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采用 C++ 编写 QML 扩展

The Qt QML module provides a set of APIs for extending QML through C++ extensions. You can write extensions to add your own QML types, extend existing Qt types, or call C/C++ functions that are not accessible from ordinary QML code.

This tutorial shows how to write a QML extension using C++ that includes core QML features, including properties, signals and bindings. It also shows how extensions can be deployed through plugins.

Many of the topics covered in this tutorial are documented in further detail in 集成 QML 和 C++ and its documentation sub-topics. In particular, you may be interested in the sub-topics 把 C++ 类的属性暴露给 QML and 从 C++ 定义 QML 类型 .

运行教程范例

The code in this tutorial is available as an example project with subprojects associated with each tutorial chapter. In Qt Creator ，打开 欢迎 模式并选择教程从 范例 。在 编辑 mode, expand the extending-qml project, right-click on the subproject (chapter) you want to run and select 运行 .

第 1 章：创建新的类型

extending-qml/chapter1-basics

A common task when extending QML is to provide a new QML type that supports some custom functionality beyond what is provided by the built-in Qt Quick 类型 . For example, this could be done to implement particular data models, or provide types with custom painting and drawing capabilities, or access system features like network programming that are not accessible through built-in QML features.

In this tutorial, we will show how to use the C++ classes in the Qt Quick module to extend QML. The end result will be a simple Pie Chart display implemented by several custom QML types connected together through QML features like bindings and signals, and made available to the QML runtime through a plugin.

To begin with, let's create a new QML type called "PieChart" that has two properties: a name and a color. We will make it available in an importable type namespace called "Charts", with a version of 1.0.

We want this PieChart type to be usable from QML like this: 

import Charts 1.0
PieChart {
    width: 100; height: 100
    name: "A simple pie chart"
    color: "red"
}

要做到这，需要 C++ 类封装此 PieChart type and its two properties. Since QML makes extensive use of Qt's 元对象系统 ，此新类必须：

这里是 PieChart 类，定义在 piechart.h : 

#include <QtQuick/QQuickPaintedItem>
#include <QColor>
class PieChart : public QQuickPaintedItem
{
    Q_OBJECT
    Q_PROPERTY(QString name READ name WRITE setName)
    Q_PROPERTY(QColor color READ color WRITE setColor)
public:
    PieChart(QQuickItem *parent = 0);
    QString name() const;
    void setName(const QString &name);
    QColor color() const;
    void setColor(const QColor &color);
    void paint(QPainter *painter);
private:
    QString m_name;
    QColor m_color;
};

类继承自 QQuickPaintedItem 因为我们想要覆写 QQuickPaintedItem::paint () in perform drawing operations with the QPainter API. If the class just represented some data type and was not an item that actually needed to be displayed, it could simply inherit from QObject . Or, if we want to extend the functionality of an existing QObject -based class, it could inherit from that class instead. Alternatively, if we want to create a visual item that doesn't need to perform drawing operations with the QPainter API, we can just subclass QQuickItem .

The PieChart 类定义了 2 个特性 名称 and color ，采用 Q_PROPERTY 宏，并覆写 QQuickPaintedItem::paint (). The class implementation in piechart.cpp simply sets and returns the m_name and m_color values as appropriate, and implements paint() to draw a simple pie chart. It also turns off the QGraphicsItem::ItemHasNoContents flag to enable painting: 

PieChart::PieChart(QQuickItem *parent)
    : QQuickPaintedItem(parent)
{
}
...
void PieChart::paint(QPainter *painter)
{
    QPen pen(m_color, 2);
    painter->setPen(pen);
    painter->setRenderHints(QPainter::Antialiasing, true);
    painter->drawPie(boundingRect().adjusted(1, 1, -1, -1), 90 * 16, 290 * 16);
}

