Flutter - Persist data with SQLite - 用 SQLite 做数据持久化

官方文档： Persist data with SQLite - Flutter

If you write an app that needs to persist and query larger amounts of data on
the local device, consider using a database instead of a local file or key-value
store. In general, databases provide faster inserts, updates, and queries
compared to other local persistence solutions.

如果你的应用程序需要再手机本地做大量的数据持久化或数据搜索,可以考虑使用数据库来替代本地文件存储或键-值对存储.
通常来说,和其他大多数的本地数据持久化方案相比数据库在数据的增删查改方面表现的更加高效.

Flutter apps can make use of the SQLite databases via the
sqflite plugin available on pub.
This recipe demonstrates the basics of using sqflite to insert, read, update,
and remove data about various Dogs!

Flutter应用程序中可以通过sqflite插件使用SQLite数据库。

本文将通过使用 sqflite 来演示插入，读取，更新，删除各种狗狗的数据。

If you are new to SQLite and SQL statements, please review the SQLite Tutorial
site to learn the basics before completing
this recipe.

如果你对于SQLite和SQL的各种语句还不熟悉，请查看SQLite官方的教程SQLite Tutorial
site，在查看本文之前掌握基本的SQL语句。

Directions

步骤

1. Add the dependencies

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   添加依赖

2. Define the Dog data model

   定义 Dog（狗） 数据模型。

3. Open the Database

   打开数据库。

4. Create the dogs table

   创建 dogs 数据表。

5. Insert a Dog into the database

   将一条 Dog 数据插入数据库。

6. Retrieve the list of Dogs

   查询所有狗狗的数据。

7. Update a Dog in the database

   更新（修改）一条 Dog 的数据.

8. Delete a Dog from the database

   删除一条 Dog 的数据。

1. Add the dependencies

1. 添加依赖

To work with SQLite databases, import the sqflite and path packages.

为了能使SQLite数据库工作，首先导入 sqflite 和 path 包。

• The sqflite package provides classes and functions that allow you to
  interact with a SQLite database.

  sqflite 提供了丰富的类和方法，以便你能便捷实用SQLite数据库。

• The path package provides functions that allow you to correctly define the
  location to store the database on disk.

  path 提供了大量方法，以便你能正确的定义数据库在磁盘上的存储位置。

dependencies:
  flutter:
    sdk: flutter
  sqflite:
  path:

2. Define the Dog data model

2. 定义狗狗的数据模型

Before you create the table to store information on Dogs, take a few moments to
define the data that needs to be stored. For this example, define a Dog class
that contains three pieces of data: A unique id, the name, and the age of
each dog.

在你准备在新建的表里存储狗狗们的信息的的时候，你需要先定义这些数据。
例如，定义一个狗类，每一条狗狗的数据将包含三个字段：一个唯一的 id ；名字 name ；年龄 age。

class Dog {
  final int id;
  final String name;
  final int age;

  Dog({this.id, this.name, this.age});
}

3. Open the Database

3. 打开数据库

Before you read and write data to the database, you need to open a connection
to the database. This involves two steps:

在你准备读写数据库的数据之前，你要先打开这个数据库。
打开一个数据库有以下两个步骤：

1. Define the path to the database file using the getDatabasesPath from the
   sqflite package combined with the path function from the path package.

使用 sqflite 包里的 getDatabasesPath 方法并配合 path 包里的 path 方法定义数据库的路径。

2. Open the database with the openDatabase function from sqflite

   使用 sqflite 包里的 openDatabase 方法打开数据库。

// Open the database and store the reference
final Future<Database> database = openDatabase(
  // Set the path to the database. Note: Using the `join` function from the
  // `path` package is best practice to ensure the path is correctly
  // constructed for each platform.
  join(await getDatabasesPath(), 'doggie_database.db'),
);

// Open the database and store the reference
final Future<Database> database = openDatabase(
  // Set the path to the database. Note: Using the `join` function from the
  // `path` package is best practice to ensure the path is correctly
  // constructed for each platform.
  join(await getDatabasesPath(), 'doggie_database.db'),
);

4. Create the dogs table

4. 创建 dogs 表

Next, you need to create a table to store information about various Dogs. For
this example, create a table called dogs that defines the data that can be
stored. In this case, each Dog contains an id, name, and age. Therefore,
these will be represented as three columns in the dogs table.

