Qt 4.8: QDeclarativeEngine Class Reference

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# QDeclarativeEngine Class Reference

The QDeclarativeEngine class provides an environment for instantiating QML components. More...

     #include <QDeclarativeEngine>

Inherits: QObject.

This class was introduced in Qt 4.7.

- List of all members, including inherited members

## Public Types

enum | ObjectOwnership { CppOwnership, JavaScriptOwnership }

## Properties

- offlineStoragePath : QString

- 1 property inherited from QObject

## Public Functions

| QDeclarativeEngine ( QObject * parent = 0 )
virtual | ~QDeclarativeEngine ()
void | addImageProvider ( const QString & providerId, QDeclarativeImageProvider * provider )
void | addImportPath ( const QString & path )
void | addPluginPath ( const QString & path )
QUrl | baseUrl () const
void | clearComponentCache ()
QDeclarativeImageProvider * | imageProvider ( const QString & providerId ) const
QStringList | importPathList () const
bool | importPlugin ( const QString & filePath, const QString & uri, QString * errorString )
QNetworkAccessManager * | networkAccessManager () const
QDeclarativeNetworkAccessManagerFactory * | networkAccessManagerFactory () const
QString | offlineStoragePath () const
bool | outputWarningsToStandardError () const
QStringList | pluginPathList () const
void | removeImageProvider ( const QString & providerId )
QDeclarativeContext * | rootContext () const
void | setBaseUrl ( const QUrl & url )
void | setImportPathList ( const QStringList & paths )
void | setNetworkAccessManagerFactory ( QDeclarativeNetworkAccessManagerFactory * factory )
void | setOfflineStoragePath ( const QString & dir )
void | setOutputWarningsToStandardError ( bool enabled )
void | setPluginPathList ( const QStringList & paths )

- 29 public functions inherited from QObject

## Signals

void | quit ()
void | warnings ( const QList<QDeclarativeError> & warnings )

- 1 signal inherited from QObject

## Static Public Members

QDeclarativeContext * | contextForObject ( const QObject * object )
ObjectOwnership | objectOwnership ( QObject * object )
void | setContextForObject ( QObject * object, QDeclarativeContext * context )
void | setObjectOwnership ( QObject * object, ObjectOwnership ownership )

- 7 static public members inherited from QObject

## Related Non-Members

QDeclarativeInfo | qmlInfo ( const QObject * object )
int | qmlRegisterInterface ( const char * typeName )
int | qmlRegisterType ( const char * uri, int versionMajor, int versionMinor, const char * qmlName )
int | qmlRegisterType ()
int | qmlRegisterTypeNotAvailable ( const char * uri, int versionMajor, int versionMinor, const char * qmlName, const QString & message )
int | qmlRegisterUncreatableType ( const char * uri, int versionMajor, int versionMinor, const char * qmlName, const QString & message )

## Macros

| QML_DECLARE_TYPE ()
| QML_DECLARE_TYPEINFO ( Type, Flags )

### Additional Inherited Members

- 1 public slot inherited from QObject
- 8 protected functions inherited from QObject

## Detailed Description

The QDeclarativeEngine class provides an environment for instantiating QML components.

Each QML component is instantiated in a QDeclarativeContext. QDeclarativeContext's are essential for passing data to QML components. In QML, contexts are arranged hierarchically and this hierarchy is managed by the QDeclarativeEngine.

Prior to creating any QML components, an application must have created a QDeclarativeEngine to gain access to a QML context. The following example shows how to create a simple Text item.

     QDeclarativeEngine engine;
     QDeclarativeComponent component(&engine);
     component.setData("import QtQuick 1.0\nText { text: \"Hello world!\" }", QUrl());
     QDeclarativeItem *item = qobject_cast<QDeclarativeItem *>(component.create());

     //add item to view, etc
     ...

In this case, the Text item will be created in the engine's root context.

See also QDeclarativeComponent and QDeclarativeContext.

## Member Type Documentation

### enum QDeclarativeEngine::ObjectOwnership

Ownership controls whether or not QML automatically destroys the QObject when the object is garbage collected by the JavaScript engine. The two ownership options are:

Constant | Value | Description
--- | --- | ---
`QDeclarativeEngine::CppOwnership` | `0` | The object is owned by C++ code, and will never be deleted by QML. The JavaScript destroy() method cannot be used on objects with CppOwnership. This option is similar to QScriptEngine::QtOwnership.
`QDeclarativeEngine::JavaScriptOwnership` | `1` | The object is owned by JavaScript. When the object is returned to QML as the return value of a method call or property access, QML will delete the object if there are no remaining JavaScript references to it and it has no QObject::parent(). This option is similar to QScriptEngine::ScriptOwnership.

