Flutter Firebase“List<Future<CustomClass>>”类型的值不能分配给“List<CustomClass>？”类型的变量

Question

我的 FutureBuilder 出现问题。它应该将自定义类“GeoPost”映射到来自 firebase 查询的“列表”，但在我的代码中的某处它没有将 Geopost 列表读取为非 Future

这是发生错误的代码：

FutureBuilder(
            future: postsRef.get().then((doc) {
              geoPosts = doc.docs.map((doc) async => await GeoPost.fromDoc(doc)).toList();
            }),
            builder: (context, snapshot) { }

自定义类和函数：

    class GeoPost {
      final String caption;
      final String postId;
      final String authorId;
      final int likesCount;
      final int commentCount;
      final Position position;
      final Timestamp timeStamp;
      final String? imageUrl;
      final Userdata authordata;
    
      GeoPost(this.caption, this.postId, this.authorId, this.likesCount, this.commentCount, this.position, this.timeStamp, this.imageUrl, this.authordata);
    
      static Future<GeoPost> fromDoc(DocumentSnapshot doc) async {
        return GeoPost.fromDocument(doc, await getPostUserDataFromFirestore(doc['userId']));
    
      }
    
      factory GeoPost.fromDocument(DocumentSnapshot doc, Userdata author) {
        return GeoPost(
            doc['caption'],
            doc['postId'],
            doc['userId'],
            doc['likeCount'] ?? 0,
            doc['commentCount'] ?? 0,
            doc['position'],
            doc['timeStamp'],
            doc['imageUrl'],
            author
        );
      }

Future<void> getPostUserDataFromFirestore (String did) async {
    return Userdata.fromDoc(await usersRef.doc(uid).get());
  }
    }

错误：

Error: A value of type 'List<Future<GeoPost>>' can't be assigned to a variable of type 'List<GeoPost>?'.

Answer 1

• 您需要等待List<Future<GeoPost> 中的所有Futures（将它们转换为List<GeoPost>），然后再将它们分配给List<GeoPost> 类型的变量。

  将您的 FutureBuilder 代码更新为：

  FutureBuilder<List<GeoPost>>(
    future: () async { 
      var doc = await FirebaseFirestore.instance.collection('collectionPath').get();
      var data = doc.docs.map((doc) async => await GeoPost.fromDoc(doc)).toList();
      geoPosts = await Future.wait(data);
      return geoPosts;
    }(),
    builder: (context, snapshot) { }
  

• 另外，您需要将getPostUserDataFromFirestore 方法的返回类型从Future<void> 更改为Future<Userdata>。没有返回值时使用void。

  将getPostUserDataFromFirestore 方法更新为：

  Future<Userdata> getPostUserDataFromFirestore (String did) async {
    return Userdata.fromDoc(await usersRef.doc(uid).get());
  }