Flutter Cloud Firestore Map<String, dynamic> error

问题

I am trying to build an app using Flutter and Firestore. When loading a Collection from Firestore using StreamBuilder to display it in a ListView, I get the following error

The following assertion was thrown building StreamBuilder<QuerySnapshot>(dirty, state:
I/flutter (26287): _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#d5638):

I/flutter (26287): type '_InternalLinkedHashMap<dynamic, dynamic>' is not a subtype of type 'Map<String, dynamic>'

I/flutter (26287): where

I/flutter (26287):   _InternalLinkedHashMap is from dart:collection

I/flutter (26287):   Map is from dart:core

I/flutter (26287):   String is from dart:core

This is how I want to get data from the DocumentSnapshot

class Creator {
  const Creator({this.creatorId, this.name});

  Creator.fromDoc(DocumentSnapshot doc) : this.fromMap(doc.data);

  Creator.fromMap(Map<String, dynamic> map) :
    assert(map.containsKey('creatorId'),
    assert(map.containsKey('name'),
    this ( creatorId: map['creatorId'], name: map['name'] );

  /*

  ...

  */
}

And how I want to use it

return Scaffold(
  appBar: AppBar(title: new Text('Creators')),
  body: StreamBuilder<QuerySnapshot>(
    stream: CreatorRepo.getCreators().map<List<Creator>>((creators) {
      return creators.documents.map<Creator>((c) => Creator.fromSnapshot(c)).toList();
    }),
    builder: (BuildContext context, snapshot) {
      if ( snapshot.hasData ) {
        return ListView.builder(
          itemCount: snapshot.data.length,
          builder: (context, index) {
            final creator = snapshot.data[index];

            return ExpansionTile(
              title: Text(creator.name),
              children: [
                Text(creator.creatorId),
              ],
            );
          },
        );
      }

      return const CircularProgressIndicator();
    },
  ),
);

Dependencies:

dependencies:
  flutter:
    sdk: flutter

  cloud_firestore: ^0.6.3
  firebase_messaging: ^0.2.4

Firestore only allows for String keys and dynamic values being, with the exception of Timestamp, core language types. The cloud_firestore plugin keeps document data in an _InternalLinkedHashMap<dynamic, dynamic>. I thought that the Map inside all DocumentSnapshot would be Map<String, dynamic>. How can I work around this? Changing all functions to take Map<dynamic, dynamic> and assume the key is a String is a rather ugly workaround to the problem.

回答1:

I believe this is because the Map returned from Firebase is not a Map<String, dynamic> but instead Map<dynamic, dynamic>.

See https://github.com/flutter/flutter/issues/17417 for a related issue.

回答2:

Use Map<String, dynamic>.from ( mapFromFirestore ). This creates a new Map<String, dynamic> from the contents of mapFromFirestore, throwing an error if it finds a key that isn't a String.

回答3:

Flutter now worked with Map<String,String>

回答4:

I too had this problem and fixed it with --

  Map<String, dynamic> var = {'item': 0, 'name': 'John'}  

instead of Map unspecified.

  Map var = {'item': 0, 'name': 'John'}  

来源：https://stackoverflow.com/questions/50159766/flutter-cloud-firestore-mapstring-dynamic-error