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In QML, properties serve many purposes but their main function is to bind to values. Values may be a \l{QML Basic Types}{basic type}, or other QML elements. The syntax for properties is: \tt{[default] property [: defaultValue]} Elements already possess useful properties but, to create custom properties, precede the property name with the keyword \c property. \snippet doc/src/snippets/declarative/properties.qml parent begin \snippet doc/src/snippets/declarative/properties.qml inherited properties \snippet doc/src/snippets/declarative/properties.qml custom properties \snippet doc/src/snippets/declarative/properties.qml parent end QML property rules coincide with many of JavaScript's property rules, for example, property names must begin with a lowercase letter. \l {JavaScript Reserved Words}{JavaScript reserved words} are not valid property names. \section1 Property Binding Property binding is a declarative way of specifying the value of a property. Binding allows a property's value to be expressed as an JavaScript expression that defines the value relative to other property values or data accessible in the application. The property value is automatically kept up to date if the other properties or data values change. Property bindings are created in QML using the colon "\c {:}" before the value: \snippet doc/src/snippets/declarative/properties.qml property binding The property binding causes the width of the \c Rectangle to update whenever the \c {parent}'s width changes. QML extends a standards compliant JavaScript engine, so any valid JavaScript expression can be used as a property binding. Bindings can access object properties, make function calls and even use built-in JavaScript objects such as \c {Date} and \c {Math}. \snippet doc/src/snippets/declarative/properties.qml JavaScript sample While syntactically bindings can be of arbitrary complexity, if a binding starts to become overly complex - such as involving multiple lines, or imperative loops - it may be better to refactor the component entirely, or at least factor the binding out into a separate function. \keyword qml-javascript-assignment \section1 Property Assignment versus Property Binding When working with both QML and JavaScript, it is important to differentiate between QML property binding and JavaScript value assignment. In QML, a property binding is created using the colon "\c {:}". \snippet doc/src/snippets/declarative/properties.qml property binding The property binding causes the width of the \c Rectangle to update whenever the \c {parent}'s width changes. Assigning a property value (using the equals sign "\c {=}") does not create a property binding. \snippet doc/src/snippets/declarative/properties.qml property assignment Instead of creating a property binding, the assignment simply sets the \c Rectangle \c width value to a number when the \c Component.onCompleted code is invoked. Assigning a value to a property that is already bound will remove the previous binding. A property can only have one value at a time (a list of property is one value), and if any code explicitly re-sets this value, the property binding is removed. There is no way to create a property binding directly from imperative JavaScript code, although it is possible to use the \l {Using the Binding Element}{Binding} element. \section1 Types of Properties Properties may bind to different types, but they are are \e type-safe. That is, properties only allow you to assign a value that matches the property type. For example, if a property is a real, and if you try to assign a string to it you will get an error. \badcode property real volume: "four" //generates an error \endcode Certain properties bind to more complex types such as other elements and objects. \keyword qml-basic-property-types \section2 Basic Property Types Basic types such as \l int, \l real, and other Qt structures may be bound to properties. For a list of types, visit the \l {QML Basic Types} document. \keyword qml-id-property \section2 The \c id Property Each QML object may be given a special unique property called an \c id. No other object within the same QML component (see \l{QML Documents}) can have the same \c id value. QML objects may then access an object using the \c id property. \snippet doc/src/snippets/declarative/properties.qml id property A component may readily access its parent's properties by using the \c parent property. Note that an \c id must begin with a lower-case letter or an underscore. The \c id cannot contain characters other than letters, numbers, underscores, and \l {JavaScript Reserved Words}{JavaScript reserved words}. \section2 Elements and Objects as Property Values Many properties bind to objects. For example, the \l Item element has a \c states property that can bind to \l State elements. This type of property binding allows elements to carry additional non-children elements. \c Item's \c transitions property behaves in a similar way; it can bind to \l Transition elements. Care must be taken when referring to the parent of an object property binding. Elements and components that are bound to properties are not necessarily set as children of the properties' component. \snippet doc/src/snippets/declarative/properties.qml object binding The code snippet has a \l Gradient element that attempts to print its parent's \c width value. However, the \c Gradient element is bound to the \c gradient property, not the \c children property of the \c Rectangle. As a result, the \c Gradient does not have the \c Rectangle as its parent. Printing the value of \c{parent.width} generates an error. Printing the \c Rectangle object's first child's \c name will print \c {childrectangle} because the second \c Rectangle is bound to the \c children property. For more information about the \c children property, please read the \l {Default Properties} section. \keyword attached-properties \section2 Attached Properties Certain objects provide additional properties by \e attaching properties to other objects. For example, the \l Keys element have properties that can \e attach to other QML objects to provide keyboard handling. \snippet doc/src/snippets/declarative/properties.qml list attached property The element \l ListView provides the delegate, \c listdelegate, the property \c isCurrentItem as an attached property. The \c ListView.isCurrentItem \e{attached property} provides highlight information to the delegate. Effectively, the \l ListView element attaches the \c ListView.isCurrentItem property to each delegate it creates. \keyword attached-signalhandlers \section2 Attached Signal Handlers \e {Attached signal