Question

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已执行。

我也尝试过使用then（）方法，这也产生了相同的结果。

这是我要在其中调用该异步函数的init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());
      });
    });
  }

运行该应用程序时的控制台输出为：

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

因此，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。列表在控制台中显示为[]，表示为空，但是如果我在以后的阶段中打印列表，则会使用值进行打印。

Answer 1

您的getUserHomes函数在foreach回调中执行了许多异步工作，它不等待，因此getUserHomes几乎在{之后立即完成了返回的Future {1}}完成。

如果您要等待ref.getDocuments中的所有工作完成，可以执行以下操作：

getUserHomes

Answer 2

我遇到了同样的问题，并且意识到，如果您使用for循环，那么可以使用预期的方法来代替

whenComplete（）方法无法正常工作-Flutter Async

2 个答案: