Flutter中snapshot参数不允许为null报错，加required后PostDetails仍报错如何解决？

问题根源 #

你遇到的报错是Flutter空安全机制的强制校验，同时代码存在多处逻辑冲突：

• PostDetails类构造函数重复定义snapshot参数：同时声明了位置参数List<DocumentSnapshot<Object?>> snapshot和命名参数required this.snapshot，两个参数类型不匹配且逻辑冲突
• PostDetails的类成员变量DocumentSnapshot snapshot未声明可空也未加late修饰，不符合空安全校验规则
• Home页传递参数时错误传递了整个snapshot列表，但是PostDetails实际需要的是单个DocumentSnapshot对象
• PostDetails里定义的get title => null会导致取值时出现空异常

修复步骤 #

第一步：修正PostDetails类代码 #

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
//ignore: must_be_immutable
class PostDetails extends StatefulWidget {
  // 加late修饰，告知运行时变量会在使用前完成初始化
  late DocumentSnapshot snapshot;
  // 删掉多余的位置参数，仅保留命名required参数传递单个snapshot
  PostDetails({required this.snapshot});

  @override
  _PostDetailsState createState() => _PostDetailsState();
}

class _PostDetailsState extends State<PostDetails> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("Post Details"),
        backgroundColor: Colors.green,
      ),
      body: Card(
        elevation: 10.0,
        margin: const EdgeInsets.all(10.0),
        child: ListView(
          children: <Widget>[
            Container(
                padding: const EdgeInsets.all(10.0),
                child: Row(
                  children: <Widget>[
                    CircleAvatar(
                      child: Text(widget.snapshot["title"][0]),
                      backgroundColor: Colors.green,
                      foregroundColor: Colors.white,
                    ),
                    const SizedBox(
                      width: 10.0,
                    ),
                    Text(
                      widget.snapshot["title"],
                      style: const TextStyle(fontSize: 22.0, color: Colors.green),
                    ),
                  ],
                )),
            const SizedBox(
              height: 7.0,
            ),
            Container(
              margin: const EdgeInsets.all(1.0),
              child: Text(
                widget.snapshot["content"],
                style: const TextStyle(fontSize: 18.0),
              ),
            )
          ],
        ),
      ),
    );
  }
}

第二步：修正Home页参数传递逻辑 #

class _HomeState extends State<Home> {
  late StreamSubscription<QuerySnapshot> subscription;

  late List<DocumentSnapshot> snapshot;
  // 新版cloud_firestore已废弃Firestore，替换为FirebaseFirestore
  CollectionReference collectionReference =
      FirebaseFirestore.instance.collection("Post");

  Color gradientStart = Colors.deepPurple[700]!;
  Color gradientEnd = Colors.purple[500]!;

  passData(DocumentSnapshot snap) {
    Navigator.of(this.context).push(MaterialPageRoute(
        // 传递当前点击的单个snap对象，补充命名参数名
        builder: (context) => PostDetails(
              snapshot: snap,
            )));
  }
}

内容的提问来源于stack exchange，提问作者zenx