path: root/src/qml/doc/src/qmllanguageref/typesystem/valuetypes.qdoc

// Copyright (C) 2022 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtqml-typesystem-valuetypes.html
\title QML Value Types
\brief Description of QML value types

QML supports built-in and custom value types.

A \e{value type} is one that is conceptually passed by value rather than by
reference, such as an \c int or a \c string. This contrasts with
\l{qtqml-typesystem-topic.html#qml-object-types}{QML Object Types}. Object types
are passed by reference. If you assign an instance of an object type to two
different properties, both properties carry the same value. Modifying the object
is reflected in both properties. If you assign an instance of a value type to
two different properties, the properties carry separate values. If you modify
one of them, the other one stays the same. Value types are only conceptually
passed by value since it must still be possible to interact with them as if they
were JavaScript objects. To facilitate this, in reality they are passed as
\l{QML Value Type and Sequence References}{Value Type References} when you access
them from JavaScript code.

Unlike an object type, a value type cannot be used to declare QML objects:
it is not possible, for example, to declare an \c int{} object or a \c size{} object.

Value types can be used to refer to:

\list
\li A single value (e.g. \l int refers to a single number)
\li A value that contains properties and methods (e.g. \l size refers to a value with \c width and \c height properties)
\li The generic type \l{var}. It can hold values of any other type but is itself a value type.
\endlist

When a variable or property holds a value type and it is assigned to another
variable or property, then a copy of the value is made.

\sa {qtqml-typesystem-topic.html}{The QML Type System}


\section1 Available Value Types

Some value types are supported by the engine by default and do not require an
\l {Import Statements}{import statement} to be used, while others do require
the client to import the module which provides them.
All of the value types listed below may be used as a \c property type in a QML
document, with the following exceptions:
\list
  \li \c void, which marks the absence of a value
  \li \c list must be used in conjunction with an object or value type as element
  \li \c enumeration cannot be used directly as the enumeration must be defined by a registered QML object type
\endlist

\section2 Built-in Value Types Provided By The QML Language

The built-in value types supported natively in the \l{The QML Reference}{QML language} are listed below:
\annotatedlist qmlvaluetypes

\section2 Value Types Provided By QML Modules

QML modules may extend the QML language with more value types.

For instance, the value types provided by the \c QtQml module are:
\annotatedlist qtqmlvaluetypes

The value types provided by the \c QtQuick module are:
\annotatedlist qtquickvaluetypes

The \l{QtQml::Qt}{Qt} global object provides \l{globalqtobjecttypes}{useful functions} for manipulating values of value
types for the \l{Qt Qml} and \l{Qt Quick} modules.

Other Qt modules will document their value types on their respective module pages.

You may define your own value types as described in
\l{qtqml-cppintegration-definetypes.html}{Defining QML Types from C++}.
In order to use types provided by a particular QML module, clients
must import that module in their QML documents.

\section1 Property Change Behavior for Value Types

Some value types have properties: for example, the \l font type has
\c pixelSize, \c family and \c bold properties. Unlike properties of
\l{qtqml-typesystem-topic.html#qml-object-types}{object types}, properties of
value types do not provide their own property change signals. It is only possible
to create a property change signal handler for the value type property itself:

\code
Text {
    // invalid!
    onFont.pixelSizeChanged: doSomething()

    // also invalid!
    font {
        onPixelSizeChanged: doSomething()
    }

    // but this is ok
    onFontChanged: doSomething()
}
\endcode

Be aware, however, that a property change signal for a value type is emitted
whenever \e any of its attributes have changed, as well as when the property itself
changes. Take the following code, for example:

\qml
Text {
    onFontChanged: console.log("font changed")

    Text { id: otherText }

    focus: true

    // changing any of the font attributes, or reassigning the property
    // to a different font value, will invoke the onFontChanged handler
    Keys.onDigit1Pressed: font.pixelSize += 1
    Keys.onDigit2Pressed: font.b = !font.b
    Keys.onDigit3Pressed: font = otherText.font
}
\endqml

In contrast, properties of an \l{qtqml-typesystem-topic.html#qml-object-types}{object type}
emit their own property change signals, and a property change signal handler for an object-type
property is only invoked when the property is reassigned to a different object value.

*/

/*!
    \qmlvaluetype int
    \ingroup qmlvaluetypes
    \brief a whole number, e.g. 0, 10, or -20.

