QtQml QML Basic Types

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)

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

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

bool Binary true/false value
double Number with a decimal point, stored in double precision
enumeration Named enumeration value
int Whole number, e.g. 0, 10, or -20
list List of QML objects
real Number with a decimal point
string Free form text string
url Resource locator
var Generic property type

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

date Date value
point Value with x and y attributes
rect Value with x, y, width and height attributes
size Value with 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.

Some basic types have properties: for example, the font type has pixelSize, family and b 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:

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
}

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.

See also The QML Type System .