Generally an application doesn't need to set an object's ownership explicitly. QML uses a heuristic to set the default object ownership. By default, an object that is created by QML has JavaScriptOwnership. The exception to this are the root objects created by calling QDeclarativeCompnent::create() or QDeclarativeComponent::beginCreate() which have CppOwnership by default. The ownership of these root-level objects is considered to have been transferred to the C++ caller.

Objects not-created by QML have CppOwnership by default. The exception to this is objects returned from a C++ method call. The ownership of these objects is passed to JavaScript.

Calling setObjectOwnership() overrides the default ownership heuristic used by QML.

## Property Documentation

### offlineStoragePath : QString

This property holds the directory for storing offline user data.

Returns the directory where SQL and other offline storage is placed.

QDeclarativeWebView and the SQL databases created with openDatabase() are stored here.

The default is QML/OfflineStorage in the platform-standard user application data directory.

Note that the path may not currently exist on the filesystem, so callers wanting to create new files at this location should create it first - see QDir::mkpath().

Access functions:

QString | offlineStoragePath () const
void | setOfflineStoragePath ( const QString & dir )

## Member Function Documentation

### QDeclarativeEngine::QDeclarativeEngine ( QObject * parent = 0 )

Create a new QDeclarativeEngine with the given parent.

### QDeclarativeEngine::~QDeclarativeEngine ()`[virtual]`

Destroys the QDeclarativeEngine.

Any QDeclarativeContext's created on this engine will be invalidated, but not destroyed (unless they are parented to the QDeclarativeEngine object).

### void QDeclarativeEngine::addImageProvider ( const QString & providerId, QDeclarativeImageProvider * provider )

Sets the provider to use for images requested via the image: url scheme, with host providerId. The QDeclarativeEngine takes ownership of provider.

Image providers enable support for pixmap and threaded image requests. See the QDeclarativeImageProvider documentation for details on implementing and using image providers.

All required image providers should be added to the engine before any QML sources files are loaded.

See also removeImageProvider().

### void QDeclarativeEngine::addImportPath ( const QString & path )

Adds path as a directory where the engine searches for installed modules in a URL-based directory structure. The path may be a local filesystem directory or a URL.

The newly added path will be first in the importPathList().

See also setImportPathList() and QML Modules.

### void QDeclarativeEngine::addPluginPath ( const QString & path )

Adds path as a directory where the engine searches for native plugins for imported modules (referenced in the `qmldir` file).

By default, the list contains only `.`, i.e. the engine searches in the directory of the `qmldir` file itself.

The newly added path will be first in the pluginPathList().

See also setPluginPathList().

### QUrl QDeclarativeEngine::baseUrl () const

Return the base URL for this engine. The base URL is only used to resolve components when a relative URL is passed to the QDeclarativeComponent constructor.

If a base URL has not been explicitly set, this method returns the application's current working directory.

See also setBaseUrl().

### void QDeclarativeEngine::clearComponentCache ()

Clears the engine's internal component cache.

Normally the QDeclarativeEngine caches components loaded from qml files. This method clears this cache and forces the component to be reloaded.

### QDeclarativeContext * QDeclarativeEngine::contextForObject ( const QObject * object )`[static]`

Returns the QDeclarativeContext for the object, or 0 if no context has been set.

When the QDeclarativeEngine instantiates a QObject, the context is set automatically.

See also setContextForObject().

### QDeclarativeImageProvider * QDeclarativeEngine::imageProvider ( const QString & providerId ) const

Returns the QDeclarativeImageProvider set for providerId.

### QStringList QDeclarativeEngine::importPathList () const

Returns the list of directories where the engine searches for installed modules in a URL-based directory structure.

For example, if `/opt/MyApp/lib/imports` is in the path, then QML that imports `com.mycompany.Feature` will cause the QDeclarativeEngine to look in `/opt/MyApp/lib/imports/com/mycompany/Feature/` for the components provided by that module. A `qmldir` file is required for defining the type version mapping and possibly declarative extensions plugins.

By default, the list contains the directory of the application executable, paths specified in the `QML_IMPORT_PATH` environment variable, and the builtin `ImportsPath` from QLibraryInfo.

See also addImportPath() and setImportPathList().

