Hybrid application using QML and Qt C++

Moved to http://www.zionsoft.net/2010/10/hybrid-application-qml-qt/

Though QML provides a nice way to design user interfaces, and JavaScript is employed there to implement the application logic and works pretty nice in many cases, we might still need Qt C++ in some situations (well, at least JavaScript has limited access outside its sandbox).

1) integrate QML into Qt C++
Suppose we have a QML file, named "myqml.qml", like this:
// this line should be "import QtQuick 1.0" since Qt 4.7.1
import Qt 4.7

Rectangle {
  id: myRectangle
  width: 800
  height: 480
  color: "lightgray"

  Text {
    id: myText
    text: "I love hybrid application!"
    anchors.centerIn: parent
    font.pointSize: 28
    font.bold: true

One easy way to integrate is to use the QDeclarativeView class, which provides a widget to display QML files. You just need the following three lines:
QDeclarativeView view;

However, QDeclarativeView consumes more resources than normal widgets. Fortunately, we can integrate QML into a graphics scene. The following lines shows the basic usage:
// provides an environment for instantiating QML components
QDeclarativeEngine engine;

// encapsulates a QML component definition
QDeclarativeComponent component(&engine, QUrl::fromLocalFile("myqml.qml"));

// creates the graphics item for QML at the engine's root context
QDeclarativeItem *item = qobject_cast<QDeclarativeItem *>(component.create());


Then with the help of the QDeclarativeItem class, you can easily access the properties of the QML element, e.g.:
qDebug() << item->property("color").typeName();
item->setProperty("color", QColor(255, 255, 255));

2) exposing Qt C++ objects to QML
You can also expose native Qt C++ objects to QML through QDeclarativeContext:
QDeclarativeContext *context = engine->rootContext();
context->setContextProperty("textFromQt", QString("I love hybrid application!"));

Then in QML, you can have e.g. the following line to access them:
text: textFromQt

You can also use QDeclarativePropertyMap to manage the exposed properties:
QDeclarativePropertyMap map;
map.insert("key1", "value1");
map.insert("key2", "value2");
context->setContextProperty("map", &map);

In a QML engine, there could be a couple of contexts, forming a tree structure. The child contexts inherit properties in the parent context. By default, there is only one root context, but you can always add more to give finer control of the exposed data, i.e. different QDeclarativeComponent inside the same context have the same exposed data set.

To expose a self-defined object, we can use the following code:
// define a class with properties
class MyObject: public QObject

  // the NOTIFY signal is needed to inform about changes
  // all properties will be exposed to QML
  Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
  MyObject(QObject *parent = 0) : QObject(parent), m_text("I love hybrid application!") {}

  QString text() const { return m_text; }
  void setText(QString &text)
    m_text = text;
    emit textChanged();

  void textChanged();

  QString m_text;

// then just expose it so QML can access it through name "myObject"
engine->rootContext()->setContextProperty("myObject", new MyObject());

Moreover, we can create new QML types:
// define the new type
class MyType : public QDeclarativeItem

  MyType(QDeclarativeItem *parent = 0) : QDeclarativeItem(parent)
    setFlag(QGraphicsItem::ItemHasNoContents, false);

  void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0)
    QPen pen(QColor(100, 100, 100), 2);
    painter->drawLine(0, 100, 100, 100);

// then register to expose it
qmlRegisterType<mychart>("com.blogspot.xizhizhu.mytype", 1, 0, "MyType");

In QML, you can use it like this:
import com.blogspot.xizhizhu.mytype 1.0

MyChart {
 id: myChart
 width: 100
 height: 200

Now let's jump to invoke a Qt C++ function from QML. Basically, QML can invoke slots and functions declared with Q_INVOKABLE. Suppose we have the following function in MyObject:
Q_INVOKABLE void showMessage()
  QMessageBox::information(NULL, "My Test", "Invoking a native function ;)");

Then you can invoke it in QML:

3) write plugins as QML extension
The benefits for using plugins as QML extensions are similar to using shared libraries, and it can be easily achieved with the help of QDeclarativeExtensionPlugin. Let's reuse the MyType class defined in the previous section. First, we need to create a plugin:
class MyPlugin : public QDeclarativeExtensionPlugin
  void registerTypes(const char *uri)
    qmlRegisterType<MyType>(uri, 1, 0, "MyType");

Q_EXPORT_PLUGIN2(myPlugin, MyPlugin);

Then create a file named "qmldir" to define which plugin to load from where (suppose the plugin is called "myplugin):
plugin myplugin /path/to/plugin

Now we can use qmlviewer to launch the QML file:
// no need to import now
MyChart {
 id: myChart
 width: 100
 height: 200

4) summary
  • Use QDeclarativeView or QDeclarativeComponent to integrate a QML file into native Qt C++.
  • Qt C++ can access the properties of QML elements through QDeclarativeItem.
  • Expose native objects to QML through QDeclarativeContext.
  • New QML types can be exported through qmlRegisterType.
  • The properties of native objects are exported as properties, and the slots or functions declared with Q_INVOKABLE can be invoked in QML.
  • Create plugins for extension using QDeclarativeExtensionPlugin.