Flutter:在读取Firebase数据时进行异步处理 [英] Flutter: doing sthg asynchronous while reading Firebase data

问题描述

我尝试从Firebase读取数据时进行异步处理，

I try to do sthg asynchronous while reading data from Firebase and it doesn't work,

这是我尝试过的:

Future<String> test() async {

  var cacheManager = await CacheManager.getInstance(); //await here is OKAY 

  DatabaseReference firebaseRef = FirebaseDatabase.instance.reference();
  firebaseRef.child('...').once().then((DataSnapshot snapshot) {
    Map<dynamic,dynamic> map = snapshot.value;
    map.forEach((key, url) {
       print('$key: $url');   //OKAY
       await precacheImage(new NetworkImage(url), context); //doesn't cache images
    });
  });
  return "";
 }

我得到了:

错误:意外的令牌等待".

Error: Unexpected token 'await'.

我也尝试过:

Future<Map> test() async {  //<---- added type Map
    ...
    map.forEach((key, url) async { //<--- added async
       //var file = await cacheManager.getFile(url);
       await precacheImage(new NetworkImage(url), context); //same, cache doesn't work
    });

但是我得到了:

E/flutter(3971):#1
__InternalLinkedHashMap&__ HashVMBase& MapMixin&__ LinkedHashMapMixin.forEach (dart:collection/runtime/libcompact_hash.dart:370:8)

E/flutter ( 3971): #1
__InternalLinkedHashMap&_HashVMBase&MapMixin&_LinkedHashMapMixin.forEach (dart:collection/runtime/libcompact_hash.dart:370:8)

有什么主意吗?

推荐答案

您将一个函数传递给then(...)，在该函数中使用await，为此功能需要标记为async. br> 仅将其他一些外部功能标记为async是不够的.

You pass a function to then(...) and in that function you use await and for that to work the function needs to be marked async.
It's not sufficient that some other outside function is marked async.

firebaseRef.child('...').once().then((DataSnapshot snapshot) async { // async here was missing
    Map<dynamic,dynamic> map = snapshot.value;
    map.forEach((key, url) {
    print('$key: $url');   //OKAY
    var file = await cacheManager.getFile(url); // doesn't work
  });

它可能仍然不起作用，但这取决于您要对file进行的操作，而您的问题尚不清楚.

It' might still not work but it depends on what you want to do with file and that's not clear from your question.

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