When extending QML with C++ code, a C++ class can be registered with the QML type system to enable the class to be used as a data type within QML code. While the properties, methods and signals of any QObject -derived class are accessible from QML, as discussed in 将 C++ 类型属性暴露给 QML , such a class cannot be used as a data type from QML until it is registered with the type system. Additionally registration can provide other features, such as allowing a class to be used as an instantiable QML object type from QML, or enabling a singleton instance of the class to be imported and used from QML.
此外， Qt QML module provides mechanisms for implementing QML-specific features such as attached properties and default properties in C++.
(Note that a number of the important concepts covered in this document are demonstrated in the 采用 C++ 编写 QML 扩展 tutorial.)
NOTE: All headers that declare QML types need to be accessible without any prefix from the project's include path.
QObject -derived class can be registered with the QML type system to enable the type to be used as a data type from within QML code.
The engine allows the registration of both instantiable and non-instantiable types. Registering an instantiable type enables a C++ class to be used as the definition of a QML object type, allowing it to be used in object declarations from QML code to create objects of this type. Registration also provides the engine with additional type metadata, enabling the type (and any enums declared by the class) to be used as a data type for property values, method parameters and return values, and signal parameters that are exchanged between QML and C++.
Registering a non-instantiable type also registers the class as a data type in this manner, but the type cannot be used instantiated as a QML object type from QML. This is useful, for example, if a type has enums that should be exposed to QML but the type itself should not be instantiable.
For a quick guide to choosing the correct approach to expose C++ types to QML, see Choosing the Correct Integration Method Between C++ and QML .
All the macros mentioned below are available from the
qqmlregistration.h
header. You need to add the following code to the files using them in order to make the macros available:
#include <QtQml/qqmlregistration.h>
Furthermore, your class declarations have to live in headers reachable via your project's include path. The declarations are used to generate registration code at compile time, and the registration code needs to include the headers that contain the declarations.
任何 QObject -derived C++ class can be registered as the definition of a QML object type . Once a class is registered with the QML type system, the class can be declared and instantiated like any other object type from QML code. Once created, a class instance can be manipulated from QML; as 将 C++ 类型属性暴露给 QML explains, the properties, methods and signals of any QObject -derived class are accessible from QML code.
To register a
QObject
-derived class as an instantiable QML object type, add
QML_ELEMENT
or
QML_NAMED_ELEMENT(<name>)
to the class declaration and
CONFIG += qmltypes
，
QML_IMPORT_NAME
，和
QML_IMPORT_MAJOR_VERSION
to your project file. This will register the class into the type namespace under the given major version, using either the class name or an explicitly given name as QML type name. The minor version(s) will be derived from any revisions attached to properties, methods, or signals. The default minor version is
0
. You can explicitly restrict the type to be available only from specific minor versions by adding the
QML_ADDED_IN_MINOR_VERSION()
macro to the class declaration. Clients can import suitable versions of the namespace in order to use the type.
For example, suppose there is a
Message
class with
author
and
creationDate
properties:
class Message : public QObject { Q_OBJECT Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged) Q_PROPERTY(QDateTime creationDate READ creationDate WRITE setCreationDate NOTIFY creationDateChanged) QML_ELEMENT public: // ... };
This type can be registered by adding an appropriate type namespace and version number to the project file. For example, to make the type available in the
com.mycompany.messaging
namespace with version 1.0:
CONFIG += qmltypes QML_IMPORT_NAME = com.mycompany.messaging QML_IMPORT_MAJOR_VERSION = 1
If the header the class is declared in is not accessible from your project's include path, you may have to amend the include path so that the generated registration code can be compiled:
INCLUDEPATH += com/mycompany/messaging
The type can be used in an object declaration from QML, and its properties can be read and written to, as per the example below:
import com.mycompany.messaging 1.0 Message { author: "Amelie" creationDate: new Date() }
Sometimes a QObject -derived class may need to be registered with the QML type system but not as an instantiable type. For example, this is the case if a C++ class:
Qt QML module provides several macros for registering non-instantiable types:
Note that all C++ types registered with the QML type system must be QObject -derived, even if they are non-instantiable.
