Qt 5.7 | Qt QML | Writing QML Extensions with C++

Qt 5.7.1 Reference Documentation

# Writing QML Extensions with C++

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 Integrating QML and C++ and its documentation sub-topics. In particular, you may be interested in the sub-topics Exposing Attributes of C++ Classes to QML and Defining QML Types from C++.

## Running the Tutorial Examples

The code in this tutorial is available as an example project with subprojects associated with each tutorial chapter. In Qt Creator, open the Welcome mode and select the tutorial from Examples. In Edit mode, expand the extending-qml project, right-click on the subproject (chapter) you want to run and select Run.

## Chapter 1: Creating a New Type

`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 types. 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"
      }

To do this, we need a C++ class that encapsulates this `PieChart` type and its two properties. Since QML makes extensive use of Qt's meta object system, this new class must:

- Inherit from QObject
- Declare its properties using the Q_PROPERTY macro

Here is our `PieChart` class, defined in `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;
      };

The class inherits from QQuickPaintedItem because we want to override 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` class defines the two properties, `name` and `color`, with the Q_PROPERTY macro, and overrides 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 basic types; for example, a string like "640x480" can be automatically converted to a QSize value.

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`.

Here is the application `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` library:

      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:

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

## Chapter 2: Connecting to C++ Methods and Signals

`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

## Chapter 3: Adding Property Bindings

`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` to the `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.

## Chapter 4: Using Custom Property Types

`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 Data Type Conversion Between QML and C++ 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)
      }

Like `PieChart`, this new `PieSlice` type inherits from 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;
      };

To use it in `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()`. The `PieSlice` is a visual item, so it must be set as a child of the `PieChart` using 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);
      }

Like the `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");
          ...
      }

## Chapter 5: Using List Property Types

`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
                  }
              ]
          }
      }

To do this, we replace the `pieSlice` property in `PieChart` with a `slices` property, declared as a QQmlListProperty type. The 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;
      };

Although the `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.

## Chapter 6: Writing an Extension Plugin

`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() method.

Here is the `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` directory.

      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 path 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`, which uses the `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` file:

      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.

## Chapter 7: Summary

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 Integrating QML and 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` property:

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

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

      PieChart {
          PieSliceRandomizer on slices {}
      }

Files:

- tutorials/extending-qml/chapter1-basics/app.qml
- tutorials/extending-qml/chapter1-basics/piechart.cpp
- tutorials/extending-qml/chapter1-basics/piechart.h
- tutorials/extending-qml/chapter2-methods/app.qml
- tutorials/extending-qml/chapter2-methods/piechart.cpp
- tutorials/extending-qml/chapter2-methods/piechart.h
- tutorials/extending-qml/chapter3-bindings/app.qml
- tutorials/extending-qml/chapter3-bindings/piechart.cpp
- tutorials/extending-qml/chapter3-bindings/piechart.h
- tutorials/extending-qml/chapter4-customPropertyTypes/app.qml
- tutorials/extending-qml/chapter4-customPropertyTypes/piechart.cpp
- tutorials/extending-qml/chapter4-customPropertyTypes/piechart.h
- tutorials/extending-qml/chapter4-customPropertyTypes/pieslice.cpp
- tutorials/extending-qml/chapter4-customPropertyTypes/pieslice.h
- tutorials/extending-qml/chapter5-listproperties/app.qml
- tutorials/extending-qml/chapter5-listproperties/piechart.cpp
- tutorials/extending-qml/chapter5-listproperties/piechart.h
- tutorials/extending-qml/chapter5-listproperties/pieslice.cpp
- tutorials/extending-qml/chapter5-listproperties/pieslice.h
- tutorials/extending-qml/chapter6-plugins/app.qml
- tutorials/extending-qml/chapter6-plugins/import/chartsplugin.cpp
- tutorials/extending-qml/chapter6-plugins/import/chartsplugin.h
- tutorials/extending-qml/chapter6-plugins/import/piechart.cpp
- tutorials/extending-qml/chapter6-plugins/import/piechart.h
- tutorials/extending-qml/chapter6-plugins/import/pieslice.cpp
- tutorials/extending-qml/chapter6-plugins/import/pieslice.h
- tutorials/extending-qml/chapter1-basics/main.cpp
- tutorials/extending-qml/extending-qml.pro
- tutorials/extending-qml/chapter1-basics/chapter1-basics.pro
- tutorials/extending-qml/chapter1-basics/chapter1-basics.qrc
- tutorials/extending-qml/chapter2-methods/chapter2-methods.pro
- tutorials/extending-qml/chapter2-methods/chapter2-methods.qrc
- tutorials/extending-qml/chapter3-bindings/chapter3-bindings.pro
- tutorials/extending-qml/chapter3-bindings/chapter3-bindings.qrc
- tutorials/extending-qml/chapter4-customPropertyTypes/chapter4-customPropertyTypes.pro
- tutorials/extending-qml/chapter4-customPropertyTypes/chapter4-customPropertyTypes.qrc
- tutorials/extending-qml/chapter5-listproperties/chapter5-listproperties.pro
- tutorials/extending-qml/chapter5-listproperties/chapter5-listproperties.qrc
- tutorials/extending-qml/chapter6-plugins/app.pro
- tutorials/extending-qml/chapter6-plugins/app.qrc
- tutorials/extending-qml/chapter6-plugins/chapter6-plugins.pro
- tutorials/extending-qml/chapter6-plugins/import/import.pro
- tutorials/extending-qml/chapter6-plugins/import/qmldir

See also Integrating QML and C++.