Now that we have defined the PieChart type, we will use it from QML. The app.qml file creates a PieChart item and display the pie chart's details using a standard QML Text item: 

import Charts 1.0
import QtQuick 2.0
Item {
    width: 300; height: 200
    PieChart {
        id: aPieChart
        anchors.centerIn: parent
        width: 100; height: 100
        name: "A simple pie chart"
        color: "red"
    }
    Text {
        anchors { bottom: parent.bottom; horizontalCenter: parent.horizontalCenter; bottomMargin: 20 }
        text: aPieChart.name
    }
}

Notice that although the color is specified as a string in QML, it is automatically converted to a QColor object for the PieChart color property. Automatic conversions are provided for various other 基本类型 ; for example, a string like "640x480" can be automatically converted to a QSize 值。

We'll also create a C++ application that uses a QQuickView to run and display app.qml . The application must register the PieChart type using the qmlRegisterType () function, to allow it to be used from QML. If you don't register the type, app.qml won't be able to create a PieChart .

这里是应用程序 main.cpp : 

#include "piechart.h"
#include <QtQuick/QQuickView>
#include <QGuiApplication>
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    qmlRegisterType<PieChart>("Charts", 1, 0, "PieChart");
    QQuickView view;
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.setSource(QUrl("qrc:///app.qml"));
    view.show();
    return app.exec();
}

This call to qmlRegisterType () registers the PieChart type as a type called "PieChart", in a type namespace called "Charts", with a version of 1.0.

Lastly, we write a .pro project file that includes the files and the declarative 库： 

QT += qml quick
HEADERS += piechart.h
SOURCES += piechart.cpp \
           main.cpp
RESOURCES += chapter1-basics.qrc
DESTPATH = $$[QT_INSTALL_EXAMPLES]/qml/tutorials/extending-qml/chapter1-basics
target.path = $$DESTPATH
qml.files = *.qml
qml.path = $$DESTPATH
INSTALLS += target qml

Now we can build and run the application:

注意： You may see a warning Expression ... depends on non-NOTIFYable properties: PieChart::name . This happens because we add a binding to the writable 名称 property, but haven't yet defined a notify signal for it. The QML engine therefore cannot update the binding if the 名称 value changes. This is addressed in the following chapters.

第 2 章：连接到 C++ 方法和信号

extending-qml/chapter2-methods

Suppose we want PieChart to have a "clearChart()" method that erases the chart and then emits a "chartCleared" signal. Our app.qml would be able to call clearChart() and receive chartCleared() signals like this: 

import Charts 1.0
import QtQuick 2.0
Item {
    width: 300; height: 200
    PieChart {
        id: aPieChart
        anchors.centerIn: parent
        width: 100; height: 100
        color: "red"
        onChartCleared: console.log("The chart has been cleared")
    }
    MouseArea {
        anchors.fill: parent
        onClicked: aPieChart.clearChart()
    }
    Text {
        anchors { bottom: parent.bottom; horizontalCenter: parent.horizontalCenter; bottomMargin: 20 }
        text: "Click anywhere to clear the chart"
    }
}

To do this, we add a clearChart() method and a chartCleared() signal to our C++ class: 

class PieChart : public QQuickPaintedItem
{
    ...
public:
    ...
    Q_INVOKABLE void clearChart();
signals:
    void chartCleared();
    ...
};

The use of Q_INVOKABLE makes the clearChart() method available to the Qt Meta-Object system, and in turn, to QML. Note that it could have been declared as a Qt slot instead of using Q_INVOKABLE , as slots are also callable from QML. Both of these approaches are valid.

The clearChart() method simply changes the color to Qt::transparent , repaints the chart, then emits the chartCleared() signal: 

void PieChart::clearChart()
{
    setColor(QColor(Qt::transparent));
    update();
    emit chartCleared();
}

Now when we run the application and click the window, the pie chart disappears, and the application outputs: 

qml: The chart has been cleared

第 3 章：添加特性绑定

extending-qml/chapter3-bindings

Property binding is a powerful feature of QML that allows values of different types to be synchronized automatically. It uses signals to notify and update other types' values when property values are changed.