### bool QDeclarativeEngine::importPlugin ( const QString & filePath, const QString & uri, QString * errorString )

Imports the plugin named filePath with the uri provided. Returns true if the plugin was successfully imported; otherwise returns false.

On failure and if non-null, *errorString will be set to a message describing the failure.

The plugin has to be a Qt plugin which implements the QDeclarativeExtensionPlugin interface.

### QNetworkAccessManager * QDeclarativeEngine::networkAccessManager () const

Returns a common QNetworkAccessManager which can be used by any QML element instantiated by this engine.

If a QDeclarativeNetworkAccessManagerFactory has been set and a QNetworkAccessManager has not yet been created, the QDeclarativeNetworkAccessManagerFactory will be used to create the QNetworkAccessManager; otherwise the returned QNetworkAccessManager will have no proxy or cache set.

See also setNetworkAccessManagerFactory().

### QDeclarativeNetworkAccessManagerFactory * QDeclarativeEngine::networkAccessManagerFactory () const

Returns the current QDeclarativeNetworkAccessManagerFactory.

See also setNetworkAccessManagerFactory().

### ObjectOwnership QDeclarativeEngine::objectOwnership ( QObject * object )`[static]`

Returns the ownership of object.

See also setObjectOwnership().

### bool QDeclarativeEngine::outputWarningsToStandardError () const

Returns true if warning messages will be output to stderr in addition to being emitted by the warnings() signal, otherwise false.

The default value is true.

See also setOutputWarningsToStandardError().

### QStringList QDeclarativeEngine::pluginPathList () const

Returns the list of directories where the engine searches for native plugins for imported modules (referenced in the `qmldir` file).

By default, the list contains only `.`, i.e. the engine searches in the directory of the `qmldir` file itself.

See also addPluginPath() and setPluginPathList().

### void QDeclarativeEngine::quit ()`[signal]`

This signal is emitted when the QML loaded by the engine would like to quit.

### void QDeclarativeEngine::removeImageProvider ( const QString & providerId )

Removes the QDeclarativeImageProvider for providerId.

Returns the provider if it was found; otherwise returns 0.

See also addImageProvider().

### QDeclarativeContext * QDeclarativeEngine::rootContext () const

Returns the engine's root context.

The root context is automatically created by the QDeclarativeEngine. Data that should be available to all QML component instances instantiated by the engine should be put in the root context.

Additional data that should only be available to a subset of component instances should be added to sub-contexts parented to the root context.

### void QDeclarativeEngine::setBaseUrl ( const QUrl & url )

Set the base URL for this engine to url.

See also baseUrl().

### void QDeclarativeEngine::setContextForObject ( QObject * object, QDeclarativeContext * context )`[static]`

Sets the QDeclarativeContext for the object to context. If the object already has a context, a warning is output, but the context is not changed.

When the QDeclarativeEngine instantiates a QObject, the context is set automatically.

See also contextForObject().

### void QDeclarativeEngine::setImportPathList ( const QStringList & paths )

Sets paths as the list of directories where the engine searches for installed modules in a URL-based directory structure.

By default, the list contains the directory of the application executable, paths specified in the `QML_IMPORT_PATH` environment variable, and the builtin `ImportsPath` from QLibraryInfo.

See also importPathList() and addImportPath().

### void QDeclarativeEngine::setNetworkAccessManagerFactory ( QDeclarativeNetworkAccessManagerFactory * factory )

Sets the factory to use for creating QNetworkAccessManager(s).

QNetworkAccessManager is used for all network access by QML. By implementing a factory it is possible to create custom QNetworkAccessManager with specialized caching, proxy and cookie support.

The factory must be set before executing the engine.

See also networkAccessManagerFactory().

### void QDeclarativeEngine::setObjectOwnership ( QObject * object, ObjectOwnership ownership )`[static]`

Sets the ownership of object.

See also objectOwnership().

### void QDeclarativeEngine::setOutputWarningsToStandardError ( bool enabled )

Set whether warning messages will be output to stderr to enabled.

If enabled is true, any warning messages generated by QML will be output to stderr and emitted by the warnings() signal. If enabled is false, on the warnings() signal will be emitted. This allows applications to handle warning output themselves.

The default value is true.

See also outputWarningsToStandardError().

### void QDeclarativeEngine::setPluginPathList ( const QStringList & paths )

Sets the list of directories where the engine searches for native plugins for imported modules (referenced in the `qmldir` file) to paths.

By default, the list contains only `.`, i.e. the engine searches in the directory of the `qmldir` file itself.

