extending-tutorial.qdoc

Go to the documentation of this file.

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qtqml-tutorials-extending-qml-example.html
\title Writing QML Extensions with C++
\brief Tutorial about extending QML with Qt C++.
 
The \l {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
\l{Overview - QML and C++ Integration} and its documentation sub-topics. In
particular, you may be interested in the sub-topics
\l{qtqml-cppintegration-exposecppattributes.html}{Exposing Attributes of C++ Classes to QML}
and \l {qtqml-cppintegration-definetypes.html}{Defining QML Types from C++}.
 
\section1 Opening the Tutorial Sources
 
The code in this tutorial is available as part of the Qt sources.
If you installed Qt with the \QOI, you can
find the sources in the Qt installation directory under
Examples/Qt-\QtVersion/qml/tutorials/extending-qml/.
 
\section1 Creating Project from Scratch
 
Alternatively, you can follow the tutorial by creating the sources from scratch:
For each chapter, create a new project using the \e {Qt Quick Application} template
in \QC, as instructed in \l {\QC: Create Qt Quick Applications}.
Then follow along by adapting and extending the generated skeleton code.
 
\section1 Chapter 1: Creating a New Type
\c 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 \l {Qt Quick QML Types}{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 \c PieChart type to be usable from QML like this:
 
\qml
    import Charts
 
    PieChart {
        width: 100; height: 100
        name: "A simple pie chart"
        color: "red"
    }
\endqml
 
To do this, we need a C++ class that encapsulates this \c PieChart type and its
properties. Since QML makes extensive use of Qt's \l{Meta-Object System}{meta object system},
this new class must:
 
\list
\li Inherit from QObject
\li Declare its properties using the Q_PROPERTY macro
\endlist
 
\section2 Class Declaration
 
Here is our \c PieChart class, defined in \c piechart.h:
 
\snippet tutorials/extending-qml/chapter1-basics/piechart.h 0
 
The class inherits from QQuickPaintedItem because we want to override
QQuickPaintedItem::paint() to 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 \c PieChart class defines the two properties, \c name and \c color, with the
Q_PROPERTY macro, and overrides QQuickPaintedItem::paint(). The \c PieChart
class is registered using the QML_ELEMENT macro, to allow it to be used from
QML. If you don't register the class, \c App.qml won't be able to create a
\c PieChart.
 
\section2 qmake Setup
 
For the registration to take effect, the \c qmltypes option is added to
\c CONFIG in the project file and a \c QML_IMPORT_NAME and
\c QML_IMPORT_MAJOR_VERSION are given:
 
\snippet tutorials/extending-qml/chapter1-basics/chapter1-basics.pro 0
 
Additionally, a \l{Module Definition qmldir Files}{qmldir} file needs to be added manually to
create a \l{Writing QML Modules}{QML module}.
 
\quotefile tutorials/extending-qml/chapter1-basics/qmldir
 
\section2 CMake Setup
 
For the registration to take effect when using CMake, use the
\l{qt6_add_qml_module} {qt_add_qml_module} command:
 
\snippet tutorials/extending-qml/chapter1-basics/CMakeLists.txt 0
 
The \l{qt6_add_qml_module}{qt_add_qml_module} API automatically generates a
\c qmldir file for the QML module.
 
\section2 Class Implementation
 
The class implementation in \c piechart.cpp simply sets and returns the
\c m_name and \c m_color values as appropriate, and implements \c paint() to
draw a simple pie chart:
 
\snippet tutorials/extending-qml/chapter1-basics/piechart.cpp 0
\dots 0
\snippet tutorials/extending-qml/chapter1-basics/piechart.cpp 1
 
\section2 QML Usage
 
Now that we have defined the \c PieChart type, we will use it from QML. The \c
App.qml file creates a \c PieChart item and displays the pie chart's details
using a standard QML \l Text item:
 
\snippet tutorials/extending-qml/chapter1-basics/App.qml 0
 
Notice that although the color is specified as a string in QML, it is automatically
converted to a QColor object for the PieChart \c color property. Automatic conversions are
provided for various other \l {QML Value Types}{value 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 \c App.qml.
 
Here is the application \c main.cpp:
 
\snippet tutorials/extending-qml/chapter1-basics/main.cpp 0
 
\section2 Project Build
 
To build the project we include the files, link against the libraries, and
define a type namespace called "Charts" with version 1.0 for any types exposed
to QML.
 
Using qmake:
 
\quotefile tutorials/extending-qml/chapter1-basics/chapter1-basics.pro
 
Using CMake:
 
\quotefile tutorials/extending-qml/chapter1-basics/CMakeLists.txt
 
Now we can build and run the application:
 
\image extending-tutorial-chapter1.png {Application creates the pie chart with
       properties defined by the pie chart type}
 
\note You may see a warning \e {Expression ... depends on non-bindable properties:
      PieChart::name}. This happens because we add a binding to the writable \c name
      property, but haven't yet defined a notify signal for it. The QML engine therefore
      cannot update the binding if the \c name value changes. This is addressed in
      the following chapters.
 
