NoSuchMethodError：吸气剂数据在null上调用，接收者：null

Question

我正在尝试创建一个详细信息屏幕，以显示数据库中的Firebase数据，并显示图像以及一些文本数据，包括项目数，日期和地理位置。有没有办法确定为什么数据返回空值？导航器应导航到DetailPage。

这是我的主文件-

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:wasteagram/pages/create_waste.dart';
import 'package:wasteagram/services/crud.dart';
import 'pages/create_waste.dart';
import 'pages/detail_screen.dart'; 


void main() => runApp(new MyApp()); 

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Wasteagram - ', 
      theme: new ThemeData(
        primarySwatch: Colors.deepOrange, 
      ), 
      home: new MyHomePage(title: 'Wasteagram - '), 
    ); 
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key); 

  final String title; 
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  CrudMethods crudMethods = new CrudMethods(); 

  Stream wasteStream; 

  Widget WasteList() {
    return Container(
      child: wasteStream != null ? Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
        StreamBuilder(
          stream: wasteStream, 
          builder: (context, snapshot) {
            return ListView.builder(
              padding: EdgeInsets.symmetric(horizontal: 16), 
              itemCount: snapshot.data.documents.length,
              shrinkWrap: true,
              itemBuilder: (context, index){
                return WidgetTile(
                  wastedate: 
                    snapshot.data.documents[index].data['wastedate'],
                  wastenumber: 
                    snapshot.data.documents[index].data['wastenumber']
              ); 
            }); 
           },)
          ], 
        ) : Container(
            alignment: Alignment.center,
            child: CircularProgressIndicator(),
        ), 
    ); 
  }

  void initState() {
    super.initState(); 
    crudMethods.getData().then((result) {
      wasteStream = result; 
    }); 

  }


  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title), 
      ), 
      body: WasteList(), 
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
         Navigator.push(
           context, 
           MaterialPageRoute(builder: (context) => CameraScreen())
         ); 
        },  
        child: new Icon(Icons.add), 
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
    ); 
  }
}

class WidgetTile extends StatelessWidget {
  String wastedate, wastenumber;
  WidgetTile({@required this.wastedate, @required this.wastenumber}); 

  @override
  Widget build(BuildContext context) {
    return ListTile(
        title: Text(wastedate), 
        trailing: Text(wastenumber),
        onTap: () {
          Navigator.push(
            context, 
            MaterialPageRoute(builder: (context) => DetailPage())
          ); 
        }
    ); 
  }
}

这是我的detail_screen.dart

import 'package:flutter/material.dart'; 
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:wasteagram/data/firestore_service.dart';



class DetailPage extends StatefulWidget {

  final DocumentSnapshot post; 

  DetailPage({this.post}); 

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

class _DetailPageState extends State<DetailPage> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.post.data["wastedate"])
      ), 
      body: Center(
        child: Container( 
          child: Column(
            children: <Widget> [
              Image.network(widget.post.data["image"]),
              Text(widget.post.data["wastedate"]), 
              Text(widget.post.data["wastenumber"]), 
              Text(widget.post.data["wastelocation"].toString()),
            ]
          )
        )
      ),
    );
  }
}

Answer 1

您没有将DocumentSnapshot传递到详细信息页面。试试这个：

Widget WasteList() {
    return Container(
      child: wasteStream != null ? Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
        StreamBuilder(
          stream: wasteStream, 
          builder: (context, snapshot) {
            return ListView.builder(
              padding: EdgeInsets.symmetric(horizontal: 16), 
              itemCount: snapshot.data.documents.length,
              shrinkWrap: true,
              itemBuilder: (context, index){
                return WidgetTile(
                  wastedate: 
                    snapshot.data.documents[index].data['wastedate'],
                  wastenumber: 
                    snapshot.data.documents[index].data['wastenumber'],
                   post:
                    snapshot.data.documents[index]
              ); 
            }); 
           },)
          ], 
        ) : Container(
            alignment: Alignment.center,
            child: CircularProgressIndicator(),
        ), 
    ); 
  }


class WidgetTile extends StatelessWidget {
  String wastedate, wastenumber;
  DocumentSnapshot post;
  WidgetTile({@required this.wastedate, @required this.wastenumber,@required this.post}); 

  @override
  Widget build(BuildContext context) {
    return ListTile(
        title: Text(wastedate), 
        trailing: Text(wastenumber),
        onTap: () {
          Navigator.push(
            context, 
            MaterialPageRoute(builder: (context) => DetailPage(post: post))
          ); 
        }
    ); 
  }
}