Let's enable property bindings for the color property. That means if we have code like this: 

import Charts 1.0
import QtQuick 2.0
Item {
    width: 300; height: 200
    Row {
        anchors.centerIn: parent
        spacing: 20
        PieChart {
            id: chartA
            width: 100; height: 100
            color: "red"
        }
        PieChart {
            id: chartB
            width: 100; height: 100
            color: chartA.color
        }
    }
    MouseArea {
        anchors.fill: parent
        onClicked: { chartA.color = "blue" }
    }
    Text {
        anchors { bottom: parent.bottom; horizontalCenter: parent.horizontalCenter; bottomMargin: 20 }
        text: "Click anywhere to change the chart color"
    }
}

The "color: chartA.color" statement binds the color value of chartB 到 color of chartA . Whenever chartA 's color value changes, chartB 's color value updates to the same value. When the window is clicked, the onClicked handler in the MouseArea changes the color of chartA , thereby changing both charts to the color blue.

It's easy to enable property binding for the color property. We add a NOTIFY feature to its Q_PROPERTY () declaration to indicate that a "colorChanged" signal is emitted whenever the value changes. 

class PieChart : public QQuickPaintedItem
{
    ...
    Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
public:
    ...
signals:
    void colorChanged();
    ...
};

Then, we emit this signal in setPieSlice() : 

void PieChart::setColor(const QColor &color)
{
    if (color != m_color) {
        m_color = color;
        update();   // repaint with the new color
        emit colorChanged();
    }
}

It's important for setColor() to check that the color value has actually changed before emitting colorChanged() . This ensures the signal is not emitted unnecessarily and also prevents loops when other types respond to the value change.

The use of bindings is essential to QML. You should always add NOTIFY signals for properties if they are able to be implemented, so that your properties can be used in bindings. Properties that cannot be bound cannot be automatically updated and cannot be used as flexibly in QML. Also, since bindings are invoked so often and relied upon in QML usage, users of your custom QML types may see unexpected behavior if bindings are not implemented.

第 4 章：使用自定义特性类型

extending-qml/chapter4-customPropertyTypes

The PieChart type currently has a string-type property and a color-type property. It could have many other types of properties. For example, it could have an int-type property to store an identifier for each chart: 

// C++
class PieChart : public QQuickPaintedItem
{
    Q_PROPERTY(int chartId READ chartId WRITE setChartId NOTIFY chartIdChanged)
    ...
public:
    void setChartId(int chartId);
    int chartId() const;
    ...
signals:
    void chartIdChanged();
};
// QML
PieChart {
    ...
    chartId: 100
}

Aside from int , we could use various other property types. Many of the Qt data types such as QColor , QSize and QRect are automatically supported from QML. (See 在 QML 和 C++ 之间的数据类型转换 documentation for a full list.)

If we want to create a property whose type is not supported by QML by default, we need to register the type with the QML engine.

For example, let's replace the use of the property with a type called " PieSlice " that has a color property. Instead of assigning a color, we assign an PieSlice value which itself contains a color : 

import Charts 1.0
import QtQuick 2.0
Item {
    width: 300; height: 200
    PieChart {
        id: chart
        anchors.centerIn: parent
        width: 100; height: 100
        pieSlice: PieSlice {
            anchors.fill: parent
            color: "red"
        }
    }
    Component.onCompleted: console.log("The pie is colored " + chart.pieSlice.color)
}

像 PieChart ，此新 PieSlice 类型继承自 QQuickPaintedItem and declares its properties with Q_PROPERTY (): 

class PieSlice : public QQuickPaintedItem
{
    Q_OBJECT
    Q_PROPERTY(QColor color READ color WRITE setColor)
public:
    PieSlice(QQuickItem *parent = 0);
    QColor color() const;
    void setColor(const QColor &color);
    void paint(QPainter *painter);
private:
    QColor m_color;
};

要使用它在 PieChart , we modify the color property declaration and associated method signatures: 

class PieChart : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(PieSlice* pieSlice READ pieSlice WRITE setPieSlice)
    ...
public:
    ...
    PieSlice *pieSlice() const;
    void setPieSlice(PieSlice *pieSlice);
    ...
};

