Update ListView.builder itemCount when firestore document is added - flutter

Question

I have a flutter app where a list is generated with ListView.Builder, and where the itemCount is the number of documents in a firestore collection. This works fine until a new document is added. When that happens I get the error (17 and 18 are just examples).
Invalid value: Not in range 0..17, inclusive: 18
I assume I would need to update the state when a new document is created, but I have no idea how i can call setState when that happens
Here is the relevant part of the code:
child: StreamBuilder(
stream: Firestore.instance.collection('contact').orderBy(sortby, descending: decending).snapshots(),
builder: (context, snapshot) {
if (!snapshot.hasData) return Container();
return ListView.builder(
itemCount: snapshot.data.documents.length,
itemBuilder: (context, index) =>
_personer(context, snapshot.data.documents[index], index),
);
},
),

Answer 1

use setState?
StreamBuilder(builder: (context, snapshot) {
return snapshot.hasData == null ? Container() : _getListView(snapshot);
} , )
_getListView(snapshot) {
setState(() {
return ListView.builder(
itemCount: snapshot.data.documents.length,
itemBuilder: (context, index) =>
_personer(context, snapshot.data.documents[index], index),
);
});
}

Answer 2

StreamBuilder use QuerySnapshot so list data can change
example code :
StreamBuilder<QuerySnapshot>(
builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
if (snapshot.hasError)
return new Text('Error: ${snapshot.error}');
switch (snapshot.connectionState) {
case ConnectionState.waiting: return new Text('Loading...');
default:
return new ListView(
children: snapshot.data.documents.map((DocumentSnapshot document) {
return ;
}).toList(),
);
}
},
)