dart - itemcount 中的 snapshot.data.length 不起作用 : "The getter ' length' was called on null"

Question

我正在尝试使用以下代码从 firestore 获取文档:

 Future getCategories() async {
    var firestore = Firestore.instance;
    QuerySnapshot qn = await firestore.collection("categories").getDocuments();
    return qn.documents;
  }

 @override
  Widget build(BuildContext context) {
    return Container(
      child:FutureBuilder(
        future:getCategories(),
        builder:(context, snapshot){
          if(snapshot.connectionState == ConnectionState.waiting){
            return Center(
              child:Text("Loading...")
            );
         }
         else
         {
           return GridView.builder(
             itemCount: snapshot.data.length,
             gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
                 crossAxisSpacing: 6.0, mainAxisSpacing: 6.0, crossAxisCount: 2),
              itemBuilder: (BuildContext context, int index) {
                return SingleCategory(
                  category_name:  snapshot.data[index].data["title"],
                  category_picture: snapshot.data[index].data["picture"],
                );
              }
           );
         }
        }
      )
    );

当我运行代码时，出现以下错误:

I/flutter ( 7555): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════ I/flutter ( 7555): The following NoSuchMethodError was thrown building FutureBuilder(dirty, state: I/flutter ( 7555): _FutureBuilderState#c3e7b): I/flutter ( 7555): The getter 'length' was called on null. I/flutter ( 7555): Receiver: null I/flutter ( 7555): Tried calling: length I/flutter ( 7555): I/flutter ( 7555): When the exception was thrown, this was the stack: I/flutter ( 7555): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)

谁能帮帮我。

Answer 1

试试这个:

 future: getData(),
              builder: (context, AsyncSnapshot<List<User>> snapshot)