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QDeclarativeEngine provides the environment in which QML is run, QDeclarativeComponent encapsulates \l {QML Documents}, and QDeclarativeContext allows applications to expose data to QML component instances. QML also includes a convenience API, QDeclarativeView, for applications that simply want to embed QML components into a new QGraphicsView. QDeclarativeView covers up many of the details discussed below. While QDeclarativeView is mainly intended for rapid prototyping it can have uses in production applications. If you are looking at retrofitting an existing Qt application with QML, read \l{Integrating QML with existing Qt UI code}. \section1 Basic Usage Every application requires at least one QDeclarativeEngine. A QDeclarativeEngine allows the configuration of global settings that apply to all the QML component instances - such as the QNetworkAccessManager that is used for network communications, and the path used for persistent storage. Multiple QDeclarativeEngine's are only needed if the application requires these settings to differ between QML component instances. \l {QML Documents} are loaded using the QDeclarativeComponent class. Each QDeclarativeComponent instance represents a single QML document. A QDeclarativeComponent can be passed a document URL, or raw text representing the content of the document. The document URL can be a local filesystem URL, or any network URL supported by QNetworkAccessManager. QML component instances can then be created by calling the QDeclarativeComponent::create() method. Here's an example of loading a QML document, and creating an object from it. \code QDeclarativeEngine *engine = new QDeclarativeEngine(parent); QDeclarativeComponent component(engine, QUrl::fromLocalFile("main.qml")); QObject *myObject = component.create(); \endcode \section1 Exposing Data QML components are instantiated in a QDeclarativeContext. A context allows the application to expose data to the QML component instance. A single QDeclarativeContext can be used to instantiate all the objects used by an application, or several QDeclarativeContext can be created for more fine grained control over the data exposed to each instance. If a context is not passed to the QDeclarativeComponent::create() method, the QDeclarativeEngine's \l {QDeclarativeEngine::rootContext()}{root context} is used. Data exposed through the root context is available to all object instances. \section1 Simple Data To expose data to a QML component instance, applications set \l {QDeclarativeContext::setContextProperty()} {context properties} which are then accessible by name from QML \l {Property Binding}s and JavaScript. The following example shows how to expose a background color to a QML file through QDeclarativeView: \table \row \o \c {// main.cpp} \snippet doc/src/snippets/declarative/qtbinding/contextproperties/main.cpp 0 \o \c {// main.qml} \snippet doc/src/snippets/declarative/qtbinding/contextproperties/main.qml 0 \endtable Or, if you want \c main.cpp to create the component without showing it in a QDeclarativeView, you could create an instance of QDeclarativeContext using QDeclarativeEngine::rootContext() instead: \snippet doc/src/snippets/declarative/qtbinding/contextproperties/main.cpp 1 Context properties work just like normal properties in QML bindings - if the \c backgroundColor context property in this example was changed to red, the component object instances would all be automatically updated. Note that it is the responsibility of the creator to delete any QDeclarativeContext it constructs. If the \c windowContext is no longer needed when the \c window component instantiation is destroyed, the \c windowContext must be destroyed explicitly. The simplest way to ensure this is to set \c window as \c windowContext's parent. QDeclarativeContexts form a tree - each QDeclarativeContext except for the root context has a parent. Child QDeclarativeContexts effectively inherit the context properties present in their parents. This gives applications more freedom in partitioning the data exposed to different QML object instances. If a QDeclarativeContext sets a context property that is also set in one of its parents, the new context property shadows that in the parent. In The following example, the \c background context property in \c {Context 1} shadows the \c background context property in the root context. \image qml-context-tree.png \section2 Structured Data Context properties can also be used to expose structured and writable data to QML objects. In addition to all the types already supported by QVariant, QObject derived types can be assigned to context properties. QObject context properties allow the data exposed to be more structured, and allow QML to set values. The following example creates