查询时如何在 Flutter (Dart) 中使用变量 Cloud Firestore

Question

这是我的工作代码：

final Stream<QuerySnapshot> products = FirebaseFirestore.instance
  .collection('products')
  .doc('men')
  .collection('tshirts')
  .snapshots();

这是我想要但不起作用的代码：

String val = 'tshirts';
final Stream<QuerySnapshot> products = FirebaseFirestore.instance
      .collection('products')
      .doc('men')
      .collection(val)
      .snapshots();

错误： 无法在初始化程序中访问实例成员“val”。 尝试用不同的表达式替换对实例成员的引用

我在构建小部件中使用的波纹管代码显示数据库查询结果和工作正常：

GridView.count(
          physics: const BouncingScrollPhysics(),
          crossAxisCount: 2,
          childAspectRatio: 0.5,
          mainAxisSpacing: 30,
          crossAxisSpacing: 10,
          children: snapshot.data!.docs.map((DocumentSnapshot document) {
            Map<String, dynamic> data =
                document.data()! as Map<String, dynamic>;

            return Visibility(
              child: GestureDetector(
                  onTap: () {
                    Navigator.push(
                        context,
                        PageTransition(
                          type: PageTransitionType.bottomToTop,
                          child: const ProductView(productId: '3453245'),
                        ));
                  },
                  child: Stack(
                    children: [
                      Container(
                        decoration: BoxDecoration(
                            image: DecorationImage(
                                image: NetworkImage('${data['img']}'),
                                fit: BoxFit.cover)),
                      ),
                      Align(
                        alignment: Alignment.bottomLeft,
                        child: SizedBox(
                          height: 70,
                          width: double.infinity,
                          child: Column(
                            crossAxisAlignment: CrossAxisAlignment.start,
                            children: [
                              Container(
                                margin: const EdgeInsets.only(
                                    right: 18, top: 15, left: 5),
                                child: Text(
                                  data['name'].toUpperCase(),
                                  textAlign: TextAlign.left,
                                  style: GoogleFonts.oswald(
                                    color: Colors.black,
                                    fontSize: 15,
                                  ),
                                  overflow: TextOverflow.ellipsis,
                                  maxLines: 1,
                                  softWrap: false,
                                ),
                              ),
                              Container(
                                margin: const EdgeInsets.only(left: 5),
                                child: Text(
                                  '\u{20B9} ${data['price']}',
                                  textAlign: TextAlign.left,
                                  style: const TextStyle(
                                    color: Colors.black,
                                    fontSize: 14,
                                  ),
                                  overflow: TextOverflow.ellipsis,
                                  maxLines: 1,
                                ),
                              )
                            ],
                          ),
                        ).frosted(
                          blur: 7,
                        ),
                      ),
                    ],
                  )),
              visible: true,
            );
          }).toList(),
        )

Answer 1

您甚至在初始化所使用的类之前就尝试使用val。 您只能在方法或构造函数中访问它，例如：

@override
void initState() {
    super.initState();
    final Stream<QuerySnapshot> products = FirebaseFirestore.instance.collection('products').doc('men').collection(val).snapshots();
}