Unhandled Exception: NoSuchMethodError: The method 'insert' was called on null. Flutter

Question

我修改了基本 sqlLite 示例以适应更动态的列表。但这可能不是最好的方法。无论如何，Database 代码如下所示：

import 'package:sqflite/sqflite.dart';
import 'package:sqflite/sqlite_api.dart';
import 'Entry.dart';
import 'package:path/path.dart';

class Database_ {

  Future<Database> database ;

  void openDBFunction() async {
    var databasesPath  = await getDatabasesPath();
    String path = join(databasesPath, 'Journal Entries.db');
    database = openDatabase(path, version: 1,
        onCreate: (Database db, int version) async {
          await db.execute(
              "CREATE TABLE entries( _entry TEXT, _date TEXT)"
          );
    });
  }

  Future<void> insertEntry(Entry entry) async {
    final Database db = await database;
    await db.insert(
      'entry',
      entry.toMap(),
      conflictAlgorithm: ConflictAlgorithm.replace,
    );
  }
  Future<List<Entry>> entries() async {
    final Database db = await database;
    final List<Map<String, dynamic>> maps = await db.query('entries');
    return List.generate(maps.length, (i) {
      return Entry(
        maps[i]["_entry"],
        maps[i]["_date"],
      );
    });
  }
}

main.dart 看起来像这样：

import 'package:flutter/material.dart';
import 'package:slide_journal/MainPage.dart';
import 'package:flutter/widgets.dart';
import 'package:slide_journal/Database_.dart';

main()async {
  WidgetsFlutterBinding.ensureInitialized();
  Database_().openDBFunction();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(),
      home: MainPage(),
    );
  }
}

然后这是输入页面：

import 'dart:ui';
import 'package:flutter/material.dart';
import 'package:slide_journal/Database_.dart';
import 'package:slide_journal/Entry.dart';
import 'List_of_Entries.dart';

// ignore: must_be_immutable
class MainPage extends StatelessWidget{
  TextEditingController _controller = TextEditingController();
  @override
  Widget build(BuildContext context) {

    return Scaffold(

      appBar: AppBar(
        shape: RoundedRectangleBorder(borderRadius: BorderRadius.vertical(bottom: Radius.circular(30))),
        title: Text("Home"),
        toolbarHeight: 50,
        automaticallyImplyLeading: false,
        actions:<Widget> [IconButton(
          icon: Icon(Icons.list_alt_sharp),
          onPressed: () =>{
            Navigator.push(context, (MaterialPageRoute(builder: (context)=>ListOfEntries())))
          },
        ),
        ]
      ),
      drawer: new Drawer(
        child: new ListView(
          children: <Widget>[
            new DrawerHeader(
              child: Text("Trisha is baby", style: new TextStyle(fontSize: 20),),
            )
          ],
        ),
      ),

      body: Center(
          child: SingleChildScrollView(
            child: Column(
              children: [
                Container(
                  height: 600,
                  width: 350,
                  child: TextField(
                    enabled: true,
                    controller: _controller,
                    enableSuggestions: true,
                    decoration: InputDecoration(
                      hintText: "Use text, emojis...",
                      suffixIcon: IconButton(
                        onPressed: () => {
                          addEntry(_controller.toString(), "DateTime.now().toString()"),
                        },
                        icon: Icon(Icons.send_outlined),
                      ),
                    ),
                    keyboardType: TextInputType.multiline,
                    style: TextStyle(fontSize: 30),
                    maxLength: 100,
                  ),
                )
              ],
            ),
          )
      ),
    );
  }
  void addEntry(String entry_, String date_)async {
    final entry = Entry(
        entry_,
        date_
    );
    Database_().insertEntry(entry);

  }
}

这是错误：

[ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: NoSuchMethodError: The method 'insert' was called on null.

Answer 1

database 未初始化。在将对象转换为它之前必须对其进行初始化。

Answer 2

数据库未在您的应用中初始化。如下操作

class DatabaseHelper {
  static String tableName = 'entries';

  static late Database _database;

  static Future<Database> get database async {
    final databasePath = await getDatabasesPath();
    final status = await databaseExists(databasePath);
    if (!status) {
      _database = await openDatabase(join(databasePath, 'notes_database.db'),
          onCreate: (database, version) {
        return database.execute(
              "CREATE TABLE entries( _entry TEXT, _date TEXT)"
          );
      }, version: 1);
    }
    return _database;
  }


  static Future<bool> insertDb(Model yourModel) async {
    final db = await database;
    try {
      await db.insert(your insert cmd);
    } on Error {
      throw Error();
    }
    return true;
  }
}