A singleton type enables properties, signals and methods to be exposed in a namespace without requiring the client to manually instantiate an object instance. QObject singleton types in particular are an efficient and convenient way to provide functionality or global property values.
Note that singleton types do not have an associated QQmlContext as they are shared across all contexts in an engine. QObject singleton type instances are constructed and owned by the QQmlEngine , and will be destroyed when the engine is destroyed.
QObject singleton type can be interacted with in a manner similar to any other QObject or instantiated type, except that only one (engine constructed and owned) instance will exist, and it must be referenced by type name rather than id. Q_PROPERTYs of QObject singleton types may be bound to, and Q_INVOKABLE functions of QObject module APIs may be used in signal handler expressions. This makes singleton types an ideal way to implement styling or theming, and they can also be used instead of ".pragma library" script imports to store global state or to provide global functionality.
Once registered, a QObject singleton type may be imported and used like any other QObject instance exposed to QML. The following example assumes that a QObject singleton type was registered into the "MyThemeModule" namespace with version 1.0, where that QObject 拥有 QColor "color" Q_PROPERTY :
import MyThemeModule 1.0 as Theme Rectangle { color: Theme.color // binding. }
A QJSValue may also be exposed as a singleton type, however clients should be aware that properties of such a singleton type cannot be bound to.
见 QML_SINGLETON for more information on how implement and register a new singleton type, and how to use an existing singleton type.
注意： Enum values for registered types in QML should start with a capital.
Properties declared final using the
FINAL
modifier to
Q_PROPERTY
cannot be overridden. This means that any properties or functions of the same name, declared either in QML or in C++ on derived types, are ignored by the QML engine. You should declare properties
FINAL
when possible, in order to avoid accidental overrides. An override of a property is visible not only in derived classes, but also to QML code executing the context of the base class. Such QML code, typically expects the original property, though. This is a frequent source of mistakes.
Properties declared
FINAL
can also not be overridden by functions in QML, or by
Q_INVOKABLE
methods in C++.
Many of the type registration functions require versions to be specified for the registered type. Type revisions and versions allow new properties or methods to exist in the new version while remaining compatible with previous versions.
Consider these two QML files:
// main.qml import QtQuick 1.0 Item { id: root MyType {} }
// MyType.qml import MyTypes 1.0 CppType { value: root.x }
where
CppType
maps to the C++ class
CppType
.
If the author of CppType adds a
root
property to CppType in a new version of their type definition,
root.x
now resolves to a different value because
root
is also the
id
of the top level component. The author could specify that the new
root
property is available from a specific minor version. This permits new properties and features to be added to existing types without breaking existing programs.
The REVISION tag is used to mark the
root
property as added in revision 1 of the type. Methods such as
Q_INVOKABLE
's, signals and slots can also be tagged for a revision using the
Q_REVISION(x)
宏：
class CppType : public BaseType { Q_OBJECT Q_PROPERTY(int root READ root WRITE setRoot NOTIFY rootChanged REVISION 1) QML_ELEMENT signals: Q_REVISION(1) void rootChanged(); };
The revisions given this way are automatically interpreted as minor versions to the major version given in the project file. In this case,
root
is only available when
MyTypes
version 1.1 or higher is imported. Imports of
MyTypes
version 1.0 remain unaffected.
For the same reason, new types introduced in later versions should be tagged with the QML_ADDED_IN_MINOR_VERSION 宏。
This feature of the language allows for behavioural changes to be made without breaking existing applications. Consequently QML module authors should always remember to document what changed between minor versions, and QML module users should check that their application still runs correctly before deploying an updated import statement.
Revisions of a base class that your type depends upon are automatically registered when registering the type itself. This is useful when deriving from base classes provided by other authors, e.g. when extending classes from the Qt Quick module.
注意： The QML engine does not support revisions for properties or signals of grouped and attached property objects.
When integrating existing classes and technology into QML, APIs will often need tweaking 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 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 class. The properties are transparently merged with the original target class when used from within QML. For example:
QLineEdit { leftMargin: 20 }
leftMargin
property is a new property added to an existing C++ type,
QLineEdit
, without modifying its source code.
