whenComplete() 方法没有按预期工作 - Flutter Async
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【中文标题】whenComplete() 方法没有按预期工作 - Flutter Async【英文标题】：whenComplete() method not working as expected - Flutter Async 【发布时间】：2019-10-29 05:50:53 【问题描述】：在我的 Flutter 应用程序中，我尝试使用 whenComplete() 方法在异步函数完成后运行一些代码。问题是我的 whenComplete() 方法中的代码甚至在异步函数完成之前就被执行了。
我也尝试过使用 then() 方法，这也产生了相同的结果。
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());
);
);
运行应用时的控制台输出为：
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
所以在我调用异步函数 getUserHomes 的 init() 函数中，我试图将 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 [] 表示为空，但如果我在稍后阶段打印该列表，它将与值一起打印。
【问题讨论】：
【参考方案1】：您的getUserHomes 函数在它不等待的foreach 回调中执行大量异步工作，因此getUserHomes 几乎在ref.getDocuments 完成后立即完成其返回的Future。
如果您想等待getUserHomes 中的所有工作完成，您可以执行以下操作：
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);
【讨论】：
【参考方案2】：我遇到了同样的问题，并意识到如果你使用 for 循环而不是使用 foreach 方法，事情会按预期工作
【讨论】：
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