Flutter - _CastError（在 null 值上使用空检查运算符）

Question

这是我的代码

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:provider/provider.dart';

class BrewList extends StatefulWidget {
  const BrewList({Key? key}) : super(key: key);

  @override
  State<BrewList> createState() => _BrewListState();
}

class _BrewListState extends State<BrewList> {
  @override
  Widget build(BuildContext context) {
    final brews = Provider.of<QuerySnapshot?>(context);
    // print(brews?.docs);
    
    for (var doc in brews!.docs) {
      print(doc.data);
    }

    return Container();
  }
}

我收到斧头异常说：

_CastError（用于 null 值的空检查运算符）

那我该怎么办？

Answer 1

我在代码中添加了注释。

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:provider/provider.dart';

class BrewList extends StatefulWidget {
  const BrewList({Key? key}) : super(key: key);

  @override
  State<BrewList> createState() => _BrewListState();
}

class _BrewListState extends State<BrewList> {
  @override
  Widget build(BuildContext context) {
    // NOTE: Before accessing Provider, make sure that it DOES EXIST on the widget tree. The provided code snippet does not have it.
    final brews = Provider.of<QuerySnapshot?>(context);
    // print(brews?.docs);
    
    // If the Provider does not exist, then brews would be NULL and you'll have the error when force dereferencing it.
    for (var doc in brews!.docs) {
      print(doc.data);
    }

    return Container();
  }
}

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