Flutter Firestore -如何只获取当月的数据？

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'dart:async';
import 'package:intl/intl.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  List<DocumentSnapshot> _documents = [];
  StreamSubscription<QuerySnapshot> _subscription;

  @override
  void initState() {
    super.initState();
    _getDocuments();
  }

  void _getDocuments() async {
    DateTime now = DateTime.now();
    DateTime firstDayOfMonth = DateTime(now.year, now.month, 1);
    DateTime lastDayOfMonth = DateTime(now.year, now.month + 1, 0);

    QuerySnapshot querySnapshot = await Firestore.instance.collection('your_collection')
        .where('date', isGreaterThanOrEqualTo: firstDayOfMonth)
        .where('date', isLessThanOrEqualTo: lastDayOfMonth)
        .getDocuments();

    setState(() {
      _documents = querySnapshot.documents;
    });

    // 订阅实时更新
    _subscription = Firestore.instance.collection('your_collection')
        .where('date', isGreaterThanOrEqualTo: firstDayOfMonth)
        .where('date', isLessThanOrEqualTo: lastDayOfMonth)
        .snapshots()
        .listen((data) {
          setState(() {
            _documents = data.documents;
          });
        });
  }

  @override
  void dispose() {
    _subscription?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: _documents.length,
      itemBuilder: (context, index) {
        Map<String, dynamic> document = _documents[index].data;
        return ListTile(
          title: Text(document['title']),
          subtitle: Text(DateFormat('yyyy-MM-dd').format(document['date'])),
        );
      },
    );
  }
}