如何在Flutter中仅加载一次异步流? [英] How to load async Stream only one time in Flutter?

问题描述

我从Firebase获得了一个普通的DocumentSnapshots流，然后使用asyncMap()方法从中生成另一个流:

I get a normal Stream of DocumentSnapshots from Firebase, and from it I generate another stream with the asyncMap() method :

    final _dataBaseReference = FirebaseFirestore.instance;

Future<Stream> getPersonalShiftsbyDay() async {
    
    //the first, normal stream 
    
     Stream firstStream =  Stream<DocumentSnapshot> _personalShifts =
            _dataBaseReference.doc("personalScheduledShift/$_empId").snapshots();
    
     // the second stream, obtained with asyncMap 
    
   return _personalShifts.asyncMap((event) async {


      _shiftDetails = {};

      await Future.forEach(event.data().keys, (key) async {
        DocumentSnapshot _shiftDoc = await _dataBaseReference
            .doc("/scheduledShift/$_busId/scheduledshift/$key")
            .get();
        DocumentSnapshot _shiftEvents = await _dataBaseReference
            .doc("/scheduledShiftEvents/$_busId/events/$key")
            .get();
        DocumentSnapshot _shiftEmployees = await _dataBaseReference
            .doc("/scheduledShiftEmployee/$_busId/employee/$key")
            .get();

          _shiftDetails["${_shiftDoc.id}"] = {
            "id": _shiftDoc.id,
            "day": _shiftDoc.data()["day"].toDate(),
            "startHour": _shiftDoc.data()["startHour"].toDate(),
            "endHour": _shiftDoc.data()["endHour"].toDate(),
            "events": _eventsId.length,
            "employees": _employeesId,
          };
        
      });
      print(_shiftDetails);
      return _shiftDetails;
    });

}

问题是，例如，当我在StreamBuilder中使用secondStream时，每次加载屏幕时，secondStream都会重新加载并等待 asyncMap 中包含的代码，加载一段时间并生成应用程序变慢.

The problem is, when I use the secondStream, for example in a StreamBuilder, every time I load the screen the secondStream reloads and awaits for the code contained in asyncMap , loading for a while and making the app slower.

所以我的问题是:如何加载一次asyncMap代码(在启动应用程序时，甚至可能仅加载一次)，然后将Stream用作普通流，仅当db中的数据发生更改时才发送事件?

推荐答案

您正在这样做吗?

build() {
  return StreamBuilder(stream: getPersonalShiftsbyDay(), ...);
}

您应该这样做

Stream _stream;
initState() {
  ...
  _stream = getPersonalShiftsbyDay();
}

build() {
  return StreamBuilder(stream: _stream, ...);
}

要了解更多信息:

如何处理不需要的小部件构建?

https://youtu.be/vPHxckiSmKY?t=4772

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