如何修复“获取器"文档被称为null."扑朔迷离 [英] How to fix 'getter "documents" was called on null.' in flutter

问题描述

我正在使用flutter和firebase创建一个移动应用程序.我在Firestore上有2个收藏集，我想阅读收藏集帖子"中的所有文档.但是，当我这样做时，在null上调用了一个说getter"documents"的错误.

I am using flutter and firebase to create a mobile app. I have 2 collections on my firestore and i wanna read all documents in a the collection "posts". However, when I do that, an error saying the getter "documents" was called on null.

    Widget getContent(BuildContext context) {
      return StreamBuilder<QuerySnapshot>(
        stream: Firestore.instance.collection("posts").snapshots(),
        builder: (context, snap) {
          return CarouselSlider(
            enlargeCenterPage: true,
            height: MediaQuery.of(context).size.height,
            items: getItems(context, snap.data.documents),
        );
        },
        );
    }

    List<Widget> getItems(BuildContext context, List<DocumentSnapshot> 
    docs){
      return docs.map(
        (doc) {
          String content = doc.data["content"];
          return Text(content);
        }
      ).toList();
    }

我希望随同所有文档中的数据一起提供，但出现了以下错误:

I expected to be delivered with the data in all documents, nut instead got this error:

I/flutter (30878): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (30878): The following NoSuchMethodError was thrown building StreamBuilder<QuerySnapshot>(dirty,
I/flutter (30878): dependencies: [MediaQuery], state: _StreamBuilderBaseState<QuerySnapshot,
I/flutter (30878): AsyncSnapshot<QuerySnapshot>>#72d38):
I/flutter (30878): The getter 'documents' was called on null.
I/flutter (30878): Receiver: null
I/flutter (30878): Tried calling: documents
I/flutter (30878): When the exception was thrown, this was the stack:
V/NativeCrypto(30878): Registering com/google/android/gms/org/conscrypt/NativeCrypto's 284 native methods...
I/flutter (30878): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter (30878): #1      PostsPageState.getContent.<anonymous closure>
package:reach_out_kef_global/main.dart:140
I/flutter (30878): #2      StreamBuilder.build 
package:flutter/…/widgets/async.dart:423
I/flutter (30878): #3      _StreamBuilderBaseState.build 
package:flutter/…/widgets/async.dart:125
I/flutter (30878): #4      StatefulElement.build 
package:flutter/…/widgets/framework.dart:3825
I/flutter (30878): #5      ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:3736
I/flutter (30878): #6      Element.rebuild 
package:flutter/…/widgets/framework.dart:3559
I/flutter (30878): #7      ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:3716
I/flutter (30878): #8      StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:3864
I/flutter (30878): #9      ComponentElement.mount 
package:flutter/…/widgets/framework.dart:3711
I/flutter (30878): #10     Element.inflateWidget 
package:flutter/…/widgets/framework.dart:2956
I/flutter (30878): #11     Element.updateChild 
package:flutter/…/widgets/framework.dart:2759
I/flutter (30878): #12     ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:3747
I/flutter (30878): #13     Element.rebuild 
package:flutter/…/widgets/framework.dart:3559
I/flutter (30878): #14     ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:3716
I/flutter (30878): #15     StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:3864
I/flutter (30878): #16     ComponentElement.mount 
package:flutter/…/widgets/framework.dart:3711
...

请帮助！

推荐答案

在调用snapshot.data.documents之前，应检查snapshot.data是否为空.这通常是我在流构建器的构建方法中要做的第一件事.如果该流为空，则类似于第一次侦听该流但在Firestore返回所请求的数据之前一样，snapshot.data将为null.在这种情况下，您可能想要显示一个容器或循环进度指示器:

You should check that the snapshot.data is not null before you call snapshot.data.documents. This is often the first thing I do in the build method of a stream builder. If the stream is empty, like it will be when it is first listened to but before firestore has returned the data being requested, the snapshot.data will be null. In that case you might want to show a container or a circular progress indicator:

Widget getContent(BuildContext context) {
  return StreamBuilder<QuerySnapshot>(
    stream: Firestore.instance.collection("posts").snapshots(),
    builder: (context, snap) {

      //just add this line
      if(snap.data == null) return CircularProgressIndicator();

      return CarouselSlider(
        enlargeCenterPage: true,
        height: MediaQuery.of(context).size.height,
        items: getItems(context, snap.data.documents),
    );
    },
    );
}

List<Widget> getItems(BuildContext context, List<DocumentSnapshot> 
docs){
  return docs.map(
    (doc) {
      String content = doc.data["content"];
      return Text(content);
    }
  ).toList();
}

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