\section1 Chapter 2: Connecting to C++ Methods and Signals
\c extending-qml/chapter2-methods
 
Suppose we want \c PieChart to have a "clearChart()" method that erases the
chart and then emits a "chartCleared" signal. Our \c App.qml would be able
to call \c clearChart() and receive \c chartCleared() signals like this:
 
\snippet tutorials/extending-qml/chapter2-methods/App.qml 0
 
\image extending-tutorial-chapter2.png {User can click anywhere on the
       application window to clear the chart. This invokes the Clear Chart
       method}
 
To do this, we add a \c clearChart() method and a \c chartCleared() signal
to our C++ class:
 
\snippet tutorials/extending-qml/chapter2-methods/piechart.h 0
\dots
\snippet tutorials/extending-qml/chapter2-methods/piechart.h 1
\dots
\snippet tutorials/extending-qml/chapter2-methods/piechart.h 2
\dots
\snippet tutorials/extending-qml/chapter2-methods/piechart.h 3
 
The use of Q_INVOKABLE makes the \c clearChart() method available to the
Qt Meta-Object system, and in turn, to QML.
 
\note You can also declare the method as a Qt slot instead of using Q_INVOKABLE,
because public and protected slots are also callable from QML (you cannot call
private slots).
 
The \c clearChart() method changes the color to Qt::transparent,
repaints the chart, then emits the \c chartCleared() signal:
 
\snippet tutorials/extending-qml/chapter2-methods/piechart.cpp 0
 
Now when we run the application and click the window, the pie chart
disappears, and the application outputs:
 
\badcode
    qml: The chart has been cleared
\endcode
 
 
 
\section1 Chapter 3: Adding Property Bindings
\c 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 \c color property. That means
if we have code like this:
 
\snippet tutorials/extending-qml/chapter3-bindings/App.qml 0
 
\image extending-tutorial-chapter3.png {Pie chart B's color property is bound
                                        to pie chart A's color property.}
 
The "color: chartA.color" statement binds the \c color value of
\c chartB to the \c color of \c chartA.
Whenever \c chartA's \c color value changes, \c chartB's \c color value
updates to the same value. When the window is clicked, the \c onClicked
handler in the MouseArea changes the color of \c chartA, thereby changing
both charts to the color blue.
 
It's easy to enable property binding for the \c color property.
We add a \l{Qt's Property System}{NOTIFY} feature to its Q_PROPERTY() declaration to indicate that a "colorChanged" signal
is emitted whenever the value changes.
 
\snippet tutorials/extending-qml/chapter3-bindings/piechart.h 0
\dots
\snippet tutorials/extending-qml/chapter3-bindings/piechart.h 1
\dots
\snippet tutorials/extending-qml/chapter3-bindings/piechart.h 2
\dots
\snippet tutorials/extending-qml/chapter3-bindings/piechart.h 3
 
Then, we emit this signal in \c setColor():
 
\snippet tutorials/extending-qml/chapter3-bindings/piechart.cpp 0
 
It's important for \c setColor() to check that the color value has actually changed
before emitting \c 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.
 
 
 
\section1 Chapter 4: Using Custom Property Types
 
\c extending-qml/chapter4-customPropertyTypes
 
The \c 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:
 
\code
    // 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
    }
\endcode
 
Aside from \c 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 \l {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 \c property with a type called
"PieSlice" that has a \c color property. Instead of assigning a color,
we assign an \c PieSlice value which itself contains a \c color:
 
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/App.qml 0
 
Like \c PieChart, this new \c PieSlice type inherits from QQuickPaintedItem and declares
its properties with Q_PROPERTY():
 
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/pieslice.h 0
 
To use it in \c PieChart, we modify the \c color property declaration
and associated method signatures:
 
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/piechart.h 0
\dots
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/piechart.h 1
\dots
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/piechart.h 2
\dots
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/piechart.h 3
 
There is one thing to be aware of when implementing \c setPieSlice(). The \c PieSlice
is a visual item, so it must be set as a child of the \c PieChart using
QQuickItem::setParentItem() so that the \c PieChart knows to paint this child
item when its contents are drawn:
 
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/piechart.cpp 0
 
Like the \c PieChart type, the \c PieSlice type has to be exposted to QML
using QML_ELEMENT.
 
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/pieslice.h 0
\dots
 
As with \c PieChart, we add the "Charts" type namespace, version 1.0, to our
build file:
 
Using qmake:
 
\quotefile tutorials/extending-qml/chapter4-customPropertyTypes/chapter4-customPropertyTypes.pro
 
Using CMake:
 
\dots
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/CMakeLists.txt 0
\snippet tutorials/extending-qml/chapter4-customPropertyTypes/CMakeLists.txt 1
\dots
 
 
 
\section1 Chapter 5: Using List Property Types
\c extending-qml/chapter5-listproperties
 
Right now, a \c PieChart can only have one \c PieSlice. Ideally a chart would
have multiple slices, with different colors and sizes. To do this, we could
have a \c slices property that accepts a list of \c PieSlice items:
 
