我想在我的应用程序中使用 sqflite。为此,我尝试遵循本教程:https://flutter.dev/docs/cookbook/persistence/sqlite 。但是,我不知道将代码放在应用程序中的何处。在本教程中,代码似乎放置在 main() 函数中 - 但是,如果我这样做,如何调用其他文件中的插入、更新和删除方法?
更新:
按照@Madhavam Shahi 的建议,我创建了一个文件databaseServices.dart
。现在,在另一个文件中,我导入 databaseServices.dart
并尝试按如下方式使用它:
import 'databaseServices.dart';
DataBaseServices db=DataBaseServices();
db.delete() //example
但是,它不起作用。我认为 databaseServices.dart
的结构不正确,但我无法发现错误。我知道我一定犯了一个非常新手的错误。以下是databaseServices.dart
的代码:
import 'dart:async';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import 'counter.dart';
class DatabaseServices {
void whatever() async {
// Open the database and store the reference.
final Future<Database> database = openDatabase(
// Set the path to the database.
join(await getDatabasesPath(), 'counter_database.db'),
// When the database is first created, create a table to store counters;
onCreate: (db, version) {
// Run the CREATE TABLE statement on the database.
return db.execute(
"CREATE TABLE counters(id INTEGER PRIMARY KEY, name TEXT, value INTEGER)",
);
},
// Set the version. This executes the onCreate function and provides a
// path to perform database upgrades and downgrades.
version: 1,
);
// Define a function that inserts counters into the database.
Future<void> insertCounter(Counter counter) async {
// Get a reference to the database.
final Database db = await database;
// Insert the Counter into the correct table. Here, if a counter is inserted twice,
// it replace any previous data.
await db.insert(
'counters',
counter.toMap(),
conflictAlgorithm: ConflictAlgorithm.replace,
);
}
// A method that retrieves all the counters from the counters table.
Future<List<Counter>> counters() async {
// Get a reference to the database.
final Database db = await database;
// Query the table for all the Counters.
final List<Map<String, dynamic>> maps = await db.query('counters');
// Counvert the List<Map<String, dynamic>> into a List<Counter>
return List.generate(maps.length, (i) {
return Counter(
id: maps[i]['id'],
name: maps[i]['name'],
value: maps[i]['value'],
);
});
}
// Method to update a Counter in the database
Future<void> updateCounter(Counter counter) async {
final db = await database;
await db.update(
'counters',
counter.toMap(),
where: "id = ?",
whereArgs: [counter.id],
);
}
//Delete a Counter from the database
Future<void> deleteCounter(int id) async {
final db = await database;
await db.delete(
'counters',
where: "id = ?",
whereArgs: [id],
);
}
}
}
最佳答案
不,在哪里创建数据库并不重要。
您可以创建一个文件databaseServices.dart
它管理数据库服务。这样您就可以轻松管理代码了。
在食谱中,他们只是展示了一个示例,说明如何使用 sqlflite
.
但是,您应该放置此行 WidgetsFlutterBinding.ensureInitialized();
在你的main()
方法之前的任何事情,如果您正在 main()
中执行异步任务方法。
更新:
要在其他文件中执行 CRUD,
- 导入您的
databaseServices.dart
归档您要在其中执行 CRUD 的文件。
import 'databaseServices.dart';
DataBaseServices db=DataBaseServices();// create an object (DataBaseServices is the name of the class)
//Now, you can access all the methods,
db.delete()//example
或者,如果您不想在 databaseServices.dart
中创建类文件,并且希望将每个函数保留为顶级函数,那么您可以执行以下操作。
import 'databaseServices.dart' as db;
//Now, you can access all the top level functions or variables.
db.delete()//example.
更新2:-
为了使每个函数都可以访问数据库,
- 将Future 数据库移到whatever() 方法之外,并将其放置在类名的正下方。 (使其成为全局的,以便每个函数都可以访问它),请注意,我删除了“final”关键字,因为我们稍后将在任何方法中初始化它。现在,在任何方法中做你想做的事情,但不是最终的Future数据库=//你的代码,而是这样做,数据库=//你的代码..通过这样做,你将初始化数据库变量,并将其作为数据库变量是一个全局变量(在任何函数外部声明,在类内部声明),任何函数都可以访问它。但你必须记住,在调用任何其他需要数据库的方法之前必须初始化数据库,因为如果你不在任何其他函数之前调用whatever()方法,那么数据库将不会被初始化,因此您的其他功能将无法运行。
示例,
import 'dart:async';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import 'counter.dart';
class DatabaseServices {
Future<Database> database;//making database global so that every function inside the class can access it.
void whatever() async {
// Open the database and store the reference.
database = openDatabase(
// Set the path to the database.
join(await getDatabasesPath(), 'counter_database.db'),
// When the database is first created, create a table to store counters;
onCreate: (db, version) {
// Run the CREATE TABLE statement on the database.
return db.execute(
"CREATE TABLE counters(id INTEGER PRIMARY KEY, name TEXT, value INTEGER)",
);
},
// Set the version. This executes the onCreate function and provides a
// path to perform database upgrades and downgrades.
version: 1,
);
}//Function whatever () ends here
// Define a function that inserts counters into the database.
Future<void> insertCounter(Counter counter) async {
// Get a reference to the database.
final Database db = await database;
// Insert the Counter into the correct table. Here, if a counter is inserted twice,
// it replace any previous data.
await db.insert(
'counters',
counter.toMap(),
conflictAlgorithm: ConflictAlgorithm.replace,
);
}
// A method that retrieves all the counters from the counters table.
Future<List<Counter>> counters() async {
// Get a reference to the database.
final Database db = await database;
// Query the table for all the Counters.
final List<Map<String, dynamic>> maps = await db.query('counters');
// Counvert the List<Map<String, dynamic>> into a List<Counter>
return List.generate(maps.length, (i) {
return Counter(
id: maps[i]['id'],
name: maps[i]['name'],
value: maps[i]['value'],
);
});
}
// Method to update a Counter in the database
Future<void> updateCounter(Counter counter) async {
final db = await database;
await db.update(
'counters',
counter.toMap(),
where: "id = ?",
whereArgs: [counter.id],
);
}
//Delete a Counter from the database
Future<void> deleteCounter(int id) async {
final db = await database;
await db.delete(
'counters',
where: "id = ?",
whereArgs: [id],
);
}
}
现在,由于没有嵌套函数,您可以轻松创建该类的对象,并根据需要轻松调用函数:)
关于sqlite - Flutter:在我的应用程序中哪里放置 sqflite 代码?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/63654317/