handlers} are similar to \l{Attached Properties}{attached properties} in that they attach to objects to provide additional functionality to objects. Two prominent elements, \l Component and \l Keys element provide \l{QML Signal and Handler Event System}{signal handlers} as attached signal handlers. \snippet doc/src/snippets/declarative/properties.qml attached signal handler Read the \l{QML Signal and Handler Event System} and the \l{Keyboard Focus in QML} articles for more information. \section2 List properties Some properties may accept a binding to a list property, where more than one component can bind to the property. List properties allow multiple \l {State}{States}, \l {Gradient}{Gradients}, and other components to bind to a single property. \snippet doc/src/snippets/declarative/properties.qml list property The list is enclosed in square brackets, with a comma separating the list elements. In cases where you are only assigning a single item to a list, you may omit the square brackets. \snippet doc/src/snippets/declarative/properties.qml single property To access the list, use the \c index property. \snippet doc/src/snippets/declarative/properties.qml print list property The snippet code simply prints the name of the first state, \c FETCH. See the \l{list}{list type} documentation for more details about list properties and their available operations. \keyword qml-grouped-properties \section2 Grouped Properties In some cases properties form a logical group and use either the \e dot notation or \e group notation. Grouped properties may be written both ways: \snippet doc/src/snippets/declarative/properties.qml grouped properties In the element documentation grouped properties are shown using the dot notation. \section2 Property Aliases Unlike a property definition, which allocates a new, unique storage space for the property, a property alias connects the newly declared property, called the \e{aliasing property} as a direct reference to an existing property, the \e{aliased property}. Read or write operations on the aliasing property results in a read or write operations on the aliased property, respectively. A property alias declaration is similar to an ordinary property definition: \tt{[default] property alias : } As the aliasing property has the same type as the aliased property, an explicit type is omitted, and the special \c alias keyword is before the property name. Instead of a default value, a property alias has a compulsory alias reference. Accessing the aliasing property is similar to accessing a regular property. In addition, the optional \c default keyword indicates that the aliasing property is a \l{Default Properties}{default property}. \snippet doc/src/snippets/declarative/Button.qml property alias When importing the component as a \c Button, the \c buttonlabel is directly accessible through the \c label property. \snippet doc/src/snippets/declarative/properties.qml alias usage In addition, the \c id property may also be aliased and referred outside the component. \snippet doc/src/snippets/declarative/Button.qml parent begin \snippet doc/src/snippets/declarative/Button.qml id alias \snippet doc/src/snippets/declarative/Button.qml parent end The \c imagebutton component has the ability to modify the child \l Image object and its properties. \snippet doc/src/snippets/declarative/properties.qml image alias Using aliases, properties may be exposed to the \l{qml-top-level-component}{top level component}. Exposing properties to the top-level component allows components to have interfaces similar to Qt widgets. \section3 Considerations for property aliases Aliases are only activated once the component \l{Component::onCompleted}{completes} its initialization. An error is generated when an uninitialized alias is referenced. Likewise, aliasing an aliasing property will also result in an error. \snippet doc/src/snippets/declarative/properties.qml alias complete When importing the component, however, aliasing properties appear as regular Qt properties and consequently can be used in alias references. It is possible for an aliasing property to have the same name as an existing property, effectively overwriting the existing property. For example, the following component has a \c color alias property, named the same as the built-in \l {Rectangle::color} property: \snippet doc/src/snippets/declarative/properties.qml alias overwrite Any object that use this component and refer to its \c color property will be referring to the alias rather than the ordinary \l {Rectangle::color} property. Internally, however, the \c coloredrectangle can correctly set its \c color property and refer to the actual defined property rather than the alias. The \l{declarative/ui-components/tabwidget}{TabWidget} example uses aliases to reassign children to the \l ListView, creating a tab effect. \keyword default-properties \section2 Default Properties When imported, QML components will bind declared children to their designated \e{default properties}. The optional \c default attribute specifies a property as the \e {default property}. For example, the State element's default property is its \l{State::changes}{changes} property. \l PropertyChanges elements may simply be placed as the \c{State}'s children and they will be bound to the \c changes property. \snippet doc/src/snippets/declarative/properties.qml state default Similarly, the \l Item element's default property is its \l{Item::data}{data} property. The \c data property manages Item's \c children and \c resources properties. This way, different data types may be placed as direct children of the \c Item. \snippet doc/src/snippets/declarative/properties.qml default property Reassigning a default property is useful when a component is reused. For example, the \l{declarative/ui-components/tabwidget}{TabWidget} example uses the \c default attribute to reassign children to the \l ListView, creating a tab effect. \section1 Using the Binding Element In some advanced cases, it may be necessary to create bindings explicitly with the\l Binding element. For example, to bind a property exposed from C++ (\c system.brightness) to a value written in QML (\c slider.value), you could use the \l Binding element as follows: \snippet doc/src/snippets/declarative/properties.qml binding element \section1 Changing Property Values in States The \l PropertyChanges element is for setting property bindings within a \l State element to set a property binding. \snippet doc/src/snippets/declarative/properties.qml PropertyChanges element The rectangle's \c color property will bind to the \c warning component's \c color property when its \c state is set to the \c WARNING state. */