\snippet tutorials/extending-qml/chapter5-listproperties/App.qml 0
 
\image extending-tutorial-chapter5.png {The slices property accepts a list of
 pie slice items. The pieslice item sets the angle and color of each pie slice}
 
To do this, we replace the \c pieSlice property in \c PieChart with a \c slices property,
declared as a \l QQmlListProperty type. The \l QQmlListProperty class enables the
creation of list properties in types exposed to QML. We replace the \c pieSlice()
function with a \c slices() function that returns a list of slices. We also use
a QList to store the internal list of slices as \c m_slices:
 
\snippet tutorials/extending-qml/chapter5-listproperties/piechart.h 0
\dots
\snippet tutorials/extending-qml/chapter5-listproperties/piechart.h 1
\dots
\snippet tutorials/extending-qml/chapter5-listproperties/piechart.h 2
 
Although the \c slices property does not have an associated \c WRITE function,
it is still modifiable because of the way \l QQmlListProperty works.
In the \c PieChart implementation, we implement \c PieChart::slices() to
return a \l QQmlListProperty value:
 
\snippet tutorials/extending-qml/chapter5-listproperties/piechart.cpp 0
 
This synthesizes the necessary functions to interact with the list from QML.
The resulting \l QQmlListProperty is a \e view into the list. Alternately, you can
manually provide the individual access functions for the list. This is necessary
if your list is not a \l QList or if you want to restrict or otherwise customize
QML access to your list. In most cases, however, the constructor taking a
\l QList pointer is the safest and easiest option.
 
The \c PieSlice class has also been modified to include \c fromAngle and \c 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.
 
 
 
\section1 Chapter 6: Writing an Extension Plugin
 
\c extending-qml/chapter6-plugins
 
Currently the \c PieChart and \c PieSlice types are used by \c 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 \c PieChart and
\c 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 \l {Creating C++ Plugins for QML}.
To start with, we create a plugin class named \c ChartsPlugin. It subclasses
QQmlEngineExtensionPlugin and uses the Q_PLUGIN_METADATA() macro to register the
plugin with the Qt meta object system.
 
Here is the \c ChartsPlugin definition in \c chartsplugin.h:
 
\snippet tutorials/extending-qml/chapter6-plugins/Charts/chartsplugin.h 0
 
Then, we configure the build file to define the project as a plugin library.
 
Using qmake:
 
\quotefile tutorials/extending-qml/chapter6-plugins/Charts/Charts.pro
 
Using CMake:
 
\quotefile tutorials/extending-qml/chapter6-plugins/Charts/CMakeLists.txt
 
When building this example on Windows or Linux, the \c 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 \c Contents/PlugIns in the the application bundle.
With qmake, this path is set in \c {chapter6-plugins/app.pro}:
 
\quotefromfile tutorials/extending-qml/chapter6-plugins/app.pro
\skipto macos
\printuntil }
 
To account for this, we also need to add this location as a
\l {QML Import Path}{QML import path} in \c main.cpp:
 
\snippet tutorials/extending-qml/chapter6-plugins/main.cpp 0
\dots
 
Defining custom import paths is useful also when there are multiple
applications using the same QML imports.
 
The \c .pro file also contains additional magic to ensure that the
\l {Module Definition qmldir Files}{module definition qmldir file} is always copied
to the same location as the plugin binary.
 
The \c qmldir file declares the module name and the plugin that is made available
by the module:
 
\quotefile tutorials/extending-qml/chapter6-plugins/Charts/qmldir
 
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
\c App.qml, which uses the \c {import Charts 1.0} statement. Alternatively, you can
load the QML file using the \l {Prototyping with the QML Runtime Tool}{qml tool},
setting the import path to the current directory so that it finds the \c qmldir file:
 
\code
    qml -I . App.qml
\endcode
 
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.
 
 
 
\section1 Chapter 7: Summary
 
In this tutorial, we've shown the basic steps for creating a QML extension:
 
\list
\li Define new QML types by subclassing QObject and registering them with
    QML_ELEMENT or QML_NAMED_ELEMENT()
\li Add callable methods using \l Q_INVOKABLE or Qt slots, and connect to Qt signals
    with an \c onSignal syntax
\li Add property bindings by defining \l{Qt's Property System}{NOTIFY} signals
\li Define custom property types if the built-in types are not sufficient
\li Define list property types using QQmlListProperty
\li Create a plugin library by defining a Qt plugin and writing a
    \l {Module Definition qmldir Files}{qmldir} file
\endlist
 
The \l{Overview - QML and C++ Integration}{QML and C++ Integration overview}
documentation shows other useful features that can be added to QML extensions.
For example, we could use \l{Default Properties}{default properties} to allow
slices to be added without using the \c slices property:
 
\badcode
    PieChart {
        PieSlice { ... }
        PieSlice { ... }
        PieSlice { ... }
    }
\endcode
 
Or randomly add and remove slices from time to time using \l{Property Value Sources}{property value sources}:
 
\badcode
    PieChart {
        PieSliceRandomizer on slices {}
    }
\endcode
 
\note To continue learning about QML extensions and features follow the
\l {Writing advanced QML Extensions with C++} tutorial.
*/