    The \c int type refers to a whole number, e.g. 0, 10, or -20.

    The possible \c int values range from -2147483648 to 2147483647,
    although most types will only accept a reduced range (which they
    mention in their documentation).

    Example:
    \qml
    NumberAnimation { loops: 5 }
    \endqml

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/

/*!
    \qmlvaluetype bool
    \ingroup qmlvaluetypes
    \brief a binary true/false value.

    The \c bool type refers to a binary true/false value.

    Properties of type \c bool have \c false as their default value.

    Example:
    \qml
    Item {
        focus: true
        clip: false
    }
    \endqml

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/

/*!
    \qmlvaluetype real
    \ingroup qmlvaluetypes

    \brief a number with a decimal point.

    The \c real type refers to a number with decimal point, e.g. 1.2 or -29.8.

    Example:
    \qml
    Item { width: 100.45; height: 150.82 }
    \endqml

    \note In QML all reals are stored in double precision, \l
    {http://en.wikipedia.org/wiki/IEEE_754} {IEEE floating point}
    format.

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/

/*!
    \qmlvaluetype double
    \ingroup qmlvaluetypes

    \brief a number with a decimal point, stored in double precision.

    The \c double type refers to a number with a decimal point and is stored in double precision, \l
    {http://en.wikipedia.org/wiki/IEEE_754} {IEEE floating point} format. It's the same as \c real.

    Properties of type \c double have \e {0.0} as their default value.

    Example:
    \qml
    Item {
        property double number: 32155.2355
    }
    \endqml

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/

/*!
    \qmlvaluetype string
    \ingroup qmlvaluetypes
    \brief A free form text string.

    The \c string type refers to a free form text string in quotes, for example
    "Hello world!". The QML language provides this value type by default.

    Example:
    \qml
    Text { text: "Hello world!" }
    \endqml

    Properties of type \c string are empty by default.

    Strings have a \c length attribute that holds the number of characters in
    the string.

    The string value type is backed by the C++ type QString. It extends the
    JavaScript String primitive type in that it provides much of the same API,
    plus some extra methods. For example, the QML string value type method
    \c {arg()} supports value substitution:

    \qml
        var message = "There are %1 items"
        var count = 20
        console.log(message.arg(count))
    \endqml

    The example above prints "There are 20 items".

    The QML string value type supports most of the ECMAScript string features,
    such as template (string) literals, string interpolation, multi-line
    strings, and looping over strings.

    In general, QML string supports most JavaScript String methods, including
    checking for inclusion using \c string.includes(), \c string.startsWith(),
    and \c string.endsWith(); repeating a string using \c string.repeats(), and
    slicing and splitting using \c string.slice() and \c string.split().

    For more information about which version of ECMAScript QML supports, see
    \l {JavaScript Host Environment}

    For more information about JavaScript String methods, see
    \l {https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String}
    {mdn JavaScript String}

    When integrating with C++, note that any QString value
    \l{qtqml-cppintegration-data.html}{passed into QML from C++} is
    automatically converted into a \c string value, and vice-versa.

    \sa {QML Value Types}, {ECMA-262}{ECMAScript Language Specification}
*/

/*!
    \qmlvaluetype url
    \ingroup qmlvaluetypes
    \brief a resource locator.

    The \c url type refers to a resource locator (like a file name, for example). It can be either
    absolute, e.g. "http://qt-project.org", or relative, e.g.  "pics/logo.png". A relative URL is
    resolved relative to the URL of the containing component.

    For example, the following assigns a valid URL to the \l {Image::source}
    property, which is of type \c url:

    \qml
    Image { source: "pics/logo.png" }
    \endqml

    When integrating with C++, note that any QUrl value
    \l{qtqml-cppintegration-data.html}{passed into QML from C++} is automatically
    converted into a \c url value, and vice-versa.

    Alternatively you may convert your \c url to a \l{https://developer.mozilla.org/en-US/docs/Web/API/URL}{URL} object
    in order to access and modify its components:
    \qml
    var urlObject = new URL(url);
    \endqml

    \note In Qt 5, URLs were automatically resolved based on the current context
    when assigning them to any \c url property. This made it impossible to
    work with relative URLs and it created inconsistent behavior when reading
    back a URL previously written to a property. Therefore, the behavior was
    changed in Qt 6: URLs are not automatically resolved on assignment anymore.
    The individual elements that use URLs have to resolve them themselves.