QML_EXTENDED (extended) macro is for registering extended types. The argument is the name of another class to be used as extension.
还可以使用 QML_EXTENDED_NAMESPACE (namespace) to register a namespace, and especially the enumerations declared within, as an extension to a type.
An extension class is a regular QObject , with a constructor that takes a QObject pointer. However, the 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 the property on the original is accessed, the corresponding property on the extension object is used instead.
Extension Objects Example demonstrates a usage of extension objects.
There may be C++ types that cannot be modified to hold the above mentioned macros. Those may be types from 3rdparty libraries, or types that need to fulfill some contract that contradicts the presence of those macros. You can still expose those types to QML, though, using the QML_FOREIGN macro. In order to do this, create a separate struct that consists entirely of the registration macros, like this:
// Contains class Immutable3rdParty #include <3rdpartyheader.h> struct Foreign { Q_GADGET QML_FOREIGN(Immutable3rdParty) QML_NAMED_ELEMENT(Accessible3rdParty) QML_ADDED_IN_VERSION(2, 4) // QML_EXTENDED, QML_SINGLETON ... };
From this code, you get a QML type with the methods and properties of Immutable3rdParty, and the QML traits (e.g.: singleton, extended) specified in Foreign.
In the QML language syntax, there is a notion of attached properties and attached signal handlers , which are additional attributes that are attached to an object. Essentially, such attributes are implemented and provided by an attaching type , and these attributes may be attached to an object of another type. This contrasts with ordinary object properties which are provided by the object type itself (or the object's inherited type).
例如， Item below uses attached properties and attached handlers:
import QtQuick 2.0 Item { width: 100; height: 100 focus: true Keys.enabled: false Keys.onReturnPressed: console.log("Return key was pressed") }
在这里，
Item
object is able to access and set the values of
Keys.enabled
and
Keys.onReturnPressed
. This allows the
Item
object to access these extra attributes as an extension to its own existing attributes.
When considering the above example, there are several parties involved:
enabled
和
returnPressed
signal, that has been attached to the
Item
object to enable it to access and set these attributes.
When the QML engine processes this code, it creates a single instance of the
attached object type
and attaches this instance to the
Item
object, thereby providing it with access to the
enabled
and
returnPressed
attributes of the instance.
The mechanisms for providing attached objects can be implemented from C++ by providing classes for the attached object type and attaching type . For the attached object type , provide a QObject -derived class that defines the attributes to be made accessible to attachee objects. For the attaching type , provide a QObject -derived class that:
static <AttachedPropertiesType> *qmlAttachedProperties(QObject *object);
This method should return an instance of the attached object type .
The QML engine invokes this method in order to attach an instance of the attached object type to the
attachee
指定通过
对象
parameter. It is customary, though not strictly required, for this method implementation to parent the returned instance to
对象
in order to prevent memory leaks.
This method is called at most once by the engine for each attachee object instance, as the engine caches the returned instance pointer for subsequent attached property accesses. Consequently the attachment object may not be deleted until the attachee
对象
被销毁。
For example, take the
Message
type described in an
earlier example
:
class Message : public QObject { Q_OBJECT Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged) Q_PROPERTY(QDateTime creationDate READ creationDate WRITE setCreationDate NOTIFY creationDateChanged) QML_ELEMENT public: // ... };
Suppose it is necessary to trigger a signal on a
Message
when it is published to a message board, and also track when the message has expired on the message board. Since it doesn't make sense to add these attributes directly to a
Message
, as the attributes are more relevant to the message board context, they could be implemented as
attached
attributes on a
Message
object that are provided through a "MessageBoard" qualifier. In terms of the concepts described earlier, the parties involved here are:
published
signal and an expired property. This type is implemented by
MessageBoardAttachedType
below
Message
object, which will be the
attachee
MessageBoard
type, which will be the
attaching type
that is used by
Message
objects to access the attached attributes
Following is an example implementation. First, there needs to be an attached object type with the necessary properties and signals that will be accessible to the attachee :
class MessageBoardAttachedType : public QObject { Q_OBJECT Q_PROPERTY(bool expired READ expired WRITE setExpired NOTIFY expiredChanged) QML_ANONYMOUS public: MessageBoardAttachedType(QObject *parent); bool expired() const; void setExpired(bool expired); signals: void published(); void expiredChanged(); };
Then the
attaching type
,
MessageBoard
, must declare a
qmlAttachedProperties()
method that returns an instance of the
attached object type
as implemented by MessageBoardAttachedType. Additionally,
MessageBoard
must be declared as an attaching type via the
QML_ATTACHED
() 宏：
class MessageBoard : public QObject { Q_OBJECT QML_ATTACHED(MessageBoardAttachedType) QML_ELEMENT public: static MessageBoardAttachedType *qmlAttachedProperties(QObject *object) { return new MessageBoardAttachedType(object); } };
Now, a
Message
type can access the properties and signals of the attached object type:
Message { author: "Amelie" creationDate: new Date() MessageBoard.expired: creationDate < new Date("January 01, 2015 10:45:00") MessageBoard.onPublished: console.log("Message by", author, "has been published!") }
Additionally, the C++ implementation may access the attached object instance that has been attached to any object by calling the qmlAttachedPropertiesObject() function.
