Qt Quick 本地存储 QML 类型

这是单例类型用于读写 SQLite 数据库。

方法

object openDatabaseSync (string name, string version, string description, int estimated_size, jsobject callback(db))

详细描述

To use the types in this module, import the module and call the relevant functions using the LocalStorage 类型：

import QtQuick 2.15
import QtQuick.LocalStorage 2.15
Item {
    Component.onCompleted: {
        var db = LocalStorage.openDatabaseSync(...)
    }
}

These databases are user-specific and QML-specific, but accessible to all QML applications. They are stored in the Databases subdirectory of QQmlEngine::offlineStoragePath (), currently as SQLite 数据库。

Database connections are automatically closed during Javascript garbage collection.

The API can be used from JavaScript functions in your QML:

import QtQuick 2.0
Rectangle {
    color: "white"
    width: 200
    height: 100
    Text {
        text: "?"
        anchors.horizontalCenter: parent.horizontalCenter
        function findGreetings() {
            var db = openDatabaseSync("QDeclarativeExampleDB", "1.0", "The Example QML SQL!", 1000000);
            db.transaction(
                function(tx) {
                    // Create the database if it doesn't already exist
                    tx.executeSql('CREATE TABLE IF NOT EXISTS Greeting(salutation TEXT, salutee TEXT)');
                    // Add (another) greeting row
                    tx.executeSql('INSERT INTO Greeting VALUES(?, ?)', [ 'hello', 'world' ]);
                    // Show all added greetings
                    var rs = tx.executeSql('SELECT * FROM Greeting');
                    var r = ""
                    for (var i = 0; i < rs.rows.length; i++) {
                        r += rs.rows.item(i).salutation + ", " + rs.rows.item(i).salutee + "\n"
                    }
                    text = r
                }
            )
        }
        Component.onCompleted: findGreetings()
    }
}

The API conforms to the Synchronous API of the HTML5 Web Database API, W3C Working Draft 29 October 2009 .

SQL 本地存储范例演示使用离线存储 API 的基础知识。

打开或创建数据库

import QtQuick.LocalStorage 2.15 as Sql
db = Sql.openDatabaseSync(identifier, version, description, estimated_size, callback(db))

The above code returns the database identified by identifier . If the database does not already exist, it is created, and the function callback is called with the database as a parameter. identifier is the name of the physical file (with or without full path) containing the database. description and estimated_size are written to the INI file (described below), but are currently unused.

May throw exception with code property SQLException.DATABASE_ERR, or SQLException.VERSION_ERR.

When a database is first created, an INI file is also created specifying its characteristics:

Key	值
Identifier	The name of the database passed to `openDatabase()`
版本	The version of the database passed to `openDatabase()`
描述	The description of the database passed to `openDatabase()`
EstimatedSize	The estimated size (in bytes) of the database passed to `openDatabase()`
Driver	Currently "QSQLITE"

通过应用程序工具可以使用此数据。

db.changeVersion(from, to, callback(tx))

This method allows you to perform a Scheme Upgrade . If it succeeds it returns a new database object of version to . Otherwise it returns undefined .

If the current version of db 不是 from , then an exception is thrown.

Otherwise, a database transaction is created and passed to callback . In this function, you can call executeSql on tx to upgrade the database.

May throw exception with code property SQLException.DATABASE_ERR or SQLException.UNKNOWN_ERR.

See example below.

var db = LocalStorage.openDatabaseSync("ActivityTrackDB", "", "Database tracking sports activities", 1000000);
if (db.version == "0.1") {
    db.changeVersion("0.1", "0.2", function(tx) {
        tx.executeSql("INSERT INTO trip_log VALUES(?, ?, ?)",
                    [ "01/10/2016","Sylling - Vikersund", "53" ]);
    }
});

db.transaction(callback(tx))

This method creates a read/write transaction and passed to callback . In this function, you can call executeSql on tx to read and modify the database.

If the callback throws exceptions, the transaction is rolled back. Below you will find an example of a database transaction which catches exceptions.

{
    var db = LocalStorage.openDatabaseSync("Activity_Tracker_DB", "", "Track exercise", 1000000)
    try {
        db.transaction(function (tx) {
            tx.executeSql('CREATE TABLE IF NOT EXISTS trip_log (date text,trip_desc text,distance numeric)')
        })
    } catch (err) {
        console.log("Error creating table in database: " + err)
    };
}

In the example you can see an insert statement where values are assigned to the fields, and the record is written into the table. That is an insert statement with a syntax that is usual for a relational database. It is however also possible to work with JSON objects and store them in a table.

Let's suppose a simple example where we store trips in JSON format using date as the unique key. An example of a table that could be used for that purpose:

create table trip_log(date text, data text)

The assignment of values to a JSON object:

var obj = {description = "Vikersund - Noresund", distance = "60"}

In that case, the data could be saved in the following way:

db.transaction(function(tx) {
    result = tx.executeSQL("insert into trip_log values (?,?)",
                           ["01/11/2016", JSON.stringify(obj)])

db.readTransaction(callback(tx))

This method creates a read-only transaction and passed to callback . In this function, you can call executeSql on tx to read the database (with select statements).

results = tx.executeSql(statement, values)

This method executes an SQL 语句 , binding the list of values to SQL positional parameters ("?").

It returns a results object, with the following properties:

类型	特性	值	Applicability
int	rows.length	结果中的行数	SELECT
var	rows.item(i)	Function that returns row i of the result	SELECT
int	rowsAffected	The number of rows affected by a modification	UPDATE, DELETE
string	insertId	插入行的 ID	INSERT

May throw exception with code property SQLException.DATABASE_ERR, SQLException.SYNTAX_ERR, or SQLException.UNKNOWN_ERR.

见以下范例：

function dbReadAll()
{
    var db = dbGetHandle()
    db.transaction(function (tx) {
        var results = tx.executeSql(
                    'SELECT rowid,date,trip_desc,distance FROM trip_log order by rowid desc')
        for (var i = 0; i < results.rows.length; i++) {
            listModel.append({
                                 id: results.rows.item(i).rowid,
                                 checked: " ",
                                 date: results.rows.item(i).date,
                                 trip_desc: results.rows.item(i).trip_desc,
                                 distance: results.rows.item(i).distance
                             })
        }
    })
}

方法文档编制

object openDatabaseSync(string name, string version, string description, int estimated_size, jsobject callback(db))

Opens or creates a local storage sql database by the given parameters.

name is the database name
version is the database version
description is the database display name
estimated_size is the database's estimated size, in bytes
callback is an optional parameter, which is invoked if the database has not yet been created.

返回创建的数据库对象。