    \note When referring to files stored with the \l{resources.html}{Qt Resource System}
    from within QML, you should use "qrc:///" instead of ":/" as QML requires URL paths.
    Relative URLs resolved from within that file will use the same protocol.

    Additionally, URLs may contain encoded characters using the 'percent-encoding' scheme
    specified by \l {https://datatracker.ietf.org/doc/html/rfc3986}{RFC 3986}.  These characters
    will be preserved within properties of type \c url, to allow QML code to
    construct precise URL values.

    For example, a local file containing a '#' character, which would normally be
    interpreted as the beginning of the URL 'fragment' element, can be accessed by
    encoding the characters of the file name:

    \qml
    Image { source: encodeURIComponent("/tmp/test#1.png") }
    \endqml

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/


/*!
    \qmlvaluetype list
    \ingroup qmlvaluetypes
    \brief a list of QML objects.

    The \c list type refers to a list of QML objects or values.

    Properties of type \c list are empty by default.

    A \c list can store QML objects or \l{QML Value Types}{value type} values.

    When integrating with C++, note that any QQmlListProperty value
    \l{qtqml-cppintegration-data.html}{passed into QML from C++} is automatically
    converted into a \c list value, and vice-versa.

    Similarly any \c{QList<T>} of a registered value type \c{T} is automatically
    converted into a \c list value, and vice-versa.

    \section1 Using the list Type

    For example, the \l Item type has a \l {Item::}{states} list-type property that
    can be assigned to and used as follows:

    \qml
    import QtQuick

    Item {
        width: 100; height: 100

        states: [
            State { name: "activated" },
            State { name: "deactivated" }
        ]

        Component.onCompleted: {
            console.log("Name of first state:", states[0].name)
            for (var i = 0; i < states.length; i++)
                console.log("state", i, states[i].name)
        }
    }
    \endqml

    The defined \l State objects will be added to the \c states list
    in the order in which they are defined.

    If the list only contains one object, the square brackets may be omitted:

    \qml
    import QtQuick

    Item {
        width: 100; height: 100
        states: State { name: "activated" }
    }
    \endqml

    You can also declare your own list properties in QML:

    \qml
    import QtQml

    QtObject {
        property list<int> intList: [1, 2, 3, 4]
        property list<QtObject> objectList
    }
    \endqml

    Lists can be used much like JavaScript arrays. For example:

    \list
    \li Values are assigned using the \c[] square bracket syntax with comma-separated values
    \li The \c length property provides the number of items in the list
    \li Values in the list are accessed using the \c [index] syntax
    \li You can use \c{push()} to append entries
    \li You can set the \c length property of the list to truncate or extend it.
    \endlist

    However, you can \e{not} automatically extend the list by assigning to an
    index currently out of range. Furthermore, if you insert \c null values
    into a list of objects, those are converted to \c nullptr entries in
    the underlying QQmlListProperty.

    A list of value types is different from a JavaScript array in one further
    important aspect: Growing it by setting its length does not produce undefined
    entries, but rather default-constructed instances of the value type.

    Similarly, growing a list of object types this way produces null entries,
    rather than undefined entries.

    This value type is provided by the QML language.

    \sa {QML Value Types}
*/

 /*!
    \qmlvaluetype var
    \ingroup qmlvaluetypes
    \brief a generic property type.

    The \c var type is a generic property type that can refer to any data type.

    It is equivalent to a regular JavaScript variable.
    For example, var properties can store numbers, strings, objects,
    arrays and functions:

    \qml
    Item {
        property var aNumber: 100
        property var aBool: false
        property var aString: "Hello world!"
        property var anotherString: String("#FF008800")
        property var aColor: Qt.rgba(0.2, 0.3, 0.4, 0.5)
        property var aRect: Qt.rect(10, 10, 10, 10)
        property var aPoint: Qt.point(10, 10)
        property var aSize: Qt.size(10, 10)
        property var aVector3d: Qt.vector3d(100, 100, 100)
        property var anArray: [1, 2, 3, "four", "five", (function() { return "six"; })]
        property var anObject: { "foo": 10, "bar": 20 }
        property var aFunction: (function() { return "one"; })
    }
    \endqml

    \section1 Change Notification Semantics

    It is important to note that changes in regular properties of JavaScript
    objects assigned to a var property will \b{not} trigger updates of bindings
    that access them.  The example below will display "The car has 4 wheels" as
    the change to the wheels property will not cause the reevaluation of the
    binding assigned to the "text" property:

    \qml
    Item {
        property var car: new Object({wheels: 4})

        Text {
            text: "The car has " + car.wheels + " wheels";
        }

        Component.onCompleted: {
            car.wheels = 6;
        }
    }
    \endqml

    If the onCompleted handler instead had \tt{"car = new Object({wheels: 6})"}
    then the text would be updated to say "The car has 6 wheels", since the
    car property itself would be changed, which causes a change notification
    to be emitted.

