内容

• 数据类型处理和所有权
• 暴露特性
• 具有对象类型的特性
• 具有对象列表类型的特性
• 分组特性
• 暴露方法 (包括 Qt 槽)
• 暴露信号

将 C++ 类型属性暴露给 QML

QML can easily be extended with functionality defined in C++ code. Due to the tight integration of the QML engine with the Qt 元对象系统 , any functionality that is appropriately exposed by a QObject -derived class is accessible from QML code. This enables C++ data and functions to be accessible directly from QML, often with little or no modification.

QML 引擎有能力自省 QObject instances through the meta-object system. This means any QML code can access the following members of an instance of a QObject 派生类：

• 特性
• 方法 (提供它们是公共槽或标记采用 Q_INVOKABLE )
• 信号

(Additionally, enums are available if they have been declared with Q_ENUMS. See 在 QML 和 C++ 之间的数据类型转换 了解更多细节)。

In general, these are accessible from QML regardless of whether a QObject -derived class has been registered with the QML type system . However, if a class is to be used in a way that requires the engine to access additional type information — for example, if the class itself is to be used as a method parameter or property, or if one of its enum types is to be used in this way — then the class may need to be registered.

Also note that a number of the important concepts covered in this document are demonstrated in the 采用 C++ 编写 QML 扩展 教程。

数据类型处理和所有权

Any data that is transferred from C++ to QML, whether as a property value, a method parameter or return value, or a signal parameter value, must be of a type that is supported by the QML engine.

By default, the engine supports a number of Qt C++ types and can automatically convert them as appropriately when used from QML. Additionally, C++ classes that are registered with the QML type system can be used as data types, as can their enums if appropriately registered. See 在 QML 和 C++ 之间的数据类型转换 for further information.

Additionally, data ownership rules are taken into consideration when data is transferred from C++ to QML. See 数据所有权 了解更多细节。

暴露特性

A property can be specified for any QObject -derived class using the Q_PROPERTY () macro. A property is a class data member with an associated read function and optional write function.

所有特性对于 QObject -derived class are accessible from QML.

例如，下文 Message 类具有 author property. As specified by the Q_PROPERTY macro call, this property is readable through the author() method, and writable through the setAuthor() 方法：

class Message : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged)
public:
    void setAuthor(const QString &a) {
        if (a != m_author) {
            m_author = a;
            emit authorChanged();
        }
    }
    QString author() const {
        return m_author;
    }
signals:
    void authorChanged();
private:
    QString m_author;
};
					

If an instance of this class was set as a context property when loading a file named MyItem.qml from C++:

int main(int argc, char *argv[]) {
    QGuiApplication app(argc, argv);
    QQuickView view;
    Message msg;
    view.engine()->rootContext()->setContextProperty("msg", &msg);
    view.setSource(QUrl::fromLocalFile("MyItem.qml"));
    view.show();
    return app.exec();
}
					

Then, the author property could be read from MyItem.qml :

// MyItem.qml
import QtQuick 2.0
Text {
    width: 100; height: 100
    text: msg.author    // invokes Message::author() to get this value
    Component.onCompleted: {
        msg.author = "Jonah"  // invokes Message::setAuthor()
    }
}
					

For maximum interoperability with QML, any property that is writable should have an associated NOTIFY signal that is emitted whenever the property value has changed. This allows the property to be used with 特性绑定 , which is an essential feature of QML that enforces relationships between properties by automatically updating a property whenever any of its dependencies change in value.

In the above example, the associated NOTIFY signal for the author 特性为 authorChanged , as specified in the Q_PROPERTY () macro call. This means that whenever the signal is emitted — as it is when the author changes in Message::setAuthor() — this notifies the QML engine that any bindings involving the author property must be updated, and in turn, the engine will update the text property by calling Message::author() 再次。

若 author property was writable but did not have an associated NOTIFY signal, the text value would be initialized with the initial value returned by Message::author() but would not be updated with any later changes to this property. In addition, any attempts to bind to the property from QML will produce a runtime warning from the engine.

注意： It is recommended that the NOTIFY signal be named <property>Changed where <property> is the name of the property. The associated property change signal handler generated by the QML engine will always take the form on<Property>Changed , regardless of the name of the related C++ signal, so it is recommended that the signal name follows this convention to avoid any confusion.

Notes on Use of Notify Signals

To prevent loops or excessive evaluation, developers should ensure that the property change signal is only emitted when the property value has actually changed. Also, 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.

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 分组特性 , 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.

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.

具有对象类型的特性

Object-type properties are accessible from QML providing that the object type has been appropriately registered with the QML type system.

例如， Message type might have a body property of type MessageBody* :

class Message : public QObject
{
    Q_OBJECT
    Q_PROPERTY(MessageBody* body READ body WRITE setBody NOTIFY bodyChanged)
public:
    MessageBody* body() const;
    void setBody(MessageBody* body);
};
class MessageBody : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE text NOTIFY textChanged)
// ...
}
					

Suppose the Message type was registered with the QML type system, allowing it to be used as an object type from QML code:

Message {
    // ...
}
					

若 MessageBody type was also registered with the type system, it would be possible to assign MessageBody 到 body property of a Message , all from within QML code:

Message {
    body: MessageBody {
        text: "Hello, world!"
    }
}
					

具有对象列表类型的特性

Properties containing lists of QObject -derived types can also be exposed to QML. For this purpose, however, one should use QQmlListProperty 而不是 QList <T> as the property type. This is because QList is not a QObject -derived type, and so cannot provide the necessary QML property characteristics through the Qt meta object system, such as signal notifications when a list is modified.

