

Future syncToCloud() async{
  final List<Map<String,dynamic>> _events = await events();
  print(_events.length);
}
  
  
Future<List<Map<String, dynamic>>> events() async {
  await Future.delayed(Duration(seconds: 2));
  return [{'hi':5}];
}
  

void main()async{
  await syncToCloud();
}