无涯教程-Flutter - 数据库

SQLite" class="css-1occaib">SQLite数据库是基于事实和标准SQL的嵌入式数据库引擎，它是小型且经过时间考验的数据库引擎，sqflite软件包提供了许多函数，可以有效地与SQLite数据库一起使用，它提供了操作SQLite数据库引擎的标准方法。

• 在Android Studio中创建一个新的Flutter应用程序product_sqlite_app。

• 用无涯教程的 product_rest_app 代码替换默认的启动代码(main.dart)。

• 将assets文件夹从 product_nav_app 复制到 product_rest_app 并在* pubspec.yaml`文件内添加assets。

flutter: assets: - assets/appimages/floppy.png - assets/appimages/iphone.png - assets/appimages/laptop.png - assets/appimages/pendrive.png - assets/appimages/pixel.png - assets/appimages/tablet.png

• 在pubspec.yaml文件中配置sqflite软件包，如下所示-

dependencies: sqflite: any

• 在pubspec.yaml文件中配置path_provider软件包，如下所示-

dependencies: path_provider: any

• 此处，path_provider软件包用于获取系统的临时文件夹路径和应用程序的路径，使用 sqflite 的最新版本号代替任何。

• Android Studio会提醒pubspec.yaml已更新。

• 单击"Get dependencies"选项。 Android studio将从互联网上获取该软件包，并为应用程序正确配置它。

• 在数据库中，无涯教程需要主键，id作为附加字段以及产品属性(如名称，价格等)，因此，请在Product类中添加id属性。另外，添加新方法toMap将产品对象转换为Map对象。 fromMap和toMap用于对Product对象进行序列化和反序列化，并用于数据库操作方法中。

class Product { final int id; final String name; final String description; final int price; final String image; static final columns = ["id", "name", "description", "price", "image"]; Product(this.id, this.name, this.description, this.price, this.image); factory Product.fromMap(Map<String, dynamic> data) {return Product( data[id], data[name], data[description], data[price], data[image], ); } Map<String, dynamic> toMap() => {"id": id, "name": name, "description": description, "price": price, "image": image }; 
}

• 在lib文件夹中创建一个新文件Database.dart，以编写SQLite的相关函数。

• 在Database.dart中导入必要的import语句。

import dart:async; 
import dart:io; 
import package:path/path.dart; 
import package:path_provider/path_provider.dart; 
import package:sqflite/sqflite.dart; 
import Product.dart;

• 请注意以下几点-

  • async                    -  用于编写异步方法。

  • io                           -  用于访问文件和目录。

  • path                      -  用于访问与文件路径相关的dart核心实用程序函数。

  • path_provider    -  用于获取临时路径和应用程序路径。

  • sqflite                   -  用于操作SQLite的数据库。

• 创建一个新的类SQLite的DbProvider

•     - 声明一个基于单例的静态SQLite的DbProvider对象，如下所示:

class SQLiteDbProvider { SQLiteDbProvider._(); static final SQLiteDbProvider db=SQLiteDbProvider._(); static Database _database; 
} 

•     - 可以通过静态db变量访问SQLite的DBProvoider对象及其方法。

SQLiteDBProvoider.db.<emthod> 

•     - 创建一个方法来获取类型为Future <Database>的数据库，创建产品表并在数据库本身创建期间加载初始数据。

Future<Database> get database async { if (_database != null) return _database; _database = await initDB(); return _database; 
}
initDB() async { Directory documentsDirectory = await getApplicationDocumentsDirectory(); String path = join(documentsDirectory.path, "ProductDB.db"); return await openDatabase(path, version: 1,onOpen: (db) {}, onCreate: (Database db, int version) async {await db.execute("CREATE TABLE Product (""id INTEGER PRIMARY KEY,""name TEXT,""description TEXT,""price INTEGER," "image TEXT" ")"); await db.execute("INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [1, "iPhone", "iPhone is the stylist phone ever", 1000, "iphone.png"]); await db.execute("INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"]); await db.execute("INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"]\); await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"]);await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [5, "Pendrive", "Pendrive is useful storage medium", 100, "pendrive.png"]);await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [6, "Floppy Drive", "Floppy drive is useful rescue storage medium", 20, "floppy.png"]); }); 
}

• 在这里，无涯教程使用了以下方法-

  •     -  getApplicationDocumentsDirectory -  返回应用程序目录路径

  •     -  join                        - 用于创建系统特定的路径，无涯教程已经使用它来创建数据库路径。

  •     -  openDatabase     - 用于打开SQLite的数据库。

  •     -  onOpen                - 用于在打开数据库时编写代码

  •     -  onCreate              - 用于在首次创建数据库时编写代码

  •     -  db.execute           - 用于执行SQL查询。它接受一个查询。如果查询具有占位符(?)，则它将接受值作为第二个参数中的列表。