QQmlListProperty is a template class that can be conveniently constructed from a QList 值。

例如， MessageBoard class below has a messages property of type QQmlListProperty that stores a list of Message instances:

class MessageBoard : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QQmlListProperty<Message> messages READ messages)
public:
    QQmlListProperty<Message> messages();
private:
    static void append_message(QQmlListProperty<Message> *list, Message *msg);
    QList<Message *> m_messages;
};
					

The MessageBoard::messages() function simply creates and returns a QQmlListProperty 从其 QList <T> m_messages member, passing the appropriate list modification functions as required by the QQmlListProperty 构造函数：

QQmlListProperty<Message> MessageBoard::messages()
{
    return QQmlListProperty<Message>(this, 0, &MessageBoard::append_message);
}
void MessageBoard::append_message(QQmlListProperty<Message> *list, Message *msg)
{
    MessageBoard *msgBoard = qobject_cast<MessageBoard *>(list->object);
    if (msg)
        msgBoard->m_messages.append(msg);
}
					

Note that the template class type for the QQmlListProperty — in this case, Message — must be registered with the QML type system.

分组特性

Any read-only object-type property is accessible from QML code as a grouped property . This can be used to expose a group of related properties that describe a set of attributes for a type.

例如，假设 Message::author property was of type MessageAuthor rather than a simple string, with sub-properties of 名称 and email :

class MessageAuthor : public QObject
{
    Q_PROPERTY(QString name READ name WRITE setName)
    Q_PROPERTY(QString email READ email WRITE setEmail)
public:
    ...
};
class Message : public QObject
{
    Q_OBJECT
    Q_PROPERTY(MessageAuthor* author READ author)
public:
    Message(QObject *parent)
        : QObject(parent), m_author(new MessageAuthor(this))
    {
    }
    MessageAuthor *author() const {
        return m_author;
    }
private:
    MessageAuthor *m_author;
};
					

The author property could be written to using the grouped property syntax in QML, like this:

Message {
    author.name: "Alexandra"
    author.email: "alexandra@mail.com"
}
					

A type that is exposed as a grouped property differs from an object-type property in that the grouped property is read-only, and is initialized to a valid value by the parent object at construction. The grouped property's sub-properties may be modified from QML but the grouped property object itself will never change, whereas an object-type property may be assigned a new object value from QML at any time. Thus, the lifetime of a grouped property object is controlled strictly by the C++ parent implementation, whereas an object-type property can be freely created and destroyed through QML code.

暴露方法 (包括 Qt 槽)

Any method of a QObject -derived type is accessible from QML code if it is:

• A public method flagged with the Q_INVOKABLE() macro
• A method that is a public Qt slot

例如， MessageBoard class below has a postMessage() method that has been flagged with the Q_INVOKABLE macro, as well as a refresh() method that is a public slot:

class MessageBoard : public QObject
{
    Q_OBJECT
public:
    Q_INVOKABLE bool postMessage(const QString &msg) {
        qDebug() << "Called the C++ method with" << msg;
        return true;
    }
public slots:
    void refresh() {
        qDebug() << "Called the C++ slot";
    }
};
					

If an instance of MessageBoard was set as the context data for a file MyItem.qml ，那么 MyItem.qml could invoke the two methods as shown in the examples below:

int main(int argc, char *argv[]) {
    QGuiApplication app(argc, argv);
    MessageBoard msgBoard;
    QQuickView view;
    view.engine()->rootContext()->setContextProperty("msgBoard", &msgBoard);
    view.setSource(QUrl::fromLocalFile("MyItem.qml"));
    view.show();
    return app.exec();
}
								

// MyItem.qml
import QtQuick 2.0
Item {
    width: 100; height: 100
    MouseArea {
        anchors.fill: parent
        onClicked: {
            var result = msgBoard.postMessage("Hello from QML")
            console.log("Result of postMessage():", result)
            msgBoard.refresh();
        }
    }
}
								

If a C++ method has a parameter with a QObject* type, the parameter value can be passed from QML using an object id or a JavaScript var value that references the object.

QML supports the calling of overloaded C++ functions. If there are multiple C++ functions with the same name but different arguments, the correct function will be called according to the number and the types of arguments that are provided.

Values returned from C++ methods are converted to JavaScript values when accessed from JavaScript expressions in QML.

暴露信号

任何公共 signal 的 QObject -derived type is accessible from QML code.

The QML engine automatically creates a 信号处理程序 for any signal of a QObject -derived type that is used from QML. Signal handlers are always named on<Signal> where <Signal> is the name of the signal, with the first letter capitalized. All parameters passed by the signal are available in the signal handler through the parameter names.

例如，假设 MessageBoard 类拥有 newMessagePosted() signal with a single parameter, subject :

class MessageBoard : public QObject
{
    Q_OBJECT
public:
   // ...
signals:
   void newMessagePosted(const QString &subject);
};
					

若 MessageBoard type was registered with the QML type system, then a MessageBoard object declared in QML could receive the newMessagePosted() signal using a signal handler named onNewMessagePosted , and examine the subject 参数值：

MessageBoard {
    onNewMessagePosted: console.log("New message received:", subject)
}
					

As with property values and method parameters, a signal parameter must have a type that is supported by the QML engine; see 在 QML 和 C++ 之间的数据类型转换 . (Using an unregistered type will not generate an error, but the parameter value will not be accessible from the handler.)

Classes may have multiple signals with the same name, but only the final signal is accessible as a QML signal. Note that signals with the same name but different parameters cannot be distinguished from one another.