例如：
Message *msg = someMessageInstance(); MessageBoardAttachedType *attached = qobject_cast<MessageBoardAttachedType*>(qmlAttachedPropertiesObject<MessageBoard>(msg)); qDebug() << "Value of MessageBoard.expired:" << attached->expired();
A property modifier type is a special kind of QML object type. A property modifier type instance affects a property (of a QML object instance) which it is applied to. There are two different kinds of property modifier types:
A property value write interceptor can be used to filter or modify values as they are written to properties. Currently, the only supported property value write interceptor is the
Behavior
type provided by the
QtQuick
导入。
A property value source can be used to automatically update the value of a property over time. Clients can define their own property value source types. The various
property animation
types provided by the
QtQuick
import are examples of property value sources.
Property modifier type instances can be created and applied to a property of a QML object through the "<ModifierType> on <propertyName>" syntax, as the following example shows:
import QtQuick 2.0 Item { width: 400 height: 50 Rectangle { width: 50 height: 50 color: "red" NumberAnimation on x { from: 0 to: 350 loops: Animation.Infinite duration: 2000 } } }
This is commonly referred to as "on" syntax.
Clients can register their own property value source types, but currently not property value write interceptors.
Property value sources
are QML types that can automatically update the value of a property over time, using the
<PropertyValueSource> on <property>
syntax. For example, the various
property animation
types provided by the
QtQuick
module are examples of property value sources.
A property value source can be implemented in C++ by subclassing
QQmlPropertyValueSource
and providing an implementation that writes different values to a property over time. When the property value source is applied to a property using the
<PropertyValueSource> on <property>
syntax in QML, it is given a reference to this property by the engine so that the property value can be updated.
For example, suppose there is a
RandomNumberGenerator
class to be made available as a property value source, so that when applied to a QML property, it will update the property value to a different random number every 500 milliseconds. Additionally, a maxValue can be provided to this random number generator. This class can be implemented as follows:
class RandomNumberGenerator : public QObject, public QQmlPropertyValueSource { Q_OBJECT Q_INTERFACES(QQmlPropertyValueSource) Q_PROPERTY(int maxValue READ maxValue WRITE setMaxValue NOTIFY maxValueChanged); QML_ELEMENT public: RandomNumberGenerator(QObject *parent) : QObject(parent), m_maxValue(100) { QObject::connect(&m_timer, SIGNAL(timeout()), SLOT(updateProperty())); m_timer.start(500); } int maxValue() const; void setMaxValue(int maxValue); virtual void setTarget(const QQmlProperty &prop) { m_targetProperty = prop; } signals: void maxValueChanged(); private slots: void updateProperty() { m_targetProperty.write(QRandomGenerator::global()->bounded(m_maxValue)); } private: QQmlProperty m_targetProperty; QTimer m_timer; int m_maxValue; };
When the QML engine encounters a use of
RandomNumberGenerator
as a property value source, it invokes
RandomNumberGenerator::setTarget()
to provide the type with the property to which the value source has been applied. When the internal timer in
RandomNumberGenerator
triggers every 500 milliseconds, it will write a new number value to that specified property.