There is one thing to be aware of when implementing setPieSlice() 。 PieSlice is a visual item, so it must be set as a child of the PieChart 使用 QQuickItem::setParentItem () so that the PieChart knows to paint this child item when its contents are drawn: 

void PieChart::setPieSlice(PieSlice *pieSlice)
{
    m_pieSlice = pieSlice;
    pieSlice->setParentItem(this);
}

像 PieChart type, the PieSlice type has to be registered using qmlRegisterType () to be used from QML. As with PieChart , we'll add the type to the "Charts" type namespace, version 1.0: 

int main(int argc, char *argv[])
{
    ...
    qmlRegisterType<PieSlice>("Charts", 1, 0, "PieSlice");
    ...
}

第 5 章：使用列表特性类型

extending-qml/chapter5-listproperties

Right now, a PieChart can only have one PieSlice . Ideally a chart would have multiple slices, with different colors and sizes. To do this, we could have a slices property that accepts a list of PieSlice items: 

import Charts 1.0
import QtQuick 2.0
Item {
    width: 300; height: 200
    PieChart {
        anchors.centerIn: parent
        width: 100; height: 100
        slices: [
            PieSlice {
                anchors.fill: parent
                color: "red"
                fromAngle: 0; angleSpan: 110
            },
            PieSlice {
                anchors.fill: parent
                color: "black"
                fromAngle: 110; angleSpan: 50
            },
            PieSlice {
                anchors.fill: parent
                color: "blue"
                fromAngle: 160; angleSpan: 100
            }
        ]
    }
}

要做到这，替换 pieSlice 特性在 PieChart 采用 slices 特性，声明成 QQmlListProperty 类型。 QQmlListProperty class enables the creation of list properties in QML extensions. We replace the pieSlice() function with a slices() function that returns a list of slices, and add an internal append_slice() function (discussed below). We also use a QList to store the internal list of slices as m_slices : 

class PieChart : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(QQmlListProperty<PieSlice> slices READ slices)
    ...
public:
    ...
    QQmlListProperty<PieSlice> slices();
private:
    static void append_slice(QQmlListProperty<PieSlice> *list, PieSlice *slice);
    QString m_name;
    QList<PieSlice *> m_slices;
};

尽管 slices property does not have an associated WRITE function, it is still modifiable because of the way QQmlListProperty works. In the PieChart implementation, we implement PieChart::slices() to return a QQmlListProperty value and indicate that the internal PieChart::append_slice() function is to be called whenever a request is made from QML to add items to the list: 

QQmlListProperty<PieSlice> PieChart::slices()
{
    return QQmlListProperty<PieSlice>(this, 0, &PieChart::append_slice, 0, 0, 0);
}
void PieChart::append_slice(QQmlListProperty<PieSlice> *list, PieSlice *slice)
{
    PieChart *chart = qobject_cast<PieChart *>(list->object);
    if (chart) {
        slice->setParentItem(chart);
        chart->m_slices.append(slice);
    }
}

The append_slice() function simply sets the parent item as before, and adds the new item to the m_slices list. As you can see, the append function for a QQmlListProperty is called with two arguments: the list property, and the item that is to be appended.

The PieSlice class has also been modified to include fromAngle and angleSpan properties and to draw the slice according to these values. This is a straightforward modification if you have read the previous pages in this tutorial, so the code is not shown here.

第 6 章：编写扩展插件

extending-qml/chapter6-plugins

Currently the PieChart and PieSlice types are used by app.qml , which is displayed using a QQuickView in a C++ application. An alternative way to use our QML extension is to create a plugin library to make it available to the QML engine as a new QML import module. This allows the PieChart and PieSlice types to be registered into a type namespace which can be imported by any QML application, instead of restricting these types to be only used by the one application.

