如何使用 object snapshot.data 在简单的 flutter-dart 代码中实现空安全？

Question

I am new to flutter/dart coding, please help me solve the following:

Here is my code trying to fetch data from the FireStore collection "DinnerNames" but I get an analysis error on the snapshot.data object at the line:

itemCount: snapshot.data.documents.length

:

The problem:

An expression whose value can be 'null' must be null-checked before it can be dereferenced. Try checking that the value isn't 'null' before dereferencing it.

Here is the code sample generates the error:

CollectionReference dinners =
        FirebaseFirestore.instance.collection('DinnerNames');

    return Scaffold(
      appBar: AppBar(
        title: Text('My Dinner Voting App'),
      ),
      body: StreamBuilder(
          stream: dinners.snapshots(),
          builder: (context, snapshot){
            if (!snapshot.hasData) return const Text('Firestore snapshot is loading..');
            
            if (!snapshot.hasError)
              return const Text('Firestore snapshot has error..');
            
            if (snapshot.data == null){
              return const Text("Snapshot.data is null..");
            }else{
               return ListView.builder(
              itemExtent: 80.0,
              itemCount:  snapshot.data.documents.length,
              itemBuilder: (context, index) =>
                  _buildListItem(context, snapshot.data.documents[index]),
            ); 
            }           
          }
          ),
    );

here is the flutter version:

dave@DaveMacBook-Pro firebasetest % flutter --version
Flutter 1.25.0-8.2.pre • channel beta • https://github.com/flutter/flutter.git
Framework • revision b0a2299859 (2 weeks ago) • 2021-01-05 12:34:13 -0800
Engine • revision 92ae191c17
Tools • Dart 2.12.0 (build 2.12.0-133.2.beta)

Answer 1

If you're sure that the object is not null by the time you're using it, just insert a Bang ! operator to cast away nullability, something like this:

ListView.builder(
  itemCount: snapshot.data!.docs.length, // <-- Notice '!'
)

Answer 2

you're already checking:

snapshot.data == null

but it seems the analysis is indicating that documents can also be null, so try to include also:

if (snapshot.data.documents != null) { ... do what you need }

or try to change this code:

if (snapshot.data == null){
  return const Text("Snapshot.data is null..");
}

to this:

if (snapshot.data == null || snapshot.data.documents == null){
  return const Text("Snapshot.data is null..");
}

Answer 3

If you explicitly tell dart/flutter that the type of the snapshot argument in the builder function will be 'AsyncSnapshot<QuerySnapshot>' you can let Flutter know that it will have certain attributes, like below. You may still have to use the '?.' operator and 'documents' has now been changed to 'docs':

    builder: (context, AsyncSnapshot<QuerySnapshot> snapshot){ // <-- here
            if (!snapshot.hasData) return const Text('Firestore snapshot is 
        loading..');
                
            if (!snapshot.hasError)
              return const Text('Firestore snapshot has error..');
                
            if (snapshot.data == null){
              return const Text("Snapshot.data is null..");
            }else{
               return ListView.builder(
                   itemExtent: 80.0,
                   itemCount:  snapshot.data?.docs.length,
                   itemBuilder: (context, index) =>
                      _buildListItem(context, snapshot.data?.docs[index]),
                   ); 
           }