// 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 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. 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 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. 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 useful functions for manipulating values of value types. 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} 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 intList: [1, 2, 3, 4] property list 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 {.}. 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} */