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While QmlView is mainly intended for rapid prototyping it can have uses in production applications. \section1 Basic Usage Every application requires at least one QmlEngine. A QmlEngine 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 QmlEngine's are only needed if the application requires these settings to differ between QML component instances. \l {QML Documents} are loaded using the QmlComponent class. Each QmlComponent instance represents a single QML document. A QmlComponent 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 QmlComponent::create() method. Here's an example of loading a QML document, and creating an object from it. \code QmlEngine *engine = new QmlEngine(parent); QmlComponent component(engine, QUrl("main.qml")); QObject *myObject = component.create(); \endcode \section1 Exposing Data QML components are instantiated in a QmlContext. A context allows the application to expose data to the QML component instance. A single QmlContext can be used to instantiate all the objects used by an application, or several QmlContext can be created for more fine grained control over the data exposed to each instance. If a context is not passed to the QmlComponent::create() method, the QmlEngine's \l {QmlEngine::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 {QmlContext::setContextProperty()} {context properties} which are then accessible by name from QML \l {Property Binding}s and \l {ECMAScript Blocks}. The following example shows how to expose a background color to a QML file. \table \row \o \code // main.cpp QmlContext *windowContext = new QmlContext(engine->rootContext()); windowContext->setContextProperty("backgroundColor", QColor(Qt::lightsteelblue)); QmlComponent component(&engine, "main.qml"); QObject *window = component.create(windowContext); \endcode \o \code // main.qml import Qt 4.6 Rectangle { color: backgroundColor Text { anchors.centerIn: parent text: "Hello Light Steel Blue World!" } } \endcode \endtable Context properties work just like normal properties in QML bindings - if the \c backgroundColor context property in the previous example was changed to red, the component object instances would all be automatically updated. QmlContexts form a tree - each QmlContext except for the root context has a parent. Child QmlContexts 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 QmlContext 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. \code class CustomPalette : public QObject { Q_OBJECT Q_PROPERTY(QColor background READ background WRITE setBackground NOTIFY backgroundChanged) Q_PROPERTY(QColor text READ text WRITE setText NOTIFY text) public: CustomPalette() : m_background(Qt::white), m_text(Qt::black) {} QColor background() const { return m_background; } void setBackground(const QColor &c) { if (c != m_background) { m_background = c; emit backgroundChanged(); } } QColor text() const { return m_text; } void setText(const QColor &c) { if (c != m_text) { m_text = c; emit textChanged(); } } private: QColor m_background; QColor m_text; }; int main(int argc, char **argv) { // ... QmlContext *windowContext = new QmlContext(engine->rootContext()); windowContext->setContextProperty("palette", new CustomPalette); QmlComponent component(&engine, "main.qml"); QObject *window = component.create(windowContext); } \endcode 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. \code // main.qml import Qt 4.6 Rectangle { width: 240 height: 320 color: palette.background Text { anchors.centerIn: parent color: palette.text text: "Hello Colorful World!" } MouseRegion { anchors.fill: parent onClicked: { palette.text = "blue"; } } } \endcode 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 QmlPropertyMap class. \section1 Network Components If the URL passed to QmlComponent is a network resource, or if the QML document references a network resource, the QmlComponent has to fetch the network data before it is able to create objects. In this case, the QmlComponent will have a \l {QmlComponent::Loading}{Loading} \l {QmlComponent::status()}{status}. An application will have to wait until the component is \l {QmlComponent::Ready}{Ready} before calling \l {QmlComponent::create()}. The following example shows how to load a QML file from a network resource. After creating the QmlComponent, it tests whether the component is loading. If it is, it connects to the QmlComponent::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 QmlComponent(engine, QUrl("http://www.example.com/main.qml")); if (component->isLoading()) QObject::connect(component, SIGNAL(statusChanged(QmlComponent::Status)), this, SLOT(continueLoading())); else continueLoading(); } void MyApplication::continueLoading() { if (component->isError()) { qWarning() << component->errors(); } else { QObject *myObject = component->create(); } } \endcode */