How to convert Future<int> to int?

So I'm trying to display a pie chart using the fl_chart plugin. The data for the chart is being retrieved from firestore. I have this function that is used to display the data:

List<PieChartSectionData> showSection(AsyncSnapshot<QuerySnapshot> snapshot) {
    return List.generate(length, (i) {
      final isTouched = i == touchedIndex;
      final double fontSize = isTouched ? 25 : 16;
      final double radius = isTouched ? 60 : 50;
      return PieChartSectionData(
        color: Color(int.parse(cerealData[i].colorVal)),
        value: cerealData[i].rating,
        title: cerealData[i].rating.toString(),
        radius: radius,
        titleStyle: TextStyle(
            fontSize: fontSize,
            fontWeight: FontWeight.bold,
            color: const Color(0xffffffff)),
      );
    });
  }

The List.generate() takes an int as an argument. Since I'm displaying realtime data, I'm trying to get the number of documents present in my collection. For that, I have a function called getLength():

  void getLength(AsyncSnapshot<QuerySnapshot> snapshot) async {
    length = await Firestore.instance.collection('cereal').snapshots().length;
    cerealData =
        snapshot.data.documents.map((e) => Cereal.fromJson(e.data)).toList();
  }

However, when I run the code, I get:

 Another exception was thrown: type 'Future<int>' is not a subtype of type 'int'

The entire code:

class _FlChartPageState extends State<FlChartPage> {
  int touchedIndex;
  var length;
  List<Cereal> cerealData;

  void getLength(AsyncSnapshot<QuerySnapshot> snapshot) async {
    length = await Firestore.instance.collection('cereal').snapshots().length;
    cerealData =
        snapshot.data.documents.map((e) => Cereal.fromJson(e.data)).toList();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: StreamBuilder<QuerySnapshot>(
          stream: Firestore.instance.collection('cereal').snapshots(),
          builder: (context, snapshot) {
            getLength(snapshot);
            if (!snapshot.hasData)
              return CircularProgressIndicator();
            else {
              return PieChart(PieChartData(
                  pieTouchData: PieTouchData(touchCallback: (pieTouchResponse) {
                    setState(() {
                      if (pieTouchResponse.touchInput is FlLongPressEnd ||
                          pieTouchResponse.touchInput is FlPanEnd) {
                        touchedIndex = -1;
                      } else {
                        touchedIndex = pieTouchResponse.touchedSectionIndex;
                      }
                    });
                  }),
                  borderData: FlBorderData(show: false),
                  sectionsSpace: 0,
                  centerSpaceRadius: 40,
                  sections: showSection(snapshot)));
            }
          }),
    );
  }

  List<PieChartSectionData> showSection(AsyncSnapshot<QuerySnapshot> snapshot) {
    return List.generate(length, (i) {
      final isTouched = i == touchedIndex;
      final double fontSize = isTouched ? 25 : 16;
      final double radius = isTouched ? 60 : 50;
      return PieChartSectionData(
        color: Color(int.parse(cerealData[i].colorVal)),
        value: cerealData[i].rating,
        title: cerealData[i].rating.toString(),
        radius: radius,
        titleStyle: TextStyle(
            fontSize: fontSize,
            fontWeight: FontWeight.bold,
            color: const Color(0xffffffff)),
      );
    });
  }
}

I read somewhere that awaiting the future gets rid of the Future. But that doesn't work here. How do I fix this?

Edit: It works if I simply pass the number of documents instead of length in List.generate(). But this won't work if there are changes to the collection. So how do I convert Future to int?

I think you aren't getting the length of the documents, you are getting the length of the snapshots if you want to get the documents length :

QuerySnapshot querySnapshot = await Firestore.instance.collection('cereal').getDocuments();
    int length = querySnapshot.documents.length;