The steps for creating a plugin are described in Creating C++ Plugins for QML . To start with, we create a plugin class named ChartsPlugin . It subclasses QQmlExtensionPlugin and registers our QML types in the inherited registerTypes() 方法。

这里是 ChartsPlugin definition in chartsplugin.h : 

#include <QQmlExtensionPlugin>
class ChartsPlugin : public QQmlExtensionPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid)
public:
    void registerTypes(const char *uri);
};

And its implementation in chartsplugin.cpp : 

#include "piechart.h"
#include "pieslice.h"
#include <qqml.h>
void ChartsPlugin::registerTypes(const char *uri)
{
    qmlRegisterType<PieChart>(uri, 1, 0, "PieChart");
    qmlRegisterType<PieSlice>(uri, 1, 0, "PieSlice");
}

Then, we write a .pro project file that defines the project as a plugin library and specifies with DESTDIR that library files should be built into a ../Charts 目录。 

TEMPLATE = lib
CONFIG += plugin
QT += qml quick
DESTDIR = ../Charts
TARGET = $$qtLibraryTarget(chartsplugin)
HEADERS += piechart.h \
           pieslice.h \
           chartsplugin.h
SOURCES += piechart.cpp \
           pieslice.cpp \
           chartsplugin.cpp
DESTPATH=$$[QT_INSTALL_EXAMPLES]/qml/tutorials/extending-qml/chapter6-plugins/Charts
target.path=$$DESTPATH
qmldir.files=$$PWD/qmldir
qmldir.path=$$DESTPATH
INSTALLS += target qmldir
CONFIG += install_ok  # Do not cargo-cult this!
OTHER_FILES += qmldir
# Copy the qmldir file to the same folder as the plugin binary
cpqmldir.files = qmldir
cpqmldir.path = $$DESTDIR
COPIES += cpqmldir

When building this example on Windows or Linux, the Charts directory will be located at the same level as the application that uses our new import module. This way, the QML engine will find our module as the default search path for QML imports includes the directory of the application executable. On macOS, the plugin binary is copied to Contents/PlugIns in the the application bundle; this path is set in chapter6-plugins/app.pro : 

osx {
    charts.files = $$OUT_PWD/Charts
    charts.path = Contents/PlugIns
    QMAKE_BUNDLE_DATA += charts
}

To account for this, we also need to add this location as a QML import 路径 in main.cpp : 

    QQuickView view;
#ifdef Q_OS_OSX
    view.engine()->addImportPath(app.applicationDirPath() + "/../PlugIns");
#endif
    ...

Defining custom import paths is useful also when there are multiple applications using the same QML imports.

The .pro file also contains additional magic to ensure that the module definition qmldir file is always copied to the same location as the plugin binary.

The qmldir file declares the module name and the plugin that is made available by the module: 

module Charts
plugin chartsplugin

Now we have a QML module that can be imported to any application, provided that the QML engine knows where to find it. The example contains an executable that loads app.qml ，使用 import Charts 1.0 statement. Alternatively, you can load the QML file using the qmlscene tool , setting the import path to the current directory so that it finds the qmldir 文件： 

qmlscene -I . app.qml

The module "Charts" will be loaded by the QML engine, and the types provided by that module will be available for use in any QML document which imports it.

第 7 章：摘要

In this tutorial, we've shown the basic steps for creating a QML extension:

  • Define new QML types by subclassing QObject and registering them with qmlRegisterType ()
  • Add callable methods using Q_INVOKABLE or Qt slots, and connect to Qt signals with an onSignal syntax
  • Add property bindings by defining NOTIFY signals
  • Define custom property types if the built-in types are not sufficient
  • Define list property types using QQmlListProperty
  • Create a plugin library by defining a Qt plugin and writing a qmldir file

The 集成 QML 和 C++ documentation shows other useful features that can be added to QML extensions. For example, we could use default properties to allow slices to be added without using the slices 特性： 

PieChart {
    PieSlice { ... }
    PieSlice { ... }
    PieSlice { ... }
}

Or randomly add and remove slices from time to time using property value sources : 

PieChart {
    PieSliceRandomizer on slices {}
}

文件：

另请参阅 集成 QML 和 C++ .