a \c CustomPalette object, and sets it as the \c palette context property. \snippet doc/src/snippets/declarative/qtbinding/custompalette/custompalette.h 0 \snippet doc/src/snippets/declarative/qtbinding/custompalette/main.cpp 0 The QML that follows references the palette object, and its properties, to set the appropriate background and text colors. When the window is clicked, the palette's text color is changed, and the window text will update accordingly. \snippet doc/src/snippets/declarative/qtbinding/custompalette/main.qml 0 To detect when a C++ property value - in this case the \c CustomPalette's \c text property - changes, the property must have a corresponding NOTIFY signal. The NOTIFY signal specifies a signal that is emitted whenever the property changes value. Implementers should take care to only emit the signal if the value \e changes to prevent loops from occuring. Accessing a property from a binding that does not have a NOTIFY signal will cause QML to issue a warning at runtime. \section2 Dynamic Structured Data If an application is too dynamic to structure data as compile-time QObject types, dynamically structured data can be constructed at runtime using the QDeclarativePropertyMap class. \section1 Calling C++ methods from QML It is possible to call methods of QObject derived types by either exposing the methods as public slots, or by marking the methods Q_INVOKABLE. The C++ methods can also have parameters and return values. QML has support for the following types: \list \o bool \o unsigned int, int \o float, double, qreal \o QString \o QUrl \o QColor \o QDate, QTime, QDateTime \o QPoint, QPointF \o QSize, QSizeF \o QRect, QRectF \o QVariant \endlist This example toggles the "Stopwatch" object on/off when the MouseArea is clicked: \table \row \o \c {// main.cpp} \snippet doc/src/snippets/declarative/qtbinding/stopwatch/stopwatch.h 0 \snippet doc/src/snippets/declarative/qtbinding/stopwatch/main.cpp 0 \o \c {// main.qml} \snippet doc/src/snippets/declarative/qtbinding/stopwatch/main.qml 0 \endtable Note that in this particular example a better way to achieve the same result is to have a "running" property in \c main.qml. This leads to much nicer QML code: \table \row \o \code // main.qml import Qt 4.7 Rectangle { MouseArea { anchors.fill: parent onClicked: stopwatch.running = !stopwatch.running } } \endcode \endtable Of course, it is also possible to call \l {Adding new methods}{functions declared in QML from C++}. \section1 Network Components If the URL passed to QDeclarativeComponent is a network resource, or if the QML document references a network resource, the QDeclarativeComponent has to fetch the network data before it is able to create objects. In this case, the QDeclarativeComponent will have a \l {QDeclarativeComponent::Loading}{Loading} \l {QDeclarativeComponent::status()}{status}. An application will have to wait until the component is \l {QDeclarativeComponent::Ready}{Ready} before calling \l {QDeclarativeComponent::create()}. The following example shows how to load a QML file from a network resource. After creating the QDeclarativeComponent, it tests whether the component is loading. If it is, it connects to the QDeclarativeComponent::statusChanged() signal and otherwise calls the \c {continueLoading()} method directly. This test is necessary, even for URLs that are known to be remote, just in case the component has been cached and is ready immediately. \code MyApplication::MyApplication() { // ... component = new QDeclarativeComponent(engine, QUrl("http://www.example.com/main.qml")); if (component->isLoading()) QObject::connect(component, SIGNAL(statusChanged(QDeclarativeComponent::Status)), this, SLOT(continueLoading())); else continueLoading(); } void MyApplication::continueLoading() { if (component->isError()) { qWarning() << component->errors(); } else { QObject *myObject = component->create(); } } \endcode \section1 Qt Resources QML content can be loaded from \l {The Qt Resource System} using the \e qrc: URL scheme. For example: \c [project/example.qrc] \quotefile doc/src/snippets/declarative/qtbinding/resources/example.qrc \c [project/project.pro] \quotefile doc/src/snippets/declarative/qtbinding/resources/resources.pro \c [project/main.cpp] \snippet doc/src/snippets/declarative/qtbinding/resources/main.cpp 0 \c [project/main.qml] \snippet doc/src/snippets/declarative/qtbinding/resources/main.qml 0 Note that the resource system cannot be accessed from QML directly. If the main QML file is loaded as a resource, all files specifed as relative paths in QML will also be loaded from the resource system. Using the resource system is completely transparent to the QML layer. This also means that if the main QML file is not loaded as a resource then files in the resource system cannot be accessed from QML. */