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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。
链接：https://www.learnfk.comhttps://www.learnfk.com/flutter/flutter-database-concepts.html

来源：LearnFk无涯教程网

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。

祝学习愉快！(内容编辑有误？请选中要编辑内容 -> 右键 -> 修改 -> 提交！)

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