Extending QML Functionalities using C++

The QML syntax declaratively describes how to construct an in-memory object tree. In Qt, QML is mainly used to describe a visual scene graph, but it is not conceptually limited to this: the QML format is an abstract description of any object tree. All the QML element types included in Qt are implemented using the C++ extension mechanisms describe on this page. Programmers can use these APIs to add new types that interact with the existing Qt types, or to repurpose QML for their own independent use.

Adding Types

 import People 1.0

 Person {
     name: "Bob Jones"
     shoeSize: 12
 }

The QML snippet shown above instantiates one Person instance and sets the name and shoeSize properties on it. Everything in QML ultimately comes down to either instantiating an object instance, or assigning a property a value.

QML relies heavily on Qt's meta object system and can only instantiate classes that derive from QObject. For visual element types, this will usually mean a subclass of QDeclarativeItem; for models used with the view elements, a subclass of QAbstractItemModel; and for arbitrary objects with properties, a direct subclass of QObject.

The QML engine has no intrinsic knowledge of any class types. Instead the programmer must register the C++ types with their corresponding QML names.

Custom C++ types are registered using a template function:

 template<typename T>
 int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

Calling qmlRegisterType() registers the C++ type T with the QML system, and makes it available in QML under the name qmlName in library uri version versionMajor.versionMinor. The qmlName can be the same as the C++ type name.

Type T must be a concrete type that inherits QObject and has a default constructor.

#include <QtDeclarative> to use qmlRegisterType().

Types can be registered by libraries, application code, or by plugins (see QDeclarativeExtensionPlugin).

Once registered, all properties of the supported types are available in QML. QML has intrinsic support for properties of the types listed in the QML Basic Types document, which includes the following:

When a property of a supported type is added to a C++ class, in a QML element based on the C++ class, a value-changed signal handler will be available. See Signal Support below.

QML is typesafe. Attempting to assign an invalid value to a property will generate an error. For example, assuming the name property of the Person element had a type of QString, this would cause an error:

 Person {
     // Will NOT work
     name: 12
 }

Extending QML - Adding Types Example shows the complete code used to create the Person type.

QML Type Versioning

In C++ adding a new method or property cannot break old applications. In QML, however, new methods and properties can change what a name previously resolved to to within a scope chain.

For example, consider these two QML files

 // main.qml
 import QtQuick 1.0
 Item {
     id: root
     MyComponent {}
 }
 // MyComponent.qml
 import MyModule 1.0
 CppItem {
     value: root.x
 }

where CppItem maps to the C++ class QCppItem.

If the author of QCppItem adds a "root" property to QCppItem in a new version of the module, it will break the above program as root.x now resolves to a different value. The solution is to allow the author of QCppItem to state that the new root property is only available from a particular version of QCppItem onwards. This permits new properties and features to be added to existing elements without breaking existing programs.

QML enables this by allowing the properties, methods and signals of a class to be tagged with a particular revision, so that they are only accessible if the relevant module version is imported. In this case, the author can tag the root property as being added in revision 1 of the class, and register that revision in version 1.1 of the module.

The REVISION tag is used to mark the root property as added in revision 1 of the class. Methods such as Q_INVOKABLE's, signals and slots can also be tagged for a revision using the Q_REVISION(x) macro:

 class CppItem : public QObject
 {
     Q_OBJECT
     Q_PROPERTY(int root READ root WRITE setRoot NOTIFY rootChanged REVISION 1)

 signals:
     Q_REVISION(1) void rootChanged();
 };

To register the new class revision to a particular version the following function is used:

 template<typename T, int metaObjectRevision>
 int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

To register CppItem version 1 for MyModule 1.1:

 qmlRegisterType<QCppItem,1>("MyModule", 1, 1, "CppItem")

root is only available when MyModule 1.1 is imported.

Object and List Property Types

 BirthdayParty {
     host: Person {
         name: "Bob Jones"
         shoeSize: 12
     }
     guests: [
         Person { name: "Leo Hodges" },
         Person { name: "Jack Smith" },
         Person { name: "Anne Brown" }
     ]
 }

The QML snippet shown above assigns a Person object to the BirthdayParty's host property, and assigns three Person objects to the guests property.

QML can set properties of types that are more complex than basic intrinsics like integers and strings. Properties can also be object pointers, Qt interface pointers, lists of object pointers, and lists of Qt interface pointers. As QML is typesafe it ensures that only valid types are assigned to these properties, just like it does for primitive types.

