如何在flutter中捕获异步函数的异常？

Question

I uses many apis to communicate with server and want to handle unexpected exception from this callings. But, It cannot catch exception at all. How to handle this?

if(event is RandomChatEventMatchStart){
  yield RandomChatState.loading();
  try {
    chatRoomID = await _api.getRoomID();
    if(chatRoomID.isEmpty){
      chatRoomID = await _api.makeChatRoom();
      yield RandomChatState.madeChatRoom(chatRoomID);
    } else {
      await _api.enterChatRoom(chatRoomID);
      yield RandomChatState.matchSucceeded();
    }
  } catch(exception) {
    yield RandomChatState.apiFailed(exception.toString());
  }
}

E/flutter (12376): PlatformException(error, Invalid document reference. Document references must have an even number of segments, but messages has 1, null) E/flutter (12376): #0      StandardMethodCodec.decodeEnvelope  package:flutter/…/services/message_codecs.dart:564 E/flutter (12376):
#1      MethodChannel.invokeMethod  package:flutter/…/services/platform_channel.dart:292 E/flutter (12376): <asynchronous suspension> E/flutter (12376): #2      DocumentReference.snapshots.<anonymous closure>  ..\…\src\document_reference.dart:126 E/flutter (12376): #3     
_runGuarded (dart:async/stream_controller.dart:805:24) E/flutter (12376): #4      _BroadcastStreamController._subscribe (dart:async/broadcast_stream_controller.dart:213:7) E/flutter (12376):
#5      _ControllerStream._createSubscription (dart:async/stream_controller.dart:818:19) E/flutter (12376): #6     
_StreamImpl.listen (dart:async/stream_impl.dart:472:9) E/flutter (12376): #7      _StreamBuilderBaseState._subscribe  package:flutter/…/widgets/async.dart:135 E/flutter (12376): #8     
_StreamBuilderBaseState.didUpdateWidget  package:flutter/…/widgets/async.dart:120 E/flutter (12376): #9      StatefulElement.update  package:flutter/…/widgets/framework.dart:3863 E/flutter (12376): #10     Element.updateChild

Answer 1

If you're using async methods like Future , you can use catchError to handle the Exception as mentioned in this guide .

Here's a sample that you can run on dartpad

void main() {
  foo()
      .whenComplete(() => print('Completed'))
      .catchError((error) => print('Error $error'));
}

Future<void> foo() async {
  for (int i = 0; i < 5; i++) {
    if(i==4) throw Exception;
    print('hello ${i + 1}');
  }
}