See also pluginPathList() and addPluginPath().

### void QDeclarativeEngine::warnings ( const QList<QDeclarativeError> & warnings )`[signal]`

This signal is emitted when warnings messages are generated by QML.

## Related Non-Members

### QDeclarativeInfo qmlInfo ( const QObject * object )

Prints warning messages that include the file and line number for the specified QML object.

When QML types display warning messages, it improves traceability if they include the QML file and line number on which the particular instance was instantiated.

To include the file and line number, an object must be passed. If the file and line number is not available for that instance (either it was not instantiated by the QML engine or location information is disabled), "unknown location" will be used instead.

For example,

     qmlInfo(object) << tr("component property is a write-once property");

prints

     QML MyCustomType (unknown location): component property is a write-once property

### int qmlRegisterInterface ( const char * typeName )

This template function registers the C++ type in the QML system under the name typeName.

#include <QtDeclarative> to use this function.

Returns the QML type id.

### int qmlRegisterType ( const char * uri, int versionMajor, int versionMinor, const char * qmlName )

This template function registers the C++ type in the QML system with the name qmlName, in the library imported from uri having the version number composed from versionMajor and versionMinor.

Returns the QML type id.

There are two forms of this template function:

     template<typename T>
     int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName);

     template<typename T, int metaObjectRevision>
     int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName);

The former is the standard form which registers the type T as a new type. The latter allows a particular revision of a class to be registered in a specified version (see QML Type Versioning).

For example, this registers a C++ class `MySliderItem` as a QML type named `Slider` for version 1.0 of a module called "com.mycompany.qmlcomponents":

     #include <QtDeclarative>

     ...

     qmlRegisterType<MySliderItem>("com.mycompany.qmlcomponents", 1, 0, "Slider");

Once this is registered, the type can be used in QML by importing the specified module name and version number:

     import com.mycompany.qmlcomponents 1.0

     Slider {
         // ...
     }

Note that it's perfectly reasonable for a library to register types to older versions than the actual version of the library. Indeed, it is normal for the new library to allow QML written to previous versions to continue to work, even if more advanced versions of some of its types are available.

### int qmlRegisterType ()

This is an overloaded function.

This template function registers the C++ type in the QML system. Instances of this type cannot be created from the QML system.

#include <QtDeclarative> to use this function.

Returns the QML type id.

### int qmlRegisterTypeNotAvailable ( const char * uri, int versionMajor, int versionMinor, const char * qmlName, const QString & message )

This function registers a type in the QML system with the name qmlName, in the library imported from uri having the version number composed from versionMajor and versionMinor, but any attempt to instantiate the type will produce the given error message.

Normally, the types exported by a module should be fixed. However, if a C++ type is not available, you should at least "reserve" the QML type name, and give the user of your module a meaningful error message.

Returns the QML type id.

Example:

     #ifdef NO_GAMES_ALLOWED
     qmlRegisterTypeNotAvailable("MinehuntCore", 0, 1, "Game", "Get back to work, slacker!");
     #else
     qmlRegisterType<MinehuntGame>("MinehuntCore", 0, 1, "Game");
     #endif

This will cause any QML which uses this module and attempts to use the type to produce an error message:

     fun.qml: Get back to work, slacker!
        Game {
        ^

Without this, a generic "Game is not a type" message would be given.

#include <QtDeclarative> to use this function.

See also qmlRegisterUncreatableType().

### int qmlRegisterUncreatableType ( const char * uri, int versionMajor, int versionMinor, const char * qmlName, const QString & message )

This template function registers the C++ type in the QML system with the name qmlName, in the library imported from uri having the version number composed from versionMajor and versionMinor.

While the type has a name and a type, it cannot be created, and the given error message will result if creation is attempted.

This is useful where the type is only intended for providing attached properties or enum values.

Returns the QML type id.

#include <QtDeclarative> to use this function.

See also qmlRegisterTypeNotAvailable().

## Macro Documentation

### QML_DECLARE_TYPE ()

Equivalent to `Q_DECLARE_METATYPE(TYPE *)` and `Q_DECLARE_METATYPE(QDeclarativeListProperty<TYPE>)`

#include <QtDeclarative> to use this macro.

### QML_DECLARE_TYPEINFO ( Type, Flags )

Declares additional properties of the given Type as described by the specified Flags.

Current the only supported type info is `QML_HAS_ATTACHED_PROPERTIES` which declares that the Type supports Attached Properties.

#include <QtDeclarative> to use this macro.