[英]About SQFlite Database in flutter
数据库类
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
[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“toMap”。 E/flutter (2893): Receiver: null E/flutter (2893): 尝试调用: toMap()
我已经创建了数据库和插入操作,但是当我调用插入操作时,它给出了一个错误,即在 null 上调用了 InsertNote 方法。
调用instructor.toMap()
时,错误意味着instructor
为空
因为你没有用任何值初始化note
所以它现在是null
Instructor note;
当调用下面的代码时,你实际上传递了一个null
给insertNote
onPressed: () async {
_databaseHelper.insertNote(note);
})
你可以做
Instructor note = Instructor();
您尚未将值设置为 note(Instructor),因此该注释为 null,因此每当您在 null 上调用任何函数时,都会出现此错误 (note(null).toMap(); 因此您必须在执行操作之前将数据设置为
note=Instructor();//set propery as you defined in the Instructor class
而且你还必须等待 onPressed 作为
onPressed: () async {
await _databaseHelper.insertNote(note);
})
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