    \section1 Property Value Initialization Semantics

    The QML syntax defines that curly braces on the right-hand-side of a
    property value initialization assignment denote a binding assignment.
    This can be confusing when initializing a \c var property, as empty curly
    braces in JavaScript can denote either an expression block or an empty
    object declaration.  If you wish to initialize a \c var property to an
    empty object value, you should wrap the curly braces in parentheses.

    Properties of type \c var are \c {undefined} by default.

    For example:
    \qml
    Item {
        property var first:  {}   // nothing = undefined
        property var second: {{}} // empty expression block = undefined
        property var third:  ({}) // empty object
    }
    \endqml

    In the previous example, the \c first property is bound to an empty
    expression, whose result is undefined.  The \c second property is bound to
    an expression which contains a single, empty expression block ("{}"), which
    similarly has an undefined result.  The \c third property is bound to an
    expression which is evaluated as an empty object declaration, and thus the
    property will be initialized with that empty object value.

    Similarly, a colon in JavaScript can be either an object property value
    assignment, or a code label.  Thus, initializing a var property with an
    object declaration can also require parentheses:

    \qml
    Item {
        property var first: { example: 'true' }    // example is interpreted as a label
        property var second: ({ example: 'true' }) // example is interpreted as a property
        property var third: { 'example': 'true' }  // example is interpreted as a property
        Component.onCompleted: {
            console.log(first.example) // prints 'undefined', as "first" was assigned a string
            console.log(second.example) // prints 'true'
            console.log(third.example) // prints 'true'
        }
    }
    \endqml

    \sa {QML Value Types}
*/

/*!
   \qmlvaluetype variant
   \ingroup qmlvaluetypes
   \brief a generic property type.

   The \c variant type is the same as the \c var type. Use \c var instead.

   \sa {QML Value Types}
*/

/*!
   \qmlvaluetype void
   \ingroup qmlvaluetypes
   \brief The empty value type.

   The \c void type is exclusively used to type-annotate JavaScript functions
   returning \c undefined. For example:

   \qml
   function doThings() : void { console.log("hello") }
   \endqml

   This is to help tooling analyze calls to such functions and compile them and
   their callers to C++.

   You cannot declare \c void properties in QML.

   \sa {QML Value Types}
*/

/*!
    \qmlvaluetype enumeration
    \ingroup qmlvaluetypes
    \brief a named enumeration value.

    The \c enumeration type refers to a named enumeration value.

    Each named value can be referred to as \c {<Type>.<value>}. For
    example, the \l Text type has an \c AlignRight enumeration value:

    \qml
    Text { horizontalAlignment: Text.AlignRight }
    \endqml

    (For backwards compatibility, the enumeration value may also be
    specified as a string, e.g. "AlignRight". This form is not
    recommended for new code.)

    When integrating with C++, note that any \c enum value
    \l{qtqml-cppintegration-data.html}{passed into QML from C++} is automatically
    converted into an \c enumeration value, and vice-versa.

    This value type is provided by the QML language.  Some enumeration values
    are provided by the QtQuick import.

    \section1 Using the enumeration Type in QML

    The \c enumeration type is a representation of a C++ \c enum type. It is
    not possible to refer to the \c enumeration type in QML itself; instead, the
    \l int or \l var types can be used when referring to \c enumeration values
    from QML code.

    For example:

    \qml
    import QtQuick 2.0

    Item {
        // refer to Text.AlignRight using an int type
        property int enumValue: textItem.horizontalAlignment

        signal valueEmitted(int someValue)

        Text {
            id: textItem
            horizontalAlignment: Text.AlignRight
        }

        // emit valueEmitted() signal, which expects an int, with Text.AlignRight
        Component.onCompleted: valueEmitted(Text.AlignRight)
    }
    \endqml

    \sa {QML Value Types}
    \sa {qtqml-syntax-objectattributes.html#enumeration-attributes}{Enumeration Attributes}
*/