Once the
RandomNumberGenerator
class has been registered with the QML type system, it can be used from QML as a property value source. Below, it is used to change the width of a
Rectangle
every 500 milliseconds:
import QtQuick 2.0 Item { width: 300; height: 300 Rectangle { RandomNumberGenerator on width { maxValue: 300 } height: 100 color: "red" } }
In all other respects, property value sources are regular QML types that can have properties, signals methods and so on, but with the added capability that they can be used to change property values using the
<PropertyValueSource> on <property>
句法。
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.
任何 QObject -derived type that is registered as an instantiable QML object type can optionally specify a default property for the type. A default property is the property to which an object's children are automatically assigned if they are not assigned to any specific property.
The default property can be set by calling the
Q_CLASSINFO
() macro for a class with a specific "DefaultProperty" value. For example, the
MessageBoard
class below specifies its
messages
property as the default property for the class:
class MessageBoard : public QObject { Q_OBJECT Q_PROPERTY(QQmlListProperty<Message> messages READ messages) Q_CLASSINFO("DefaultProperty", "messages") QML_ELEMENT public: QQmlListProperty<Message> messages(); private: QList<Message *> m_messages; };
This enables children of a
MessageBoard
object to be automatically assigned to its
messages
property if they are not assigned to a specific property. For example:
MessageBoard { Message { author: "Naomi" } Message { author: "Clancy" } }
若
messages
was not set as the default property, then any
Message
objects would have to be explicitly assigned to the
messages
property instead, as follows:
MessageBoard { messages: [ Message { author: "Naomi" }, Message { author: "Clancy" } ] }
(Incidentally, the
Item::data
property is its default property. Any
Item
objects added to this
data
property are also added to the list of
Item::children
, so the use of the default property enables visual children to be declared for an item without explicitly assigning them to the
children
property.)
Additionally, you can declare a "ParentProperty" Q_CLASSINFO () to inform the QML engine which property should denote the parent object in the QML hierarchy. For example, the Message type might be declared as follows:
class Message : public QObject { Q_OBJECT Q_PROPERTY(QObject* board READ board BINDABLE boardBindable) Q_PROPERTY(QString author READ author BINDABLE authorBindable) Q_CLASSINFO("ParentProperty", "board") QML_ELEMENT public: Message(QObject *parent = nullptr) : QObject(parent) { m_board = parent; } QObject *board() const { return m_board.value(); } QBindable<QObject *> boardBindable() { return QBindable<QObject *>(&m_board); } QString author() const { return m_author.value(); } QBindable<QString> authorBindable() { return QBindable<QString>(&m_author); } private: QProperty<QObject *> m_board; QProperty<QString> m_author; };
Defining the parent property affords qmllint and other tools better insight into the intention of your code and avoids false positive warnings on some property accesses.
When building user interfaces with the Qt Quick module, all QML objects that are to be visually rendered must derive from the Item type, as it is the base type for all visual objects in Qt Quick 。此 Item type is implemented by the QQuickItem C++ class, which is provided by the Qt Quick module. Therefore, this class should be subclassed when it is necessary to implement a visual type in C++ that can be integrated into a QML-based user interface.
见 QQuickItem documentation for more information. Additionally, the 采用 C++ 编写 QML 扩展 tutorial demonstrates how a QQuickItem -based visual item can be implemented in C++ and integrated into a Qt Quick-based user interface.
For some custom QML object types, it may be beneficial to delay the initialization of particular data until the object has been created and all of its properties have been set. For example, this may be the case if the initialization is costly, or if the initialization should not be performed until all property values have been initialized.
Qt QML module provides the QQmlParserStatus to be subclassed for these purposes. It defines a number of virtual methods that are invoked at various stages during component instantiation. To receive these notifications, a C++ class should inherit QQmlParserStatus and also notify the Qt meta system using the Q_INTERFACES () 宏。
例如：
class MyQmlType : public QObject, public QQmlParserStatus { Q_OBJECT Q_INTERFACES(QQmlParserStatus) QML_ELEMENT public: virtual void componentComplete() { // Perform some initialization here now that the object is fully created } };