Null 检查用于 null 值的运算符

Question

body: StreamBuilder(
  stream: db.collection('products').where('category', isEqualTo: 'category').snapshots(),
  builder: (context, AsyncSnapshot<QuerySnapshot> snapshot) {
    if(snapshot.hasError){
      Fluttertoast.showToast(msg: 'Something error');
    }
    if(snapshot.connectionState == ConnectionState.waiting) {
      CircularProgressIndicator();
    }
    final values = snapshot.data!.docs;
    return ListView.builder(
        itemCount: values!.length,
        itemBuilder: (BuildContext context, int index) {
          return Padding(
            padding: const EdgeInsets.all(8.0),
            child: Container(
              child: Padding(
                padding: const EdgeInsets.all(8.0),
                child: Text(values![index]['name']),
              ),
            ),
          );
        }
    );
  },
),

The following _CastError was thrown building StreamBuilder<QuerySnapshot<Map<String, dynamic>>>(dirty, state: _StreamBuilderBaseState<QuerySnapshot<Map<String, dynamic>>, AsyncSnapshot<QuerySnapshot<Map<String, dynamic>>>>#c87c1):

Answer 1

you missed return in your if blocks, so those widgets are not being returned, this redirect to the snapshot.data..docs widget which is null initially, change it to this:

 body: StreamBuilder(
        stream: db.collection('products').where('category', isEqualTo: 'category').snapshots(),
        builder: (context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if(snapshot.hasError){
            Fluttertoast.showToast(msg: 'Something error');
            return Text("error"); // add this to showcase the error
          }
          if(snapshot.connectionState == ConnectionState.waiting) {
            return CircularProgressIndicator(); // add this to show loading when data is fetching
          }
          final values = snapshot.data!.docs;
          return ListView.builder(
              itemCount: values!.length,
              itemBuilder: (BuildContext context, int index) {
                return Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Container(
                    child: Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: Text(values![index]['name']),
                    ),
                  ),
                );
              }
          );
        },
      ),