接下来，你需要创建一个表用以存储各种狗狗的信息。在这个示例中，创建一个名为 dogs 数据库表，它定义了可以被存储的数据。
这样，每条 Dog 数据就包含了一个 id， name 和 age。
因此，在 dogs 数据库表中将有三列，分别是 id， name 和 age。

1. The id is a Dart int, and will be stored as an INTEGER SQLite
   Datatype. It is also good practice to use an id as the primary key for the
   table to improve query and update times.

   id 是Dart的 int 类型，在数据表中是SQLite的 INTEGER 数据类型。最佳实践是将 id 作为数据库表的主键，用以改善查询和修改的时间。

2. The name is a Dart String, and will be stored as a TEXT SQLite
   Datatype

   name 是Dart的 String类型，在数据表中是SQLite的 TEXT 数据类型。

3. The age is also a Dart int, and will be stored as an INTEGER
   Datatype

   age 也是Dart的 int 类型，在数据表中是SQLite的 INTEGER 数据类型。

For more information about the available Datatypes that can be stored in a
SQLite database, please see the official SQLite Datatypes
documentation.

关于SQLite数据库能够存储的更多的数据类型信息请查阅the official SQLite Datatypes
documentation。

final Future<Database> database = openDatabase(
  // Set the path to the database. 
  join(await getDatabasesPath(), 'doggie_database.db'),
  // When the database is first created, create a table to store dogs
  onCreate: (db, version) {
    // Run the CREATE TABLE statement on the database
    return db.execute(
      "CREATE TABLE dogs(id INTEGER PRIMARY KEY, name TEXT, age INTEGER)",
    );
  },
  // Set the version. This executes the onCreate function and provides a
  // path to perform database upgrades and downgrades.
  version: 1,
);

5. 插入一条狗狗的数据

5. Insert a Dog into the database

Now that you have a database with a table suitable for storing information
about various dogs, it’s time to read and write data!

现在你已经准备好了一个数据库用于存储各种狗狗的信息数据，是时候开始读写数据了！

First, insert a Dog into the dogs table. This involves two steps:

首先，在 dogs 数据表中插入一条 Dog 数据。分以下两步：

1. Convert the Dog into a Map

   把 Dog 转换成一个 Map 数据类型。

2. Use the
   insert
   method to store the Map in the dogs table

   使用insert方法把 Map 保存到 dogs 数据表中。

// First, update the Dog class to include a `toMap` method.
class Dog {
  final int id;
  final String name;
  final int age;

  Dog({this.id, this.name, this.age});

  // Convert the dog into a Map. The keys must correspond to the names of the 
  // columns in the database.
  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'age': age,
    };
  }
}

// Next, define a function that inserts dogs into the database
Future<void> insertDog(Dog dog) async {
  // Get a reference to the database
  final Database db = await database;

  // Insert the Dog into the correct table. You may also specify the 
  // `conflictAlgorithm` to use in case the same dog is inserted twice. 
  // 
  // In this case, replace any previous data.
  await db.insert(
    'dogs',
    dog.toMap(),
    conflictAlgorithm: ConflictAlgorithm.replace,
  );
}

// Now, you can create a Dog to and add it to the dogs table!
final fido = Dog(
  id: 0, 
  name: 'Fido', 
  age: 35,
);

await insertDog(fido);

6. Retrieve the list of Dogs

6. 查询狗狗列表

Now that you have a Dog stored in the database, you can query the database
for a specific dog or a list of all dogs! This involves two steps:

现在你已经有了一条 Dog 存储在数据库里。
你可以通过查询数据库，检索到一只狗狗的数据或者所有狗狗的数据。分为以下两步:

1. Run a query against the dogs table. This will return a List<Map>

   调用dogs表对像的query方法。这将返回一个List <Map>。

2. Convert the List<Map> into a List<Dog>

   将 List<Map> 转换成 List<Dog> 数据类型。

// A method that will retrieve all the dogs from the dogs table
Future<List<Dog>> dogs() async {
  // Get a reference to the database
  final Database db = await database;

  // Query the table for All The Dogs.
  final List<Map<String, dynamic>> maps = await db.query('dogs');

  // Convert the List<Map<String, dynamic> into a List<Dog>.
  return List.generate(maps.length, (i) {
    return Dog(
      id: maps[i]['id'],
      name: maps[i]['name'],
      age: maps[i]['age'],
    );
  });
}

// Now, you can use the method above to retrieve all the dogs!
print(await dogs()); // Prints a list that include Fido

7. Update a Dog in the database

7. 修改一条 Dog 数据

After you’ve inserted some information into the database, you may want to update
that information at a later time. To do so, use the
update
method from the sqflite library.

