我如何从Flutter引用Firestore中文档，并在应用程序的不同位置获取数据

dependencies:
  cloud_firestore: ^2.5.3

import 'package:cloud_firestore/cloud_firestore.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  FirebaseFirestore.instance.settings = Settings(
    persistenceEnabled: true, // 启用本地持久化
    cacheSizeBytes: FirebaseFirestore.instance.collection('dummy').snapshots().length, // 设置缓存大小
  );
  runApp(MyApp());
}

Future<DocumentSnapshot<Map<String, dynamic>>> getDocumentData() async {
  DocumentSnapshot<Map<String, dynamic>> snapshot = await FirebaseFirestore.instance
      .collection('collectionName') // 替换为集合名称
      .doc('documentId') // 替换为文档ID
      .get();
  return snapshot;
}

void fetchData() async {
  DocumentSnapshot<Map<String, dynamic>> snapshot = await getDocumentData();
  if (snapshot.exists) {
    Map<String, dynamic> data = snapshot.data()!;
    String fieldValue = data['fieldName']; // 替换为字段名称
    // 在这里处理数据
  }
}