- Qt 5.15
- Qt Quick
- Qt Quick Local Storage QML Types
- Qt 5.15.19 Reference Documentation

# Qt Quick Local Storage QML Types

This is a singleton type for reading and writing to SQLite databases.

## Methods

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

## Detailed Description

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

     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 databases.

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.

The SQL Local Storage example demonstrates the basics of using the Offline Storage API.

#### Open or Create a Database

     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 | Value
--- | ---
Identifier | The name of the database passed to `openDatabase()`
Version | The version of the database passed to `openDatabase()`
Description | The description of the database passed to `openDatabase()`
EstimatedSize | The estimated size (in bytes) of the database passed to `openDatabase()`
Driver | Currently "QSQLITE"

This data can be used by application tools.

#### 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 is not 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 statement, binding the list of values to SQL positional parameters ("?").

It returns a results object, with the following properties:

Type | Property | Value | Applicability
--- | --- | --- | ---
int | rows.length | The number of rows in the result | 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 | The id of the row inserted | INSERT

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

See below for an example:

     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
                                  })
             }
         })
     }

## Method Documentation

     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.

Returns the created database object.