Properties that are pointers to objects or Qt interfaces are declared with the Q_PROPERTY() macro, just like other properties. The host property declaration looks like this:

     Q_PROPERTY(Person *host READ host WRITE setHost)

As long as the property type, in this case Person, is registered with QML the property can be assigned.

QML also supports assigning Qt interfaces. To assign to a property whose type is a Qt interface pointer, the interface must also be registered with QML. As they cannot be instantiated directly, registering a Qt interface is different from registering a new QML type. The following function is used instead:

 template<typename T>
 int qmlRegisterInterface(const char *typeName)

This registers the C++ interface T with the QML system as typeName.

Following registration, QML can coerce objects that implement this interface for assignment to appropriately typed properties.

The guests property is a list of Person objects. Properties that are lists of objects or Qt interfaces are also declared with the Q_PROPERTY() macro, just like other properties. List properties must have the type QDeclarativeListProperty<T>. As with object properties, the type T must be registered with QML.

The guest property declaration looks like this:

     Q_PROPERTY(QDeclarativeListProperty<Person> guests READ guests)

Extending QML - Object and List Property Types Example shows the complete code used to create the BirthdayParty type.

Inheritance and Coercion

 BirthdayParty {
     host: Boy {
         name: "Bob Jones"
         shoeSize: 12
     }
     guests: [
         Boy { name: "Leo Hodges" },
         Boy { name: "Jack Smith" },
         Girl { name: "Anne Brown" }
     ]
 }

The QML snippet shown above assigns a Boy object to the BirthdayParty's host property, and assigns three other objects to the guests property.

QML supports C++ inheritance hierarchies and can freely coerce between known, valid object types. This enables the creation of common base classes that allow the assignment of specialized classes to object or list properties. In the snippet shown, both the host and the guests properties retain the Person type used in the previous section, but the assignment is valid as both the Boy and Girl objects inherit from Person.

To assign to a property, the property's type must have been registered with QML. Both the qmlRegisterType() and qmlRegisterInterface() template functions already shown can be used to register a type with QML. Additionally, if a type that acts purely as a base class that cannot be instantiated from QML needs to be registered, the following function can be used:

     template<typename T>
     int qmlRegisterType()

This registers the C++ type T with the QML system. The parameterless call to the template function qmlRegisterType() does not define a mapping between the C++ class and a QML element name, so the type is not instantiable from QML, but it is available for type coercion.

Type T must inherit QObject, but there are no restrictions on whether it is concrete or the signature of its constructor.

QML will automatically coerce C++ types when assigning to either an object property, or to a list property. Only if coercion fails does an assignment error occur.

Extending QML - Inheritance and Coercion Example shows the complete code used to create the Boy and Girl types.

Default Property

 BirthdayParty {
     host: Boy {
         name: "Bob Jones"
         shoeSize: 12
     }

     Boy { name: "Leo Hodges" }
     Boy { name: "Jack Smith" }
     Girl { name: "Anne Brown" }
 }

The QML snippet shown above assigns a collection of objects to the BirthdayParty's default property.

The default property is a syntactic convenience that allows a type designer to specify a single property as the type's default. The default property is assigned to whenever no explicit property is specified. As a convenience, it is behaviorally identical to assigning to the default property explicitly by name.

From C++, type designers mark the default property using a Q_CLASSINFO() annotation:

 Q_CLASSINFO("DefaultProperty", "property")

This marks property as the class's default property. property must be either an object property, or a list property.

A default property is optional. A derived class inherits its base class's default property, but may override it in its own declaration. property can refer to a property declared in the class itself, or a property inherited from a base class.

Extending QML - Default Property Example shows the complete code used to specify a default property.

Grouped Properties

 Boy {
     name: "Jack Smith"
     shoe {
         size: 8
         color: "blue"
         brand: "Puma"
         price: 19.95
     }
 }

The QML snippet shown above assigns a number of properties to the Boy object, including four properties using the grouped property syntax.

Grouped properties collect similar properties together into a single named block. Grouped properties can be used to present a nicer API to developers, and may also simplify the implementation of common property collections across different types through implementation reuse.