使用 sqflite 库中的update方法，你可以对已经插入到数据库中的数据进行修改（更新）。

This involves two steps:

修改数据操作包含以下两步：

1. Convert the Dog into a Map

   将一条狗狗的数据转换成 Map 数据类型。

2. Use a where clause to ensure you update the correct Dog

   使用 where 语句定位到具体将要被修改的数据。

Future<void> updateDog(Dog dog) async {
  // Get a reference to the database
  final db = await database;

  // Update the given Dog
  await db.update(
    'dogs',
    dog.toMap(),
    // Ensure we only update the Dog with a matching id
    where: "id = ?",
    // Pass the Dog's id through as a whereArg to prevent SQL injection
    whereArgs: [dog.id],
  );
}

// Now, you can update Fido's age!
await updateDog(Dog(
  id: 0, 
  name: 'Fido', 
  age: 42,
));

// And you could print the updated results
print(await dogs()); // Prints Fido with age 42.

{{site.alert.warning}}
Always use whereArgs to pass arguments to a where statement. This
helps safeguard against SQL injection attacks.

使用 whereArgs 将参数传递给 where 语句。有助于防止 SQL 注入攻击。

Do not use string interpolation, such as where: "id = ${dog.id}"!

这里不要使用字符串模板，比如： where: "id = ${dog.id}"！

{{site.alert.end}}

8. Delete a Dog from the database

In addition to inserting and updating information about Dogs, you can also
remove dogs from the database. To delete data, use the
delete
method from the sqflite library.

除了插入和修改狗狗们的数据，你还可以从数据库中删除狗狗的数据。 删除数据用到了 sqflite 库中的delete
方法

In this portion, create a function that takes in an id and deletes the dog with
a matching id from the database. To make this work, you must provide a where
clause to limit the records being deleted.

在这一小节，新建一个方法用来接收一个 id 并且删除数据库中与这个 id 匹配的那一条数据。
为了达到这个目的，你必须使用 where 语句限定哪一条才是被删除的数据。

Future<void> deleteDog(int id) async {
  // Get a reference to the database
  // 获得数据库引用
  final db = await database;

  // Remove the Dog from the Database
  // 将狗狗从数据库移除
  await db.delete(
    'dogs',
    // Use a `where` clause to delete a specific dog
    // 使用 `where` 语句删除指定的狗狗
    where: "id = ?",
    // Pass the Dog's id through as a whereArg to prevent SQL injection
    // 通过 `whereArg` 将狗狗的 id 传递给 `delete` 方法，以防止 SQL 注入。
    whereArgs: [id],
  );
}

Example

示例

To run the example:

运行示例需要以下几步：

1. Create a new Flutter project

2. Add the sqfite and path packages to your pubspec.yaml

3. Paste the following code into a new file called lib/db_test.dart

4. Run the code with flutter run lib/db_test.dart

5. 创建一个新的 Flutter 工程。

6. 将 sqfite 和 path 包添加到 pubspec.yaml 文件里。

7. 将以下代码粘贴在 lib/db_test.dart 文件里（若无则新建，若有则覆盖）。

import 'dart:async';

import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';

void main() async {
  final database = openDatabase(
    // Set the path to the database. Note: Using the `join` function from the
    // `path` package is best practice to ensure the path is correctly
    // constructed for each platform.
    // 设置数据库的路径。注意：使用 `path` 包中的 `join` 方法是
    // 确保在多平台上路径都正确的最佳实践。
    join(await getDatabasesPath(), 'doggie_database.db'),
    // When the database is first created, create a table to store dogs
    // 当数据库第一次被创建的时候，创建一个数据表，用以存储狗狗们的数据。
    onCreate: (db, version) {
      return db.execute(
        "CREATE TABLE dogs(id INTEGER PRIMARY KEY, name TEXT, age INTEGER)",
      );
    },
    // Set the version. This executes the onCreate function and provides a
    // path to perform database upgrades and downgrades.
    // 设置版本。 它将执行 onCreate 方法，同时提供数据库升级和降级的路径。
    version: 1,
  );

