QML Basic Types#

Description of basic types provided by the Qt QML module

QML supports a number of basic types.

A basic type is one that refers to a simple value, such as an int or a string. This contrasts with a QML Object Types , which refers to an object with properties, signals, methods and so on. Unlike an object type, a basic type cannot be used to declare QML objects: it is not possible, for example, to declare an int{} object or a size{} object.

Basic types can be used to refer to:

  • A single value (e.g. int refers to a single number, var can refer to a single list of items)

  • A value that contains a simple set of property-value pairs (e.g. size refers to a value with width and height attributes)

When a variable or property holds a basic type and it is assigned to another variable or property, then a copy of the value is made. In JavaScript, this value is called a primitive value.

Supported Basic Types#

Some basic types are supported by the engine by default and do not require an import statement to be used, while others do require the client to import the module which provides them. All of the basic types listed below may be used as a property type in a QML document, with the following exceptions:

  • list must be used in conjunction with a QML object type

  • enumeration cannot be used directly as the enumeration must be defined by a registered QML object type

Basic Types Provided By The QML Language#

The basic types supported natively in the QML language are listed below:

qml-int.html

a whole number, e.g. 0, 10, or -20.

qml-bool.html

a binary true/false value.

qml-real.html

a number with a decimal point.

qml-double.html

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

qml-string.html

a free form text string.

qml-url.html

a resource locator.

qml-list.html

a list of QML objects.

qml-var.html

a generic property type.

qml-enumeration.html

a named enumeration value.

Basic Types Provided By QML Modules#

QML modules may extend the QML language with more basic types. For example, the basic types provided by the QtQuick module are listed below:

qml-date.html

a date value.

qml-point.html

a value with x and y attributes.

qml-size.html

a value with width and height attributes

qml-rect.html

a value with x, y, width and height attributes.

The Qt global object provides useful functions for manipulating values of basic types.

Currently only QML modules which are provided by Qt may provide their own basic types, however this may change in future releases of Qt QML. In order to use types provided by a particular QML module, clients must import that module in their QML documents.

Property Change Behavior for Basic Types#

Some basic types have properties: for example, the font type has pixelSize, family and bold properties. Unlike properties of object types , properties of basic types do not provide their own property change signals. It is only possible to create a property change signal handler for the basic type property itself:

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

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

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

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

In contrast, properties of an 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.