A grouped property block is implemented as a read-only object property. The shoe property shown is declared like this:

     Q_PROPERTY(ShoeDescription *shoe READ shoe)

The ShoeDescription type declares the properties available to the grouped property block - in this case the size, color, brand and price properties.

Grouped property blocks may declared and accessed be recusively.

Extending QML - Grouped Properties Example shows the complete code used to implement the shoe property grouping.

Attached Properties

 Boy {
     name: "Leo Hodges"
     shoe { size: 10; color: "black"; brand: "Reebok"; price: 59.95 }

     BirthdayParty.rsvp: "2009-07-06"
 }

The QML snippet shown above assigns a date to the rsvp property using the attached property syntax.

Attached properties allow unrelated types to annotate other types with some additional properties, generally for their own use. Attached properties are identified through the use of the attacher type name, in the case shown BirthdayParty, as a prefix to the property name.

In the example shown, BirthdayParty is called the attaching type, and the Boy instance the attachee object instance.

For the attaching type, an attached property block is implemented as a new QObject derived type, called the attachment object. The properties on the attachment object are those that become available for use as the attached property block.

Any QML type can become an attaching type by declaring the qmlAttachedProperties() public function and declaring that the class has QML_HAS_ATTACHED_PROPERTIES:

 class MyType : public QObject {
     Q_OBJECT
 public:

     ...

     static AttachedPropertiesType *qmlAttachedProperties(QObject *object);
 };

 QML_DECLARE_TYPEINFO(MyType, QML_HAS_ATTACHED_PROPERTIES)

This returns an attachment object, of type AttachedPropertiesType, for the attachee object instance. It is customary, though not strictly required, for the attachment object to be parented to object to prevent memory leaks.

AttachedPropertiesType must be a QObject derived type. The properties on this type will be accessible through the attached properties syntax.

This method will be called at most once for each attachee object instance. The QML engine will cache the returned instance pointer for subsequent attached property accesses. Consequently the attachment object may not be deleted until object is destroyed.

Conceptually, attached properties are a type exporting a set of additional properties that can be set on any other object instance. Attached properties cannot be limited to only attaching to a sub-set of object instances, although their effect may be so limited.

For example, a common usage scenario is for a type to enhance the properties available to its children in order to gather instance specific data. Here we add a rsvp field to all the guests coming to a birthday party:

 BirthdayParty {
     Boy { BirthdayParty.rsvp: "2009-06-01" }
 }

However, as a type cannot limit the instances to which the attachment object must attach, the following is also allowed, even though adding a birthday party rsvp in this context will have no effect.

 GraduationParty {
     Boy { BirthdayParty.rsvp: "2009-06-01" }
 }

From C++, including the attaching type implementation, the attachment object for an instance can be accessed using the following method:

 template<typename T>
 QObject *qmlAttachedPropertiesObject<T>(QObject *attachee, bool create = true);

This returns the attachment object attached to attachee by the attaching type T. If type T is not a valid attaching type, this method always returns 0.

If create is true, a valid attachment object will always be returned, creating it if it does not already exist. If create is false, the attachment object will only be returned if it has previously been created.

Extending QML - Attached Properties Example shows the complete code used to implement the rsvp attached property.

Memory Management and QVariant types

It is an element's responsibility to ensure that it does not access or return pointers to invalid objects. QML makes the following guarentees:

  • An object assigned to a QObject (or QObject-derived) pointer property will be valid at the time of assignment.

    Following assignment, it is the responsibility of the class to subsequently guard this pointer, either through a class specific method or the generic QPointer class.

  • An object assigned to a QVariant will be valid at the time of assignment.

    When assigning an object to a QVariant property, QML will always use a QMetaType::QObjectStar typed QVariant. It is the responsibility of the class to guard the pointer. A general rule when writing a class that uses QVariant properties is to check the type of the QVariant when it is set and if the type is not handled by your class, reset it to an invalid variant.

  • An object assigned to a QObject (or QObject-derived) list property will be valid at the time of assignment.

    Following assignment, it is the responsibility of the class to subsequently guard this pointer, either through a class specific method or the generic QPointer class.

Elements should assume that any QML assigned object can be deleted at any time, and respond accordingly. If documented as such an element need not continue to work in this situation, but it must not crash.

Signal Support

 BirthdayParty {
     onPartyStarted: console.log("This party started rockin' at " + time);
 }

The QML snippet shown above associates the evaluation of a JavaScript expression with the emission of a Qt signal.

