Flutter：为什么我无法从 sqflite 中查看列表？

Question

我是颤振的初学者。我创建了一个方法，从“todos”表中选择特定列，因此我希望在列表类型“todos”中接收该查询。

但是当我调用它时，这不会出现在屏幕上。我做错了什么？

证据：

这是我的数据库助手：

import 'dart:developer';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';
import 'lista_model.dart';

class DatabaseHandler {
  Future<Database> initializeDB() async {
    String path = await getDatabasesPath();
    return openDatabase(
      join(path, 'courses.db'), 
      onCreate: (database, version) async {
        final String sql = ''
            'CREATE TABLE todos ('
            ' id INTEGER PRIMARY KEY AUTOINCREMENT,'
            ' title TEXT,'
            ' entidad TEXT,'
            ' categoria TEXT,'
            ' emision TEXT,'
            ' imgcourse TEXT,'
            ' urlcourse TEXT,'
            ' description TEXT'
            ')';
        await database.execute(sql);

        final String addCourse = ''
            'INSERT INTO todos(title, entidad, categoria, emision, imgcourse, urlcourse, description) VALUES ('
          '"Control" , "Slim", "TIC", "Certificable", "https://dinahosting.com", "https://aprende.org/", "+Entu formación académica y profesional."),'
          '("Transparencia" , "Defensoría", "Ciencias", "Certificable", "https://www.notinetlegal.com", "https://campusvirtual", "+El accesopara tu formación académica y profesional.")'
            ;
        await database.execute(addCourse);
      },
      version: 1,
    );
  }

//this is the method where i make the column select query and it works fine
  Future<List<Map>> queryCat() async {

    Database db = await initializeDB();

    db.rawQuery('SELECT categorias FROM todos WHERE categoria="TIC"');
    List<Map> result =
        await db.rawQuery('SELECT * FROM todos WHERE categoria=?', ['TIC']);

    // print the results
    result.forEach((row) => print(row));
    return result;
  }

//method to make all query
 Future<List<todo>> todos() async {
    final db = await initializeDB();
    final List<Map<String, dynamic>> queryResult = await db.query('todos');
    return queryResult.map((e) => todo.fromMap(e)).toList();
  }


//this method was antoher way to make the query to my column, i dont kwnow if it works
  Future<List<todo>> cattic() async {
    final db = await initializeDB();
    List<String> columnCategoria = ['categoria'];

    final List<Map<String, dynamic>> queryResult = await db.query('todos',
        columns: columnCategoria, //selecciona columna, en este caso = categoria
        where: 'categoria = ?', //donde categoria sea "TIC");
        whereArgs: ['TIC']);

    return queryResult.map((e) => todo.fromMap(e)).toList();
  }

}

这是我的班级屏幕，我应该在其中查看列查询中的列表，但没有显示任何内容：

import 'package:cursin/model/user_model.dart';
import 'package:cursin/screens/course_detail.dart';
import 'package:cursin/screens/dbhelper.dart';
import 'package:cursin/screens/lista_model.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:google_mobile_ads/google_mobile_ads.dart';
import 'package:share_plus/share_plus.dart';
import 'package:shared_preferences/shared_preferences.dart';

class categorias extends StatefulWidget {
  categorias({required this.cat});
  late String cat;

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

class _categoriasState extends State<categorias> {
  late DatabaseHandler handler;
  Future<List<todo>>? _todo;

  @override
  void initState() {
    switch (widget.cat) {
      case "TIC":
        {
          handler = DatabaseHandler();
          handler.initializeDB().whenComplete(() async {
            setState(() async {
              _todo = (await handler.queryCat()) as Future<List<todo>>;
            });
          });
        }
        break;
    }
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey[850],
      appBar: AppBar(
        title: Text(
          "Welcome "
        ),
      ),
      body: FutureBuilder<List<todo>>(
        future: _todo,
        builder: (BuildContext context, AsyncSnapshot<List<todo>> snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const Center(
              child: const CircularProgressIndicator(),
            );
          } else if (snapshot.hasError) {
            return Text('Error: ${snapshot.error}');
          } else {
            var items = snapshot.data ?? <todo>[];

            return Container(
              margin: EdgeInsets.symmetric(
                horizontal: 5.0,
                vertical: 5.0,
              ),
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(12),
              ),
              child: ListView.builder(
                itemCount: items.length,
                itemBuilder: (BuildContext context, int index) {
                  return GestureDetector(
                    onTap: () {
                      Navigator.push(
                        context,
                        MaterialPageRoute(
                          builder: (context) => CourseDetail(td: items[index]),
                        ),
                      );
                    },
                    child: Column(
                      children: [

                        Padding(

                          child: Container(
                              child: ListTile(
                                leading: SizedBox(
                                  height: 100.0,
                                  width: 80.0,
                                  child: Image.network(items[index].imgcourse),
                                ),

                                shape: RoundedRectangleBorder(
                                  borderRadius: BorderRadius.circular(15),
                                ),

                                title: Text(
                                  items[index].title,
                                  style: TextStyle(
                                    fontSize: 23,
                                    color: Colors.blueAccent,
                                  ),
                                ),
                              )),
                        )
                      ],
                    ),
                  );
                },
              ),
            );
          }
        },
      ),
    );
  }
}