  Future<void> insertDog(Dog dog) async {
    // Get a reference to the database
    // 获得数据库引用
    final Database db = await database;

    // Insert the Dog into the correct table. We will also specify the
    // `conflictAlgorithm` to use in this case. If the same dog is inserted
    // multiple times, it will replace the previous data.
    // 在正确的数据表里插入狗狗的数据。 我们也要在这个操作中指定 `conflictAlgorithm` 策略。
    // 如果同样的狗狗数据被多次插入，后一次插入的数据将会覆盖之前的数据。
    await db.insert(
      'dogs',
      dog.toMap(),
      conflictAlgorithm: ConflictAlgorithm.replace,
    );
  }

  Future<List<Dog>> dogs() async {
    // Get a reference to the database
    // 获得数据库引用
    final Database db = await database;

    // Query the table for All The Dogs.
    // 查询数据表，获取所有的狗狗们。
    final List<Map<String, dynamic>> maps = await db.query('dogs');

    // Convert the List<Map<String, dynamic> into a List<Dog>.
    // 将 List<Map<String, dynamic> 转换成 List<Dog> 数据类型。
    return List.generate(maps.length, (i) {
      return Dog(
        id: maps[i]['id'],
        name: maps[i]['name'],
        age: maps[i]['age'],
      );
    });
  }

  Future<void> updateDog(Dog dog) async {
    // Get a reference to the database
    // 获得数据库引用
    final db = await database;

    // Update the given Dog
    // 修改给定的狗狗的数据
    await db.update(
      'dogs',
      dog.toMap(),
      // Ensure we only update the Dog with a matching id
      // 确定给定的狗狗id是否匹配
      where: "id = ?",
      // Pass the Dog's id through as a whereArg to prevent SQL injection
      // 通过 whereArg 传递狗狗的 id 可以防止 SQL 注入。
      whereArgs: [dog.id],
    );
  }

  Future<void> deleteDog(int id) async {
    // Get a reference to the database
    // 获得数据库引用
    final db = await database;

    // Remove the Dog from the Database
    // 将狗狗从数据库移除
    await db.delete(
      'dogs',
      // Use a `where` clause to delete a specific dog
      // 使用 `where` 语句删除指定的狗狗
      where: "id = ?",
      // Pass the Dog's id through as a whereArg to prevent SQL injection
      // 通过 `whereArg` 将狗狗的 id 传递给 `delete` 方法，以防止 SQL 注入。
      whereArgs: [id],
    );
  }

  var fido = Dog(
    id: 0,
    name: 'Fido',
    age: 35,
  );

  // Insert a dog into the database
  // 在数据库插入一条狗狗的数据
  await insertDog(fido);

  // Print the list of dogs (only Fido for now)
  // 打印一个列表的狗狗们（现在列表里只有一只叫 Fido 的狗狗）
  print(await dogs());

  // Update Fido's age and save it to the database
  // 修改数据库中 Fido 的 年龄并且保存
  fido = Dog(
    id: fido.id,
    name: fido.name,
    age: fido.age + 7,
  );
  await updateDog(fido);

  // Print Fido's updated information
  // 打印 Fido 的修改后的信息
  print(await dogs());

  // Delete Fido from the Database
  // 从数据库中删除 Fido
  await deleteDog(fido.id);

  // Print the list of dogs (empty)
  // 打印一个列表的狗狗们（这里已经空了）
  print(await dogs());
}

class Dog {
  final int id;
  final String name;
  final int age;

  Dog({this.id, this.name, this.age});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'age': age,
    };
  }

  // Implement toString to make it easier to see information about each dog when
  // using the print statement.
  // 重写 toString 方法，以便使用 print 方法查看每个狗狗信息的时候能更清晰。
  @override
  String toString() {
    return 'Dog{id: $id, name: $name, age: $age}';
  }
}