firebase - whenComplete() 方法未按预期工作 - Flutter Async

Question

在我的 Flutter 应用程序中，我尝试在使用 whenComplete() 方法完成异步函数后运行一些代码。问题是我的 whenComplete() 方法中的代码甚至在异步函数完成之前就开始执行了。

我也尝试过使用 then() 方法，结果也是一样。

这是我调用异步函数的初始化函数:

@override
  void initState() {
    super.initState();

    FirebaseAuth.instance.currentUser().then((user) {
      setState(() {
        mUid = user.uid;

        getUserName();
        getUserHomes().whenComplete(() {
          print(mUserHomes);
          setState(() {
            dropdownVal = mUserHomes[0];
          });
        });
      });
    }).catchError((e) {
      print(e);
    });
  }

这是异步函数的函数体:

Future getUserHomes() async {
    CollectionReference ref = Firestore.instance
        .collection('users')
        .document(mUid)
        .collection('accessibleHomes');
    QuerySnapshot eventsQuery = await ref.getDocuments();

    eventsQuery.documents.forEach((document) {
      DocumentReference homeReference = document["homeReference"];

      getHomeDevices("home", homeReference).whenComplete(() {
        setState(() {});
      });

      homeReference.get().then((DocumentSnapshot ds) {
        mUserHomes.add(ds["alias"].toString());
      });
    });
  }

运行应用程序的控制台输出是:

Performing hot restart...
Syncing files to device ASUS Z01RD...
Restarted application in 2,521ms.
I/flutter ( 5718): []
E/flutter ( 5718): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: RangeError (index): Invalid value: Valid value range is empty: 0
E/flutter ( 5718): #0      List.[] (dart:core-patch/growable_array.dart:145:60)
E/flutter ( 5718): #1      _TabOneState.initState.<anonymous closure>.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:smart_switch/home/tabs/tab_one/tab_one_content.dart:52:37)
E/flutter ( 5718): #2      State.setState (package:flutter/src/widgets/framework.dart:1117:30)
E/flutter ( 5718): #3      _TabOneState.initState.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:smart_switch/home/tabs/tab_one/tab_one_content.dart:51:11)
E/flutter ( 5718): #4      _rootRun (dart:async/zone.dart:1120:38)
E/flutter ( 5718): #5      _CustomZone.run (dart:async/zone.dart:1021:19)
E/flutter ( 5718): #6      _FutureListener.handleWhenComplete (dart:async/future_impl.dart:150:18)
E/flutter ( 5718): #7      Future._propagateToListeners.handleWhenCompleteCallback (dart:async/future_impl.dart:609:39)
E/flutter ( 5718): #8      Future._propagateToListeners (dart:async/future_impl.dart:665:37)
E/flutter ( 5718): #9      Future._complete (dart:async/future_impl.dart:473:7)
E/flutter ( 5718): #10     _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
E/flutter ( 5718): #11     _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:28:18)
E/flutter ( 5718): #12     _completeOnAsyncReturn (dart:async-patch/async_patch.dart:294:13)
E/flutter ( 5718): #13     _TabOneState.getUserHomes (package:smart_switch/home/tabs/tab_one/tab_one_content.dart)
E/flutter ( 5718): #14     _asyncThenWrapperHelper.<anonymous closure> (dart:async-patch/async_patch.dart:77:64)
E/flutter ( 5718): #15     _rootRunUnary (dart:async/zone.dart:1132:38)
E/flutter ( 5718): #16     _CustomZone.runUnary (dart:async/zone.dart:1029:19)
E/flutter ( 5718): #17     _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
E/flutter ( 5718): #18     Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
E/flutter ( 5718): #19     Future._propagateToListeners (dart:async/future_impl.dart:668:32)
E/flutter ( 5718): #20     Future._complete (dart:async/future_impl.dart:473:7)
E/flutter ( 5718): #21     _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
E/flutter ( 5718): #22     _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:28:18)
E/flutter ( 5718): #23     _completeOnAsyncReturn (dart:async-patch/async_patch.dart:294:13)
E/flutter ( 5718): #24     Query.getDocuments (package:cloud_firestore/src/query.dart)
E/flutter ( 5718): #25     _asyncThenWrapperHelper.<anonymous closure> (dart:async-patch/async_patch.dart:77:64)
E/flutter ( 5718): #26     _rootRunUnary (dart:async/zone.dart:1132:38)
E/flutter ( 5718): #27     _CustomZone.runUnary (dart:async/zone.dart:1029:19)
E/flutter ( 5718): #28     _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
E/flutter ( 5718): #29     Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
E/flutter ( 5718): #30     Future._propagateToListeners (dart:async/future_impl.dart:668:32)
E/flutter ( 5718): #31     Future._complete (dart:async/future_impl.dart:473:7)
E/flutter ( 5718): #32     _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
E/flutter ( 5718): #33     _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:28:18)
E/flutter ( 5718): #34     _completeOnAsyncReturn (dart:async-patch/async_patch.dart:294:13)
E/flutter ( 5718): #35     MethodChannel.invokeMapMethod (package:flutter/src/services/platform_channel.dart)
E/flutter ( 5718): #36     _asyncThenWrapperHelper.<anonymous closure> (dart:async-patch/async_patch.dart:77:64)
E/flutter ( 5718): #37     _rootRunUnary (dart:async/zone.dart:1132:38)
E/flutter ( 5718): #38     _CustomZone.runUnary (dart:async/zone.dart:1029:19)
E/flutter ( 5718): #39     _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
E/flutter ( 5718): #40     Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
E/flutter ( 5718): #41     Future._propagateToListeners (dart:async/future_impl.dart:668:32)
E/flutter ( 5718): #42     Future._complete (dart:async/future_impl.dart:473:7)
E/flutter ( 5718): #43     _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
E/flutter ( 5718): #44     _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:28:18)
E/flutter ( 5718): #45     _completeOnAsyncReturn (dart:async-patch/async_patch.dart:294:13)
E/flutter ( 5718): #46     MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart)
E/flutter ( 5718): #47     _asyncThenWrapperHelper.<anonymous closure> (dart:async-patch/async_patch.dart:77:64)
E/flutter ( 5718): #48     _rootRunUnary (dart:async/zone.dart:1132:38)
E/flutter ( 5718): #49     _CustomZone.runUnary (dart:async/zone.dart:1029:19)
E/flutter ( 5718): #50     _FutureListener.handleValue (dart:async/future_impl.dart:126

因此，在调用异步函数 getUserHomes 的 init() 函数中，我尝试将 dropdownVal 变量设置为从 Firestore DB 获取的列表 mUserHomes 的第一个值。该列表在控制台中打印为 []，这意味着空，但如果我在稍后阶段打印该列表，它会打印出值。

Answer 1

你的 getUserHomes 函数在它不等待的 forEach 回调中做了一堆异步工作，所以 getUserHomes 完成了它返回的Future 几乎在 ref.getDocuments 完成后立即执行。

如果你想等待 getUserHomes 中的所有工作完成，你可以这样做:

Future<void> getUserHomes() async {
  CollectionReference ref = Firestore.instance
      .collection('users')
      .document(mUid)
      .collection('accessibleHomes');
  QuerySnapshot eventsQuery = await ref.getDocuments();

  final waitList = <Future<void>>[];

  for (var document in eventsQuery.documents) {
    DocumentReference homeReference = document["homeReference"];

    waitList.add(getHomeDevices("home", homeReference));
    waitList.add(
      homeReference.get().then((DocumentSnapshot ds) {
        mUserHomes.add(ds["alias"].toString());
      });
    );
  }

  await Future.wait(waitList);
}