关于flutter中的SQFlite数据库

Question

import 'package:sqflite/sqflite.dart';
import 'dart:async';
import 'dart:io';
import 'package:path_provider/path_provider.dart';
import 'package:sqflite_project/instructors/instructors.dart';

class DatabaseHelper {
  static DatabaseHelper _databaseHelper;
  static Database _database;

  String istrcuctTable = 'instruct_table';
  String colID = 'id';
  String colName = 'name';
  String colPass = 'password';

  DatabaseHelper._createInstance(); // Named constructor to create instance of DatabaseHelper

  factory DatabaseHelper() {
    if (_databaseHelper == null) {
      _databaseHelper = DatabaseHelper
          ._createInstance(); // This is executed only once, singleton object
    }
    return _databaseHelper;
  }

  Future<Database> get database async {
    if (_database == null) {
      _database = await initializeDatabase();
    }
    return _database;
  }

  Future<Database> initializeDatabase() async {
    // Get the directory path for both Android and iOS to store database.
    Directory directory = await getApplicationDocumentsDirectory();
    String path = directory.path + 'notes.db';

    // Open/create the database at a given path
    var notesDatabase =
        await openDatabase(path, version: 1, onCreate: _createDb);
    return notesDatabase;
  }

  void _createDb(Database db, int newVersion) async {
    await db.execute(
        'CREATE TABLE $istrcuctTable($colID INTEGER PRIMARY KEY AUTOINCREMENT, $colName TEXT, '
        '$colPass TEXT)');
  }

  // Insert Operation: Insert a Note object to database
  Future<int> insertNote(Instructor instructor) async {
    Database db = await this.database;
    var result = await db.insert(istrcuctTable, instructor.toMap());
    return result;
  }
}


Main Class

import 'package:flutter/material.dart';
import 'package:sqflite_project/instructors/instructors.dart';
import 'package:sqflite_project/utils/database_helper.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController nameController = TextEditingController();
  TextEditingController passController = TextEditingController();
  int result;
  Instructor note;
  DatabaseHelper _databaseHelper = DatabaseHelper();
  String name;
  String password;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Form(
      child: Column(children: <Widget>[
        TextFormField(
          controller: nameController,
        ),
        TextFormField(
          controller: passController,
        ),
        RaisedButton(
            child: Text('Login'),
            onPressed: () async {
              _databaseHelper.insertNote(note);
            })
      ]),
    ));
  }
}

[错误:flutter/lib/ui/ui_dart_state.cc(157)]未处理的异常：nosuchmethoderror：在null上调用了方法“to map”。E/Flutter（2893）：接收器：null E/Flutter（2893）：尝试调用：toMap（）

我已经创建了数据库和insert操作，但是当我调用insert操作时，它给出了一个错误，即InsertNote方法是在NULL上调用的。

Answer 1

该错误表示调用instructor.tomap()
时instructor为空，因为您没有用任何值初始化note，所以现在为null

Instructor note;

当调用下面的代码时，实际上是将null传递给insertnote

onPressed: () async {
          _databaseHelper.insertNote(note);
        })

你可以做

Instructor note = Instructor();