All Qt signals on a registered class become available as special "signal properties" within QML to which the user can assign a single JavaScript expression. The signal property's name is a transformed version of the Qt signal name: "on" is prepended, and the first letter of the signal name upper cased. For example, the signal used in the example above has the following C++ signature:

 signals:
     void partyStarted(const QTime &time);

In classes with multiple signals with the same name, only the final signal is accessible as a signal property. Note that signals with the same name but different parameters cannot be distinguished.

Signal parameters become accessible by name to the assigned script. An unnamed parameter cannot be accessed, so care should be taken to name all the signal parameters in the C++ class declaration. The intrinsic types listed in Adding Types, as well registered object types are permitted as signal parameter types. Using other types is not an error, but the parameter value will not be accessible from script.

Extending QML - Signal Support Example shows the complete code used to implement the onPartyStarted signal property.

If you want to use signals from items not created in QML, you can access their signals with the Connections element.

Additionally, if a property is added to a C++ class, all QML elements based on that C++ class will have a value-changed signal handler for that property. The name of the signal handler is on<Property-name>Changed, with the first letter of the property name being upper case.

Note: The QML signal handler will always be named on<Property-name>Changed, regardless of the name used for the NOTIFY signal in C++. We recommend using <property-name>Changed() for the NOTIFY signal in C++.

See also Importing Reusable Components

Methods

Slots and methods marked Q_INVOKABLE may be called as functions in QML.

 BirthdayParty {
     host: Person {
         name: "Bob Jones"
         shoeSize: 12
     }
     guests: [
         Person { name: "Leo Hodges" },
         Person { name: "Jack Smith" },
         Person { name: "Anne Brown" }
     ]

     Component.onCompleted: invite("William Green")
 }

In this example an invitation is added via an invite() invokable method of the BirthdayParty element. This function is available in QML by marking the invite() method with Q_INVOKABLE in the BirthdayParty class:

     Q_INVOKABLE void invite(const QString &name);

Extending QML - Methods Example shows the complete code used to implement the invite() method.

The invite() method is similarly available if it is declared as a slot.

Property Value Sources

 BirthdayParty {
     HappyBirthdaySong on announcement { name: "Bob Jones" }
 }

The QML snippet shown above applies a property value source to the announcement property. A property value source generates a value for a property that changes over time.

Property value sources are most commonly used to do animation. Rather than constructing an animation object and manually setting the animation's "target" property, a property value source can be assigned directly to a property of any type and automatically set up this association.

The example shown here is rather contrived: the announcement property of the BirthdayParty object is a string that is printed every time it is assigned and the HappyBirthdaySong value source generates the lyrics of the song "Happy Birthday".

     Q_PROPERTY(QString announcement READ announcement WRITE setAnnouncement)

Normally, assigning an object to a string property would not be allowed. In the case of a property value source, rather than assigning the object instance itself, the QML engine sets up an association between the value source and the property.

Property value sources are special types that derive from the QDeclarativePropertyValueSource base class. This base class contains a single method, QDeclarativePropertyValueSource::setTarget(), that the QML engine invokes when associating the property value source with a property. The relevant part of the HappyBirthdaySong type declaration looks like this:

 class HappyBirthdaySong : public QObject, public QDeclarativePropertyValueSource
 {
     Q_OBJECT
     Q_INTERFACES(QDeclarativePropertyValueSource)
 public:
     HappyBirthdaySong(QObject *parent = 0);

     virtual void setTarget(const QDeclarativeProperty &);
 };

In all other respects, property value sources are regular QML types. They must be registered with the QML engine using the same macros as other types, and can contain properties, signals and methods just like other types.

When a property value source object is assigned to a property, QML first tries to assign it normally, as though it were a regular QML type. Only if this assignment fails does the engine call the setTarget() method. This allows the type to also be used in contexts other than just as a value source.

Extending QML - Property Value Source Example shows the complete code used to implement the HappyBirthdaySong property value source.

Property Binding

 BirthdayParty {
     id: theParty

     HappyBirthdaySong on announcement { name: theParty.host.name }

     host: Boy {
         name: "Bob Jones"
         shoe { size: 12; color: "white"; brand: "Nike"; price: 90.0 }
     }
 }

The QML snippet shown above uses a property binding to ensure the HappyBirthdaySong's name property remains up to date with the host.