• 编写getAllProducts来获取数据库中的所有产品-

Future<List<Product>> getAllProducts() async { final db = await database; List<Map> results = await db.query("Product", columns: Product.columns, orderBy: "id ASC"); List<Product> products = new List(); results.forEach((result) { Product product = Product.fromMap(result); products.add(product); }); return products; 
}

• 编写getProductById来获取特定于 id的产品

Future<Product> getProductById(int id) async {final db = await database; var result = await db.query("Product", where: "id=", whereArgs: [id]); return result.isNotEmpty ? Product.fromMap(result.first) : Null; 
}

• 在这里，无涯教程使用了where和whereArgs来应用过滤器。

• 创建三种方法-插入，更新和删除方法，以从数据库中插入，更新和删除产品。

insert(Product product) async { final db = await database; var maxIdResult = await db.rawQuery("SELECT MAX(id)+1 as last_inserted_id FROM Product");var id = maxIdResult.first["last_inserted_id"]; var result = await db.rawInsert("INSERT Into Product (id, name, description, price, image)" " VALUES (?, ?, ?, ?, ?)", [id, product.name, product.description, product.price, product.image] ); return result; 
}
update(Product product) async { final db = await database; var result = await db.update("Product", product.toMap(), where: "id=?", whereArgs: [product.id]); return result; 
} 
delete(int id) async { final db = await database; db.delete("Product", where: "id=?", whereArgs: [id]); 
}

• Database.dart的最终代码如下-

import dart:async; 
import dart:io; 
import package:path/path.dart; 
import package:path_provider/path_provider.dart; 
import package:sqflite/sqflite.dart; 
import Product.dart; class SQLiteDbProvider {SQLiteDbProvider._(); static final SQLiteDbProvider db = SQLiteDbProvider._(); static Database _database; Future<Database> get database async {if (_database != null) return _database; _database = await initDB(); return _database; } initDB() async {Directory documentsDirectory = await getApplicationDocumentsDirectory(); String path = join(documentsDirectory.path, "ProductDB.db"); return await openDatabase(path, version: 1, onOpen: (db) {}, onCreate: (Database db, int version) async {await db.execute("CREATE TABLE Product (" "id INTEGER PRIMARY KEY," "name TEXT," "description TEXT," "price INTEGER," "image TEXT"")"); await db.execute("INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [1, "iPhone", "iPhone is the stylist phone ever", 1000, "iphone.png"]); await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"]);await db.execute("INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"]); await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"]); await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [5, "Pendrive", "Pendrive is useful storage medium", 100, "pendrive.png"]);await db.execute( "INSERT INTO Product (id, name, description, price, image) values (?, ?, ?, ?, ?)", [6, "Floppy Drive", "Floppy drive is useful rescue storage medium", 20, "floppy.png"]); }); }Future<List<Product>> getAllProducts() async {final db = await database; List<Map> results = await db.query("Product", columns: Product.columns, orderBy: "id ASC"); List<Product> products = new List();   results.forEach((result) {Product product = Product.fromMap(result); products.add(product); }); return products; } Future<Product> getProductById(int id) async {final db = await database; var result = await db.query("Product", where: "id=", whereArgs: [id]); return result.isNotEmpty ? Product.fromMap(result.first) : Null; } insert(Product product) async { final db = await database; var maxIdResult = await db.rawQuery("SELECT MAX(id)+1 as last_inserted_id FROM Product"); var id = maxIdResult.first["last_inserted_id"]; var result = await db.rawInsert("INSERT Into Product (id, name, description, price, image)" " VALUES (?, ?, ?, ?, ?)", [id, product.name, product.description, product.price, product.image] ); return result; } update(Product product) async { final db = await database; var result = await db.update("Product", product.toMap(), where: "id=?", whereArgs: [product.id]); return result; } delete(int id) async { final db = await database; db.delete("Product", where: "id=?", whereArgs: [id]);} 
}

• 更改主要方法以获取产品信息。

void main() {runApp(MyApp(products: SQLite的DbProvider.db.getAllProducts())); 
}

• 在这里，无涯教程使用了getAllProducts方法来从数据库中获取所有产品。

• 运行该应用程序并查看结果。它与先前的示例访问产品服务API相似，不同之处在于，产品信息是从本地SQLite的数据库存储和获取的。

Flutter - 数据库 - 无涯教程网无涯教程网提供SQLite" class="css-1occaib">SQLite数据库是基于事实和标准SQL的嵌入式数据库引擎...https://www.learnfk.com/flutter/flutter-database-concepts.html