Property binding is a core feature of QML. In addition to assigning literal values, property bindings allow the developer to assign an arbitrarily complex JavaScript expression that may include dependencies on other property values. Whenever the expression's result changes - through a change in one of its constituent values - the expression is automatically reevaluated and the new result assigned to the property.

All properties on custom types automatically support property binding. However, for binding to work correctly, QML must be able to reliably determine when a property has changed so that it knows to reevaluate any bindings that depend on the property's value. QML relies on the presence of a NOTIFY signal for this determination.

Here is the host property declaration:

     Q_PROPERTY(Person *host READ host WRITE setHost NOTIFY hostChanged)

The NOTIFY attribute is followed by a signal name. It is the responsibility of the class implementer to ensure that whenever the property's value changes, the NOTIFY signal is emitted. The signature of the NOTIFY signal is not important to QML.

To prevent loops or excessive evaluation, developers should ensure that the signal is only emitted whenever the property's value is actually changed. If a property, or group of properties, is infrequently used it is permitted to use the same NOTIFY signal for several properties. This should be done with care to ensure that performance doesn't suffer.

To keep QML reliable, if a property does not have a NOTIFY signal, it cannot be used in a binding expression. However, the property can still be assigned a binding as QML does not need to monitor the property for change in that scenario.

Consider a custom type, TestElement, that has two properties, "a" and "b". Property "a" does not have a NOTIFY signal, and property "b" does have a NOTIFY signal.

 TestElement {
     // This is OK
     a: b
 }
 TestElement {
     // Will NOT work
     b: a
 }

The presence of a NOTIFY signal does incur a small overhead. There are cases where a property's value is set at object construction time, and does not subsequently change. The most common case of this is when a type uses Grouped Properties, and the grouped property object is allocated once, and only freed when the object is deleted. In these cases, the CONSTANT attribute may be added to the property declaration instead of a NOTIFY signal.

     Q_PROPERTY(ShoeDescription *shoe READ shoe CONSTANT)

Extreme care must be taken here or applications using your type may misbehave. The CONSTANT attribute should only be used for properties whose value is set, and finalized, only in the class constructor. All other properties that want to be used in bindings should have a NOTIFY signal instead.

Extending QML - Binding Example shows the BirthdayParty example updated to include NOTIFY signals for use in binding.

Extension Objects

 QLineEdit {
     leftMargin: 20
 }

The QML snippet shown above adds a new property to an existing C++ type without modifying its source code.

When integrating existing classes and technology into QML, their APIs will often need to be tweaked to fit better into the declarative environment. Although the best results are usually obtained by modifying the original classes directly, if this is either not possible or is complicated by some other concerns, extension objects allow limited extension possibilities without direct modifications.

Extension objects are used to add additional properties to an existing type. Extension objects can only add properties, not signals or methods. An extended type definition allows the programmer to supply an additional type - known as the extension type - when registering the target class whose properties are transparently merged with the original target class when used from within QML.

An extension class is a regular QObject, with a constructor that takes a QObject pointer. When needed (extension class creation is delayed until the first extended property is accessed) the extension class is created and the target object is passed in as the parent. When an extended property on the original is accessed, the appropriate property on the extension object is used instead.

When an extended type is installed, one of the

 template<typename T, typename ExtendedT>
 int qmlRegisterExtendedType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

 template<typename T, typename ExtendedT>
 int qmlRegisterExtendedType()

functions should be used instead of the regular qmlRegisterType() variations. The arguments are identical to the corresponding non-extension registration functions, except for the ExtendedT parameter which is the type of the extension object.

Optimization

Often to develop high performance elements it is helpful to know more about the status of the QML engine. For example, it might be beneficial to delay initializing some costly data structures until after all the properties have been set.

The QML engine defines an interface class called QDeclarativeParserStatus, which contains a number of virtual methods that are invoked at various stages during component instantiation. To receive these notifications, an element implementation inherits QDeclarativeParserStatus and notifies the Qt meta system using the Q_INTERFACES() macro.

For example,

 class Example : public QObject, public QDeclarativeParserStatus
 {
     Q_OBJECT
     Q_INTERFACES(QDeclarativeParserStatus)
 public:
     virtual void componentComplete()
     {
         qDebug() << "Woohoo!  Now to do my costly initialization";
     }
 };
Notes provided by the Qt Community

No notes